Dephosphorization of Steels Aug15

Dephosphorization of Steels...

Dephosphorization of Steels  The effects of phosphorus (P) on the properties of steels are summarized in Tab 1. It can be seen that P has both positive and negative effects on the steel’s properties. Tab 1 Effects of phosphorus on properties of steels Sl.No. Property Effect of phosphorus 1 Strength Strong positive (strengthens ferrite) 2 Bake hardenability Positive 3 Ductility Strong negative 4 Galvanneal Can improve resistance to powdering 5 Phosphatability Positive 6 Enameling a. Fish scaling Negative b. Pickling Positive 7 Weldability Not harmful for contents less than 0.1 % 8 Core loss in motor lamination Strong negative 9 Fracture toughness Strong negative   Steels having low content of P are necessary for applications where high ductility is needed, such as thin sheets, deep drawn steel, and pipelines etc. In the earlier days, P control was not considered a big challenge in steel production since iron ores with low P contents were readily and cheaply available. However, in the recent past, because of high iron ore prices, lower priced iron ores from sources which normally have higher P content are being used and this has made P control an important activity during the steelmaking. In addition to P from in the iron ores, P also enters the liquid steel due to the recycling of the BOF (basic oxygen furnace) slag. The recycling of the BOF slag is being done through the sinter plant or directly into the blast furnace in order to retrieve the iron and lime content of the slag and to minimize the issues related to slag disposal. The sinter or the BOF slag fed to the blast furnace inevitably increases the P content of the hot metal and hence the P loads on the steelmaking process. In integrated steel plants,...

Desulphurization of Liquid Steel Jul30

Desulphurization of Liquid Steel...

Desulphurization of Liquid Steel Solubility of sulphur (S) in liquid iron (Fe) is quite high. But the solubility of S in solid iron is limited. It is 0.002 % in ferrite at room temperature and 0.013 % in austenite at around 1000 deg C. Hence, when liquid steel cools down, sulphur is liberated from the solution in the form of iron sulphide (FeS) which forms a eutectic with the surrounding iron. The eutectic is segregated at the iron grain boundaries. The eutectic temperature is comparatively low at around 988 deg C. Fe-FeS eutectic weakens the bonding between the grains and causes sharp drop in the properties of steel at the temperatures of hot deformation. During the continuous casting of liquid steel, sulphur present in liquid steel (i) causes the formation of undesirable sulphides which promotes granular weaknesses and cracks in steel during solidification, (ii) lowers the melting point and inter-granular strength, (iii) contributes to the brittleness of steel and thus acts as stress raiser in steel, and (iv) results in the hot shortness. Sulphur, present in solid steel as FeS inclusions, has several detrimental effects on steel processing. During deformation, FeS inclusions act as crack initiation sites and zones of weakness. Such inclusions from sulphur adversely affect the toughness, ductility, formability, weldability, and corrosion resistance of steel. An increase in manganese (Mn) content (not less than 0.2 %) however, helps prevent formation of FeS. Sulphur is thus an undesirable element in steel. Manganese actively reacts with iron sulphides during solidification of steel transforming FeS to MnS according to the following reaction. FeS (slag) + Mn (steel) = MnS (slag) + Fe The melting temperature of manganese sulphide (MnS) is comparatively high (around 1610 deg C). Hence steel containing manganese can be deformed in hot state. However...

Weldability of Steels...

Weldability of Steels There are several factors which control the weldability of carbon (C) and low alloy steels in electric arc welding. A good understanding of the chemical and physical phenomena which occurs in the weldments is necessary for the proper welding of the different steels. Operational parameters, thermal cycles, and metallurgical factors affecting the weld metal transformations and the susceptibility to hot and cold cracking are some of the factors which have marked influence on the weldability of steels. There are also some common tests which determine the weldability of steel. The C and low alloy steels represents a large number of steels which differ in chemical composition, strength, heat treatment, corrosion resistance, and weldability. These steels can be categorized as (i) plain C steels, (ii) high strength low alloy (HSLA) steels, (iii) quenched and tempered (QT) steels, (iv) heat treatable low alloy (HTLA) steels, and (v) pre-coated steels. To understand weldability of steels, it is necessary to have knowledge about the various weld regions. Characteristic features of welds Single pass weldments In the case of a single pass bead, the weldment is generally divided into two main regions namely (i) the fusion zone, or weld metal, and (ii) the heat affected zone (HAZ) as shown in Fig 1. Within the fusion zone, the peak temperature exceeds the melting point of the base steel, and the chemical composition of the weld metal depends on the choice of welding consumables, the base steel dilution ratio, and the operating conditions. Under conditions of rapid cooling and solidification of the weld metal, alloying and impurity elements segregate extensively to the centre of the inter-dendritic or inter-cellular regions and to the centre parts of the weld, resulting in significant local chemical in-homogeneities. Therefore, the transformation behaviour of...

Induction Furnace and Important Operational Aspects Feb14

Induction Furnace and Important Operational Aspects...

Induction Furnace and Important Operational Aspects   The development of the induction furnace for steel making has been a boon to the small steel producers. These furnaces are easy to install, operate and maintain. These furnaces are smaller in heat size with a low cost investment and preferred by lower capacity steel plants. In these furnaces, steel is produced by melting the charge material using the heat produced by electromagnetic field. The induction furnace consists basically of a crucible, inductor coil, and shell, cooling system and tilting mechanism. The crucible is formed from refractory material, which the furnace coils is lined with. This crucible holds the charge material and subsequently the melt. The choice of refractory material depends on the type of the charge and basically consist of either acidic, basic or neutral refractories. The inductor coil is a tubular copper coil with specific number of turns. An alternating current (AC) passes through it and magnetic flux is generated within the conductor. The magnetic flux generated induces eddy currents that enable the heating and subsequently the melting process in the crucible. The shell is the outer part of the furnace. This houses the crucible and the inductor coils, and has higher thermal capacity. It is made of rectangular parallelepiped with low carbon steel plate and joined at the corners by edge carriers from angular pieces and strips of non-magnetic metal. The cooling system is normally a through one way flow system with the tubular copper coils connected to water source through flexible rubber hoses. The cooling process is important because the circuit of the furnace appears resistive, and the real power is not only consumed in the charged material but also in the resistance of the coil. This coil loss as well as the loss...

Argon Rinsing of Steels Oct26

Argon Rinsing of Steels...

Argon Rinsing of Steels Gas rinsing process is a method where rinsing of liquid steel in the teeming ladle is carried out through injection of inert gas into the steel bath. Argon (Ar) gas is preferred for rinsing since it is not only inert in nature but its solubility in steel is also very low. Rinsing results from the expansion of gas due to heating and decrease in pressure as the gas rises. The Ar rinsing of liquid steel is carried out for obtaining homogenous temperature, composition, and promotion of slag metal refining reaction.  The Ar rinsing of liquid steel is reported to be an excellent process for floatation and separation of non-metallic inclusions. The liquid steel after tapping is stratified in the teeming ladle due to the additions of the ferro alloys and the carburizer in the teeming ladle at the time of tapping of the steel. This stratified steel is agitated by purging of Ar gas in the Ar rinsing station. Ar gas purging through the liquid steel bath help generate enough bath turbulence to effect rapid thermal homogenization. Stirring with Ar also enhances mixing rate for chemical additions. Different variables for argon rinsing include gas purging rate, amount of liquid steel (heat size), amount of superheat available in the liquid steel, amount of carry over slag, amount of synthetic slag or ladle covering compound added, amount of mixing needed for chemical additions. Experienced operators and metallurgists recognize the importance of accurate and consistent Ar gas rinsing in the teeming ladle. Clean steel and good castability in the continuous casting machine (CCM) depend on a consistent and gentle rinse stir. A good Ar rinsing control system at the Ar rinsing station facilitates reproducible and accurate argon rinsing rates and durations. Ar gas can...