Cold Rolled Steel:

Cold Rolled Steel:

Cold rolling takes place below re-crystallization temperature of steel. Cold rolled sheet products have been available for many, many years, and have been successfully used for a multitude of applications. They offer better control of thickness, shape, width, surface finish, and other special quality features that compliment the emerging need for highly engineered end use applications.

Cold rolled sheet products are used in a wide variety of end applications such as appliances - refrigerators, washers, dryers, and other small appliances, automobiles - exposed as well as unexposed parts, electric motors, and bathtubs. To meet the various end use requirements, cold-rolled sheet products are metallurgically designed to provide specific attributes such as high formability, deep drawability, high strength, high dent resistance, good magnetic properties, enamelability, and paintability.

The primary feature of cold reduction is to reduce the thickness of hot-rolled coils into thinner, but also becomes much harder, less ductile, and very difficult to form. However, after the cold-reduced product is annealed (heated to high temperatures), it becomes very soft and formable. In fact, the combination of cold reduction and annealing lead to a refinement of the steel that provides very desirable and unique forming properties for subsequent use by the customer.

The primary feature of cold reduction is to reduce the thickness of hot-rolled coils into thinner thicknesses that are not generally attainable in the hot rolled state. Clearly, controlling the sheet thickness along the entire length of the coil is very important to ensure that the product will perform consistently during the processing by the end user. In addition, there are a number of other product attributes that need to be controlled in the cold reduction process. Flatness (deviation from a flat plane) is one of the more important attributes. Very sophisticated strip-shape controlling technology is used to maintain good flatness. Surface finish is another product attribute that needs to be controlled during the cold-reduction process.

One of the important operations is the pickling operation, which must be well-controlled to assure that all the oxides formed during hot rolling are removed. The thickness of the hot-rolled strip is important in that the properties of the final cold rolled and annealed product is influenced by the percent cold reduction. This means that the thickness of each hot-rolled coil is carefully controlled to provide the mill with a specific thickness to achieve the proper percent cold reduction.

Steel chemistry, hot strip mill processing variables, pickling practices, cold-rolling mill practices, annealing practices, and finally, temper rolling practices all have a role in achieving the manufacture of top quality cold-rolled sheet products.

Hot rolling of steel:

Hot rolling of steel:

Hot rolling of steel is the metallurgical process when billets are reduced to rolled products in high temperature condition. Hot rolling is used mainly to produce sheet metal or simple cross sections from steel billets. In this method metal is passed or deformed between a set of work rolls and the temperature of the metal is generally above its re-crystallization temperature.

It permits easily large deformations of steel to be achieved with a low number of rolling cycles. Because the steel is worked before the formation of crystal structures, it does not itself affect its micro-structural properties. Hot rolling is primarily concerned with manipulating material shape and product’s geometry. It does not affect the mechanical properties of the steel.

Hot rolling is done by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.

Steel making:

Steel making:

After iron is obtained either from blast furnace in liquid / melt form (pig iron) or from DRI (sponge iron) process; it is sent for steel production. The iron that emerges from the blast furnace contains 4 - 4.5 % carbon by wt., and other impurities which makes the metal too brittle for most engineering applications. The Basic Oxygen Steelmaking (BOS) process takes this liquid iron plus recycled scrap steel, and reduces the carbon content to between 0 and 1.5% by blowing oxygen through the metal.

Steel is generally made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes.

In Bessemer (BOS) process molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter. In the Siemens process, the necessary heat for melting and working the charge is supplied by oil or gas. Both the gas and air are preheated by regenerators. The regenerators are chambers filled with checker brickwork, brick and space alternating.

The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air.

The least costly method of making steel uses scrap metal as its base. Steel scrap from many sources—such as old bridges, refrigerators, and automobiles—and other additives are placed in an electric arc furnace, where the intense heat produced by carbon electrodes and chemical reactions melts the scrap, converting it into molten steel.

Most of these steel plants have finishing mills on site that convert iron and steel into both finished and intermediate products. Some of the goods produced in finishing mills are steel wire, pipe, bars, rods, and sheets. While wire, steel reinforcing bars, and pipes are considered finished products, rolled steel is intermediates, meaning it is normally shipped to companies, such as automotive plants, that stamp, shape, and machine the rolled steel into car parts. In these finishing mills, products also may be coated with chemicals, paints, or other metals that give the steel desired characteristics for various industries and consumers.

Steel manufacturing is an intensely competitive global industry. By continually improving its manufacturing processes and consolidating businesses many of the steel companies increased productivity sufficiently to remain competitive in the global market for steel.

Direct reduction of iron (DRI) – ‘Sponge iron’

Direct reduction of iron (DRI) – ‘Sponge iron’, another method of producing iron:

All steelmaking processes require the input of iron bearing materials as process feedstock. For making steel in a basic oxygen furnace, the iron bearing feed materials are usually blast furnace hot metal and steel scrap. A broadly used iron source is also a product known as Direct Reduced Iron ("DRI") which is produced by the solid state reduction of iron ore to highly metallized iron without the formation of liquid iron. This solid state reduction of iron ore is also called ‘sponge iron’.

Sponge iron is the product created when iron ore is reduced to metallic iron, in the presence of coal, at temperatures below the melting point of iron. The external shape of the ore is retained with 30% reduction in weight due to oxide reduction resulting in change in true density from 4.4 gm/cc to 7.8 gm/cc in this product. This paves the way for 54% reduction in volume which is manifested in pore formation through out the interior of reduced product and hence the name “Sponge Iron”. This spongy mass sometimes called a bloom. This makes for an energy-efficient feedstock for specialty steel manufacturers which used to rely upon scrap metal. The advantage of this technique is that iron can be obtained at a lower furnace temperature (only about 1,100°C or so). Only small quantities of sponge iron can be made at a time as compare to blast furnace process, is the major disadvantage.

In this method, the iron ore along with coal is charged to the top portion of the reduction zone of a rotary kiln or furnace, wherein the bed of particles which descend by gravity is reduced by a hot reducing gas largely composed by carbon monoxide (CO) and hydrogen (H2). Finally, the product sponge iron is discharged from the bottom portion of the discharge zone of the furnace and conveyed (after cooling), for example, to be melted in an electric arc furnace or to be briquetted in a briquetting machine coupled to the reduction reactor. The evolution of sponge iron as a metallic feed in electric steel making has been mainly due to reduced availability of high quality scrap and its increasing cost.

Quality of sponge iron for steel making: There are several parameters to be monitored for improving the quality of sponge iron for steel making operation, such as – (a) Size, (b) Density, (c) Unit weight, (d) Crushing strength, (e) Weather resistance, (f) Carbon contents, (g) Metallization.

(a) Size - The size of sponge iron is very important especially with regard to continuous feeding. A very fine sized material (1 mm to 2 mm) would be quickly oxidized during falling to the slag or may be lost in fume extraction system. Extremely large size (exceeding 30 mm) poses problem during continuous feeding. The size fraction less than 2 mm needs to be limited for continuous feeding.

(b) Density - Sponge iron after falling should have the ability to penetrate into the slag layer and reside at the slag/metal interface for effective heat transfer and chemical reaction. Sponge iron with lower density tend to float on the slag while, high density material readily penetrates into the metal. Hence, it is desirable to have the density of sponge iron in the range 4 - 6 gm/cc.

(c) Unit weight – The transition time of the sponge iron pellets through the slag is dependant on the momentum. If the pellet stays in the slag layer for too long a time, the phenomenon of slag boiling occurs. Slag fluidity is highly important. However, a heavier sponge iron pellet does not require close control in slag fluidity.

(d) Crushing strength - Sponge iron should possess good crushing strength to prevent generation of large amounts of fines.

(e) Weather resistance - Sponge iron is prone to oxidation and heat builds up in contact with atmosphere. The storage of Sponge Iron for long periods of time affects its metallization, partially due to surface re-oxidation caused by the porous structure of sponge iron pellets or lumps.

(f) Carbon contents - During continuous feeding, an active carbon — oxygen boil is necessary to shield the arcs. It has been observed that to achieve the aforesaid, sponge iron should possess a minimum of 0.60% carbon.

(g) Metallization - High metallization helps in lower power consumption but severely reduces the bath activity and results in flat bath conditions. For low metallization levels, increased carburization is required to compensate for the extra oxygen in sponge iron.

Extraction of Iron using blast furnace and various types of Steel:

Extraction of Iron using blast furnace and various types of Steel:

Iron ore is reduced to iron by heating them with coke (a form of carbon) in blast furnace. As mentioned earlier, common iron ores are hematite (Fe2O3) and magnetite (Fe3O4). The air blown into the bottom of the blast furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important to conserve heat energy. The coke burns in the blast of hot air to form carbon dioxide; exothermic reaction releases heat. This reaction is the main source of heat in the furnace.

C + O2 = CO2

At the high temperature at the bottom of the furnace, carbon dioxide reacts with carbon to produce carbon monoxide.

C + CO2 = 2CO

It is this carbon monoxide which is the main reducing agent in the furnace to produce iron.

Fe2O3 + 3CO = 2Fe + 3CO2

In the hotter parts of the furnace, the carbon itself also acts as a reducing agent. Notice that at these temperatures, the other product of the reaction is carbon monoxide, not carbon dioxide.

Fe2O3 + 3C = 2Fe + 3CO

The temperature of the furnace is hot enough to melt the iron which trickles down to the bottom as ‘pig iron’, where it can be tapped off. The limestone is added to convert siliceous impurities into ‘slag’ ( as calcium silicate, CaSiO3), which melts and runs to the bottom. The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron.

CaCO3 + O2 = CaO + CO2. CaO + SiO2 = CaSiO3

Pig iron – The molten iron from the bottom of the blast furnace is pig iron. It contains 3.5 - 4.5% carbon and varying amount of contamination such as, sulfur, silicon and phosphorus. Pig iron is the intermediate step on the way to cast iron and steel.

Cast Iron – Some time pig iron from the blast furnace can be used as cast iron. It is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle.

Steel - Most of the pig iron is used to make one of a number of types of steel. There isn't just one substance called steel - they are a family of alloys of iron with carbon or various other metals after removal of impurities from molten iron.

Removal of impurities - Impurities in the pig iron from the Blast Furnace include carbon, sulfur, phosphorus and silicon. Sulfur is removed by reacting with magnesium (Mg) as magnesium sulfate (MgS).

Mg + S = MgS

Carbon is removed by blowing oxygen in molten iron. The impure molten iron is mixed with scrap iron (from recycling) and oxygen is blown on to the mixture. The oxygen reacts with the remaining impurities to form various oxides. The carbon forms carbon monoxide. Since this is a gas it removes itself from the iron! This carbon monoxide can be cleaned and used as a fuel gas.

Elements like phosphorus and silicon react with the oxygen to form acidic oxides. These are removed using quicklime (calcium oxide), which is added to the furnace during the oxygen blow. They react to form compounds such as calcium silicate or calcium phosphate which form a slag on top of the iron.

Various steel products: The various steel products used include Wrought iron, Mild steel, High carbon steel and other specialized steel.

Wrought iron: When all the carbon is removed from the molten iron to give high purity iron, it is known as wrought iron. Wrought iron is quite soft and has little structural strength. It was once used to make decorative gates and railings, but these days mild steel is normally used instead.

Mild steel: Mild steel is iron containing up to about 0.25% of carbon. The presence of the carbon makes the steel stronger and harder than pure iron. The higher the percentage of carbon, the harder the steel becomes. Mild steel is used for lots of things - nails, wire, car bodies, ship building, girders and bridges amongst others.

High carbon steel: High carbon steel contains up to about 1.5% of carbon. The presence of the extra carbon makes it very hard, but it also makes it more brittle.

Specialized steel: These are iron alloyed with other metals, such as -

	iron mixed with	special properties	uses include
stainless steel	chromium and nickel	resists corrosion	cutlery, cooking utensils, kitchen sinks, industrial equipment for food and drink processing
titanium steel	titanium	withstands high temperatures	gas turbines, spacecraft
manganese steel	manganese	very hard	rock-breaking machinery, some railway track (e.g. points), military helmets

Manganese, manganese ore, ferromanganese and manganese dioxide:

Manganese, manganese ore, ferromanganese and manganese dioxide:

Manganese (Mn) is a hard, silvery white metal with a melting point of 1,244° C. Ordinarily too brittle to be of structural value itself, it is an essential agent in steelmaking. It has a properties to remove impurities such as sulfur and oxygen and adds important physical properties to steel.

The most important manganese ores are the oxides pyrolusite, romanechite, manganite, and hausmannite and the carbonate ore rhodochrosite. Rhodonite and braunite, both silicate ores, are frequently found with the oxides. Only ores containing greater than 35 percent manganese are considered commercially exploitable.

Some manganese ores are upgraded by washing, jigging and undersized ores can be agglomerated by sintering.

Manganese is used principally in the form of alloys with iron. The most important of these alloys, which are used in steelmaking, are ferromanganese. Ferromanganese, an alloy of iron and manganese, used in the production of steel. This is a product of the blast furnace, obtained by treating pyrolusite (manganese ore) in a blast furnace with iron ore and carbon. It is containing, besides iron, 74 to 82% of manganese and some silicon, phosphorus, sulfur and carbon. It is used as a deoxidizer and for the introduction of manganese into steel.

It is made by heating a mixture of the oxides MnO2 and Fe2O3, with carbon in a furnace. They undergo thermal decomposition reaction. Standard ferromanganese specifications given below:

Mn - 74.0-82.0%

C - 7.5% max.

Si - 1.2% max.

P - 0.35% max.

S - 0.05% max.

In cast iron, manganese is used mainly to counteract the bad effects of sulfur. In steel, manganese acts as a deoxidizer and combines with sulfur, thereby improving the hot-working properties of the steel. Also improves the strength, toughness of steel.

Other use of manganese ore is in batteries as manganese dioxide. Manganese dioxide (MnO2) is blackish or brown solid in color, occurs naturally as the mineral pyrolusite, which is the main ore of manganese. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the zinc-carbon battery.

Iron ore and its beneficiation:

Iron ore and its beneficiation:

Most useful metal in the world, Iron, is extracted from iron ore. It is the rock from which metallic iron can be economically extracted. Iron ore is a mineral substance which, when heated in the presence of a reductant, will yield metallic iron (Fe). The iron ore, usually, very rich in iron oxides (Fe3O4 and Fe2O3). Iron ores are mostly dark grey to rusty red in color and high specific gravity. Two main types of iron ore used for iron making – Magnetite (Fe3O4) and Hematite (Fe2O3). Common iron ores include:

• Hematite - Fe₂O₃ - 70 percent iron
• Magnetite - Fe₃O₄ - 72 percent iron
• Limonite - Fe₂O₃ + H₂O - 50 percent to 66 percent iron
• Siderite - FeCO₃ - 48 percent iron

Iron ore is the source of primary iron for the world's iron and steel industries. It is therefore essential for the production of steel, which in turn is essential to maintain a strong industrial base. Almost all (98%) iron ore is used in steelmaking. Iron ore is mined in about 50 countries. The seven largest of these producing countries account for about three-quarters of total world production. Australia and Brazil together dominate the world's iron ore exports, each having about one-third of total exports.

Hematite deposits are mostly sedimentary in origin, such as the banded iron formations (BIFs). BIFs consist of alternating layers of chert (a variety of the mineral quartz), hematite and magnetite. They are found throughout the world and are the most important iron ore in the world today. Their formation is not fully understood, though it is known that they formed by the chemical precipitation of iron from shallow seas about 1.8-1.6 billion years ago, during the Proterozoic period.

Magnetite also mostly found in Banded iron formations (BIF). They are fine grained metamorphosed sedimentary rocks composed predominantly of magnetite and silica. Mining and processing of BIF formations involves coarse crushing and screening. Magnetite is beneficiated by crushing and then separating the magnetite from the gangue minerals with a magnet. This is usually so efficient that lower grade ore can be treated when it is magnetite than a comparable grade of hematite ore, especially when the magnetite is quite coarse.

Magnetic separation and flotation are the most widely accepted technologies for the upgrading of iron ore particles, but these processes result in iron concentrate with high amounts of very fine and/or interlocked silica particles.

Inferior sources of iron ore generally required beneficiation. Due to the high density of hematite relative to silicates, beneficiation usually involves a combination of crushing and milling as well as heavy liquid separation. This is achieved by passing the finely crushed ore over a bath of solution containing bentonite or other agent which increases the density of the solution. When the density of the solution is properly calibrated, the hematite will sink and the silicate mineral fragments will float and can be removed.