Coated Steel – Galvanized steel:

Coated Steel – Galvanized steel:

Steel sheet coated through a heat process or through electrolysis with a layer of substance to protect the base metal (substrate) against corrosion. The most commonly used material is zinc which can be applied either using the heat process (hot-dip galvanizing) or using electrolysis (electro-galvanizing).

A. Galvanized steel products - Zinc coated galvanized GP and GC steel products are sturdy, light weight, bright and corrosion resistant. The functions of the zinc layer are mainly three folds:

(a) To retain the steel intact with its full initial strength

(b) To provide the surface a more pleasing appearance

(c) To increase the life of any suitable organic finishing system applied over it

In fact, zinc coatings provide the most effective and economic way of protecting steel against corrosion. Zinc coated steel is very environment friendly. Zinc is an integral part of our environment and occurs naturally in rocks, soil, air and water. Zinc is also an essential element for all life, from humans and animals to plants and micro – organisms. All types of zinc coated steel products are recyclable. In the last few decades, both the recycling technology and its capacity has been developed considerably in response to increasing environmental awareness and the cost-effective alternative supplies of raw materials for the steel and zinc industries. Presently, about 80 percent of the zinc is available from zinc coated steel that is recycled in India.

B. Galvanizing process: There are two main processes used in the production galvanized product namely: (i) Hot – dipped galvanizing and (ii) Electrolytic Galvanizing

(i) Hot – dipped Galvanizing – It is one of the basic and efficient corrosion resistant technique for producing galvanized steel. During the galvanizing process, steel coils are previously cleaned and pickled and then dipped into a bath of molten zinc to form a series of zinc / iron alloys integrated with the steel surface. As steel is removed from the bath, a layer of relatively pure zinc is deposited on the top of alloy layers. On solid deification, the zinc assumes a crystalline metallic luster, often known as “spangling”. The spangles can be enhanced and reduced depending on the end use. The life of galvanized coating in steel structures permanently immersed in liquids depends on the corrosion properties of the liquid and the thickness of the galvanized coating in the structure. Hot-dipped galvanizing technology has been greatly improved over the years and by the introduction of the sendzimir process or one of its variants, HDG products are introducing on the market share of electrolytic galvanized products in the western world because it is more cost effective.

(ii) Electrolytic Galvanized Steel - In the electrolytic galvanizing process, zinc ions from the electrolytic is deposited on the strip surface (cathode) under the influence of electric current using either soluble or insoluble anodes. The electrolyte is usually zinc sulfate or zinc chloride or mixed. The electro–galvanized strips are post treated with passivation solutions such as phosphate or chromate. The plating process controls the coating thickness resulting in much thinner coatings as well as double-side coatings. The automobile industries in Japan and the U.S. are using between 35 to 45 percent of the total steel consumed in the form of electro-galvanized steel sheets. These sheets have excellent surface finish and press formability for protection from corrosion of auto body, fuel tanks, exhaust pipes etc.

Stainless Steel - Rust and stain-resistant steel

Stainless Steel - Rust and stain-resistant steel:

The resistance of stainless steel to corrosion and staining, low maintenance, relative inexpensive and familiar luster make it an ideal base material for a host of commercial applications.Stainless steels are distinguished from carbon steel by their content of chromium and, in certain cases, nickel. Adding chromium to carbon steel makes it more rust and stain-resistant and adding nickel to chromium stainless steel enhances the mechanical properties of the steel.

Stainless steels are iron-base alloys containing Chromium. Stainless steels usually contain less than 30% Cr and more than 50% Fe. They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film. This oxide establishes on the surface and heals itself in the presence of oxygen. Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen. Carbon is usually present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades. Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application. The resistance of stainless steel to many corrosive factors, such as exposure to water, air, acid and alkalis, is provided by a transparent protective chromium oxide film that forms on its exterior.

Stainless steels are manufactured in different types of grade, but all types contain at least 10% chromium, along with other elements added to develop specific properties. Depending on the quantity of the various elements present in a stainless steel alloy, it has a metallurgical structure that is characteristic of one of three basic stainless steel groups – (a) Martenistic, (b) Ferritic and (c) Austenitic. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, a structural alloy in automotive and aerospace assembly and building material in skyscrapers and other large buildings.

There are a large number of stainless steels produced. Corrosion resistance, physical properties, and mechanical properties are generally among the properties considered when selecting stainless steel for an application. A more detailed list of selection criteria is listed below:

(i) Corrosion resistance;

(ii) Resistance to oxidation and sulfidation;

(iii) Toughness;

(iv) Cryogenic strength;

(v) Resistance to abrasion and erosion;

(vi) Resistance to galling and seizing;

(v) Surface finish;

(vi) Magnetic properties;

(vii) Retention of cutting edge;

(viii) Ambient strength;

(ix) Ductility;

(x) Elevated temperature strength;

(xi) Suitability for intended cleaning procedures;

(xii) Stability of properties in service;

(xiii) Thermal conductivity;

(xiv) Electrical resistivity;

(xv) Suitability for intended fabrication techniques

Environment friendliness of modern steel:

Environment friendliness of modern steel:

Steel is the world's most recycled material. Steel's unique magnetic properties make it an easy material to recover from the waste stream, i.e., it can be recycled. The properties of steel remain unchanged no matter how many times the steel is recycled. The electric arc furnace (EAF) method of steel production can use recycled steel exclusively.

Most steel is made via one of two basic routes: (1) Integrated (blast furnace and basic oxygen furnace); (2) Electric arc furnace (EAF).

The integrated route uses raw materials (that is, iron ore, limestone and coke) and scrap to create steel. On the other hand, the EAF method uses scrap as its principal input.

The EAF method is much easier and faster since it only requires scrap steel. Recycled steel is introduced into a furnace and re-melted along with some other additions to produce the end product.

Steel can be produced by other methods such as open hearth. However, the amount of steel produced by these methods decreases every year.

Of the steel produced recently, about 65.0% was produced via the integrated route, 32.0% via EAF and 3.0% via the open hearth and other methods.

Steel is not a single product. There are currently more than 3,500 different grades of steel with many different physical, chemical, and environmental properties. Approximately 75% of modern steels have been developed in the last 20 years. If the Eiffel Tower were to be rebuilt today the engineers would only need one-third of the amount of steel. Modern cars are built with new steels that are stronger, but up to 25% lighter than in the past.

Steel is very friendly to the environment. It is completely recyclable, possesses great durability, and, compared to other materials, requires relatively low amounts of energy to produce. Innovative lightweight steel construction (such as in automobile and rail vehicle construction) help to save energy and resources. The steel industry has made immense efforts to limit environmental pollution in the last decades. Energy consumption and carbon dioxide emissions have decreased by one-half of what they were in the 1960s. Dust emissions have been reduced by even more.

Finished steel product descriptions:

Finished steel product descriptions:

The following list shows 15 finished steel products descriptions generally used and available in the market (alternate names for the products are shown in brackets).

• Plate (Heavy Steel Plate)
• Pipe (Heavy Pipe)
• Hot-Rolled Coil (Hot Rolled Steel)
• Pickled Hot Rolled Coil (Pickled Hot Rolled Steel)
• Cold-rolled Coil (Cold Rolled Steel or Full hard Steel)
• Finished Cold Rolled Coil (Annealed and Tempered Cold Rolled Steel)
• Electro-galvanized Coil (Electro-galvanized Steel)
• Hot Dip Galvanized Coil (Hot Dip Galvanized Steel)
• Tin Plate Coil (Electrolytic Tin Plated Steel)
• Tin-free Coil (Electrolytic Chrome Coated Steel)
• Organic Coated Coil (Organic Coated Steel)
• Cold Formed Sections
• Section Rolling
• Rebar (Reinforcing Bar)
• Engineering Steel (Tool Steel)

1. Plate (Heavy Steel Plate) - A flat Steel sheet rolled on a Hot Rolling mill. It can be found on the market in sheets and is further processed into finished products by the manufacturer. Heavy plate is used in a large number of sectors including structural Steels, shipbuilding, pipes, pressure vessels, boilers, heavy metal structures, and offshore structures. Typical thickness is from 2 to 20 mm. The maximum width is 1,860 mm.

2. Pipe (Heavy Pipe) - A flat plate Steel coil that is bent and welded into a tube. It can be found on the market for final use. A heavy-wall pipe is technically used to transport fluids (for example, oil, gases, water, and chemicals).

3. Hot-Rolled Coil (Hot Rolled Steel) - A coil of Steel rolled on a hot-strip mill (hot-rolled coil). It can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. The various types of hot rolled Steel have applications in virtually all sectors of industry. For example, transport, construction, shipbuilding, gas containers, pressure vessels, energy pipelines. Hot rolled Steel sheet with an anti-slip surface and a diamond or teardrop pattern is typically used for stairs, industrial floors and tailboards for goods vehicles. Typical thickness is from 2 to 7 mm. Typical width is from 600 to 2,100 mm.

4. Pickled Hot Rolled Coil (Pickled Hot Rolled Steel) - Hot-rolled Steel from which the iron oxides present at the surface have been removed in a pickling process. Pickled Hot Rolled Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. The various types of pickled hot rolled Steel have applications in virtually all sectors of industry. For example, transport, construction, shipbuilding, gas containers, pressure vessels, energy pipelines. Typical thickness is from 2 to 7 mm. Typical width is from 600 to 2,100 mm.

5. Cold-rolled Coil (Cold Rolled Steel or Full hard Steel) - Obtained by a further thickness reduction of a pickled hot rolled coil. This step is achieved at low temperature in a cold-reduction mill. Cold-rolled Coil can be found on the market in coil, but requires a further heating process before being manufactured into finished products. The various types of Cold-rolled Steel are used as primary material for finished cold rolled coils and coated coils.Typical thickness is from 0.15 to 3 mm. Typical width is from 600 to 2,100 mm.

6. Finished Cold Rolled Coil (Annealed and Tempered Cold Rolled Steel) - Obtained by heat treatment (annealing) and strain-hardening of a Cold Rolled Steel in a way to achieve final mechanical properties that make the Steel suitable for further uses (forming and bending). Finished Cold Rolled Steel includes a wide range of different Steels which can be classified into the following main groups: formable Steels, high strength formable Steels, weathering structural Steels, structural Steels, hardenable Steels. It can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer.

Finished Cold-rolled Steel features excellent forming properties, electromagnetic properties, paintability, weldability, and is suitable for fabrication by forming, pressing and bending. Applications include domestic applications, automotive applications, lighting fixtures, electrical components (stators, rotors), various kinds of sections roofing applications, profiled sheets, and wall elements.Typical thickness is from 0.3 to 3 mm. Typical width is from 600 to 2,100 mm.

7. Electrogalvanized Coil (Electrogalvanized Steel) - Obtained by electro-plating Finished Cold Rolled Steel with a thin layer of zinc or zinc-nickel to provide corrosion resistance. Electro-galvanized Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. Electro-galvanized Coil features excellent forming properties, paintability, weldability, and is suitable for fabrication by forming, pressing and bending. Applications include domestic applications, building applications (for example, wall elements, roofing applications, automotive applications (for example, body in white for vehicles underbody auto parts), lighting fixtures, drums and various kinds of sections applications, profiled sheets. Typical thickness is from 0.3 to 3 mm. Typical width is from 600 to 2,100 mm.

8. Hot Dip Galvanized Coil (Hot Dip Galvanized Steel) - Obtained by passing Cold Rolled Coil through a molten zinc bath, in order to coat the steel with a thin layer of zinc to provide corrosion resistance. Hot Dip Galvanized Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer.

Hot Dip Galvanized Coil features excellent forming properties, paintability, weldability, and is suitable for fabrication by forming, pressing and bending. Applications include domestic applications, building applications (for example, wall elements, roofing applications), automotive applications (for example, body in white for vehicle underbody auto parts), lighting fixtures, drums and various kinds of sections applications, profiled sheets. Typical thickness is from 0.3 to 3 mm. Typical width is from 600 to 2,100 mm.

9. Tin Plate Coil (Electrolytic Tin Plated Steel) - Obtained by electro-plating a thin Finished Cold Rolled Coil with a thin layer of tin. Tin Plate Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. Electrolytic Tin Plated Steel is used primarily in food cans, and industrial packaging (for example, small drums). Typical thickness is from 0.13 to 0.49 mm. Typical width is from 600 to 1,100 mm.

10. Tin-free Coil (Electrolytic Chrome Coated Steel) - Obtained by electro plating a thin Finished Cold Rolled Coil with a thin layer of Chrome. Tin-free Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. Tin-free Coil is used primarily in food cans, and industrial packaging (for example, small drums). Typical thickness is from 0.13 to 0.49 mm. Typical width is from 600 to 1,100 mm.

11. Organic Coated Coil (Organic Coated Steel) - Obtained by coating a steel substrate with organic layers such as paint or laminated film. The substrate is mainly Hot Dip Galvanized Coil but may also be Electro-galvanized Coil or Finished Cold Rolled coil or Tin Free Steel. Organic Coated Coil can be found on the market in coil or in sheets and is further processed into finished products by the manufacturer. Organic Coated Coil is used in all activity sectors such as construction (for example, roof, wall and ceiling claddings, lighting, radiators), general industry (for example, office furniture, heating, ventilating, air conditioning), domestic appliances (for example, refrigerators, washing machines, small kitchen appliances, computer casings, VCR and DVD casings), and packaging. Typical thickness is from 0.15 to 1.5 mm. Typical width is from 600 to 1,300 mm.

12. Cold Formed Sections - Cold Formed Sections are made by roll forming narrow Finished Cold Rolled Coil or Hot Dip Galvanized Coil into a final shape such as a box section. Typical applications include light steel framing, automotive, and roadside crash barriers.

13. Section Rolling - A Steel Section rolled on a Hot Rolling mill. Steel Section includes I-beams, H-beams, wide-flange beams, and sheet piling. It can be found on the market for direct use. This product is used in construction, multi-story buildings, industrial buildings, bridge trusses, vertical highway supports, and riverbank reinforcement.

14. Rebar (Reinforcing Bar) - A Steel Reinforcing Bar is rolled on a Hot Rolling mill. It can be found on the market for direct use or is further processed into finished products by the manufacturer. This product is used to strengthen concrete in highway and building construction also as primary product for the Wire Rod process.

15. Engineering Steel (Tool Steel) - Engineering Steel is rolled on a Hot Rolling mill. It can be found on the market and is further processed into finished products by the manufacturer. This steel is used in the manufacture of tools, dies, components for engines, drives, equipment, transmissions.

Iron ore, its beneficiation and agglomeration - as required for iron and steel industry:

Iron ore, its beneficiation and agglomeration - as required for iron and steel industry:

A. Iron Ore: A naturally occurring mineral from which iron (Fe) metal is extracted in various forms viz Hot metal/ DRI etc.

B. Types of Ore: Two major varieties used for iron making are Haematite Ore( containing Ferric Oxide - Fe2O3) and Magnetite Ore (containing Ferro-Ferric Oxide – Fe3O4). When chemically pure, Haematite contains apprx 70% and Magnetite 72.4% iron . But usually iron content of ores ranges between 50-65/67% (rich ores) and 30-35% (lean ores); the remains being impurities known as Gangue (such as Alumina, silica etc.) and Moisture.

C. Grades of Ore: Iron ore is typically classified as High grade (+65% Fe), Medium grade (+62 – 65% Fe) and Low grade (-62% Fe). Typically, the Integrated Steel Plants(ISPs) use medium/High grade Iron Ore whereas the Sponge Iron plants require only High Grade iron ore, preferably, with +67% Fe.

(i) Lumpy/Fine Ore: Iron Ore is traded in lumps (i.e. sized ore) or in fines. Production/availability of lumps is limited by virtue of the natural occurrence and also because of generation of lot of fines during crushing of large lumps present in the run-of –mines (ROM).

(ii) Natural pellet: It is a term coined by producers in some Asian counties, to designate sized iron ore used directly in Sponge Iron production.

(iii) Blue Dust: Blue Dust is the name given to naturally occurring, extremely friable, high grade Haematite Iron Ore powder.

D. Beneficiation of Ore: Very low grade Iron ore cannot be used in metallurgical plants and needs to be upgraded to increase the iron content and reduce the Gauge content. A process adopted to upgrade ore is called Beneficiation.

Asian, specially of Indian origin of ore, is generally rich in iron (Fe) content but theAlumina content is very high which call for special adjustments/techniques for production of iron/steel at the cost of productivity and quality and hence money. Some of the mines have set up a Beneficiation plant to beneficiate Magnetite ore with approximately 35% Fe to high grade Iron Ore Concentrates.

E. Agglomeration of Iron Ore: Iron Ore Fines/blue dust cannot be charged in the blast furnace directly since they block the passage for ascending gas inside the fee. So, they are agglomerated (by igniting at lower temperature causing only interfacial fusion) into larger lumpy pieces with/without addition of additives like limestone, dolomite etc. Two types of agglomerated products are commonly produced/used in the industry namely Sinter and Pellet. Accordingly the processes are known as Sintering and Pelletising respectively:

(a) Sinter : Sinter is a clinker like aggregate which is normally produced from relatively coarser fine iron ore (normally –3mm) mixed with coke breeze (-3mm), limestone dolomite fines (-3mm) and other metallurgical return wastes from the plant. Sinter is a much preferred input/raw material in blast furnaces. It improves BF operation and productivity and reduces coke consumption in blast furnace. Presently, more than 70% hot metal in the world (in India 50%) is produced through the sinter.

(b) Pellet: Pellets are normally produced in the form of Globules from very fine iron ore (normally –100 mesh) and mostly used for production of Sponge Iron in gas based plants, though they are also used in blast furnaces in some countries in place of sized iron ore.

Types of finished steel products and their terminologies:

Types of finished steel products and their terminologies:

Steel is not a single product. There are currently more than 3,500 different grades of steel with many different properties - physical, chemical, environmental, 75% of which have been developed in the last 20 years. If the Eiffel Tower were to be rebuilt today the engineers would only need one-third of the amount of steel, modern cars have new steels with higher strength reducing the overall shell weight by 25%. Some of the finished steel terminologies have been discussed below:

(1) Annealing: Annealing is a heat treatment process. With this, steel products are reheated to a suitable temperature to remove stresses, resulting from previous processing and to soften them and/or improve their machinability and cold forming properties.

(2) Billet: Billet is a semi-finished steel product with a square cross section up to 155mm x 155mm. This product is either rolled or continuously cast and is further processed by rolling to produce finished products like wire rods, merchant bars and other sections. The range of semi-finished products above 155 mm x 155 mm is called blooms.

(3) Carbon Steel: A type of steel generally having no specified minimum quantity of any alloying element and containing only an incidental amount of any element other than carbon, silicon, manganese, copper, sulfur and phosphorus.

(4) Coated Steel (Galvanized & Color coated): Steel sheet coated through a heat process or through electrolysis with a layer of substance to protect the base metal (substrate) against corrosion. The most commonly used material is zinc which can be applied either using the heat process (hot-dip galvanizing) or using electrolysis (electro-galvanizing). An organic coating (paint, plastic) can also be deposited on the layer of zinc. The zinc-coated steel is often referred to as "galvanized steel".

(5) Electric Sheets: These sheets are produced from steel alloyed with silicon (up to 3.5%) and are used in the manufacture of alternators, transformers and motors. Grain oriented electrical sheets have a high silicon content and are used for the manufacture of large transformers. Non-oriented products are used in electric motors and small transformers.

(6) Flat Steel Products: A flat steel product is a plate product or a (hot or cold) rolled strip product. Typically steel is rolled between sets of rollers to produce the final thickness. Plate products vary in dimensions from 10 mm to 200 mm and thin flat rolled products from 1 mm to 10 mm. Plate products are used for ship building, construction, large diameter welded pipes and boiler applications. Thin flat products find end use applications in automotive body panels, domestic 'white goods' products, 'tin cans' and whole host of other products from office furniture to heart pacemakers.

(7) Long Steel Products: A long product is a rod, a bar or a section - typical rod products are the reinforcing rods for concrete, engineering products, gears, tools etc. are typical of bar products and sections are the large rolled steel joists (RSJ) that are used in building construction projects. Wire-drawn products and seamless pipes are also part of the long products group.

(8) Refining Stand: A stage in the process of making crude steel, during which the crude steel is further refined (i.e., most residual impurities are removed) and additions of other metals may be made before it is cast.

(9) Semi-finished Products: Steel products such as billet, blooms and slabs. These products can be made by direct continuous casting of hot steel or by pouring the liquid steel into ingots, which are then hot rolled into semi-finished products.

(10) Slab: A semi-finished steel product obtained by rolling ingots on a rolling mill or processed through a continuous caster and cut into various lengths. The slab has a rectangular cross section and is used as a starting material in the production process of flat products, i.e., hot rolled coils.

(11) Strip: Flat steel coil products, with widths of less than 600mm for hot rolled products and less than 500mm for cold rolled products.

(12) Thin Strip Continuous Casting: Casting technology that takes liquid steel and casts it into solid strip in one step, thereby eliminating the need for a continuous slab caster and hot strip mill.

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.

Uranium ore, Yellow cake and Uranium:

Uranium ore, Yellow cake and Uranium:

An ore mineral is a mineral which may be used for extraction of one or more than one metals. A uranium ore mineral is therefore a mineral possessing such physical and chemical properties and occurring in a deposit in such concentrations that it may be used for the profitable extraction of uranium, either alone or together with one or more other metals.

Pitchblende and Uraninite are used for extraction of uranium, contain theoretically up to 85 per cent uranium (average from 50 to 80%). Other minerals such as carnotite, torbernite, tyuyamunite, autunite, uranophane, and brannerite, contain 45 to 60 per cent uranium. The majorities of uranium-bearing minerals, however, contain uranium in small or trace amounts as an accessory to other major constituents.

In uranium ore, the presence of uranium is in combinations which are extremely difficult to break down chemically in order to recover the uranium. These minerals also usually occur scattered sparsely throughout the deposit so that recovery difficult and expensive. Primary uranium minerals have been found most commonly in veins or pegmatites, although in recent years extensive, flat-lying deposits of pitchblende in sedimentary rocks have also been discovered. The primary uranium minerals are generally black or dark brown, noticeably heavy, and often have a shiny or pitch-like luster. At the present time, there are only three known primary uranium ore minerals, and the most important of these, uraninite and pitchblende, are really varieties of the same mineral.

Uraninite is a naturally occurring uranium oxide with cubic or octahedral crystal form. It has a specific gravity of 8-10.5 (iron = 7.85), a grayish-black color and a hardness of 5-6, about the same as steel.

Pitchblende is the massive variety of uraninite, without apparent crystal form, that occurs most abundantly in the rich primary vein deposits of uranium. It is the chief constituent of nearly all high-grade uranium ores and has provided the largest part of all uranium produced throughout the world.

Yellowcake is milled uranium oxide, known to chemists as U3O8. When uranium ore comes out of the mine, it actually contains fairly little of the precious radioactive element. The milling process gets rid of the useless minerals that dominate the ore. First, raw ore is passed through a series of industrial-sized crushers and grinders. The resulting "pulped" ore is then bathed in sulfuric acid (H2SO4), a process which leaches out the uranium. After some drying and filtering, the end product is yellowcake: a coarse, oxidized powder that is often yellow in color but can also have a red or gray tint, depending on the number and type of impurities that may remain. Yellowcake is a first step toward enriched uranium. At the refinery, the yellow cake is dissolved in nitric acid. The initial separation and refining processes generate large volumes of acid and organic waste.

It is necessary to enrich the U-235 isotope concentration from its natural composition of 0.7% for use in either reactors or bombs. Reactor grade uranium contains from 3.5 to 4.0% U-235, while the Hiroshima uranium bomb contained more than 80% of the lighter U-235. The process used for enrichment involves ‘gaseous diffusion’ and thus the uranium must be converted to a gaseous compound, uranium hexafluoride (UF6).

Major economic minerals require for production of metals:

Major economic minerals require for production of metals:

Earth crust consists of rocks, and all the rocks have minerals. An ore is a rock containing metalliferous minerals of economic value, are mined for profitable extraction of metals. Ore is formed by concentration of low-abundance elements. 99% of the Earth’s crust is made up of oxygen, silica, aluminum, iron, calcium, sodium, potassium, magnesium and titanium. These are called major elements.

Many other elements are useful in modern society. Concentration of these elements in average crust is very small (all together they’re only 1%). Geologic processes are must to concentrate these elements hundreds to thousands of times to make ore. Most ore minerals belong to these four non-silicate mineral groups: (i) Native Elements (none); (ii) Sulfides (S); (iii) Oxides (O2); (iv) Hydroxides (OH). Examples are given below:

(i) Native Elements:

• Metals - metallic bonds, metallic luster

• Semimetals - have some, but not all, properties of metals: arsenic, antimony and bismuth

• Non-metals - covalent bonding, lack metallic properties

Principal Native Metals

• Gold (Au)

• Silver (Ag)

• Copper (Cu)

• Platinum (Pt, rarest and most valuable)

Native Non-metals

• Sulfur (S)

• Graphite (C) – Low pressure polymorph of Carbon

• Diamond (C) – High pressure polymorph of Carbon >30kbar, – Formed deep in the Earth’s mantle >90km, – Brought to the surface by violent, explosive igneous eruptions of magmas called kimberlites

(ii) Common Sulfide Minerals (reduced, formed in low oxygen environments)

• Pyrite (FeS2) Grows in cubes.

• Chalcopyrite (CuFeS2) is the most important ore mineral of Copper.

• Sphalerite (ZnS) is the most important ore mineral of zinc.

• Galena (PbS) is the most important ore mineral of Pb (lead).

(iii) Common Oxide Minerals (oxidized, formed in hi oxygen environments)

• Magnetite (Fe3O4)

• Hematite (Fe2O3)

• Zincite (ZnO)

• Franklinite (ZnFe2O4)

• Pyrolusite (MnO2)

• Chromite (FeCr2O4)

• Cassiterite (SnO2)

Hematite (Fe2O3) and Magnetite (Fe3O4) are Fe (iron) ores

Hematite and Magnetite occur together with red chert in BIFs

(iv) Common Hydroxide Minerals

• Goethite FeO(OH)

• Al hydroxides - gibbsite Al(OH)3, boehmite AlO(OH), diaspore AlO(OH),

Together these make up bauxite (actually a rock name, multiple minerals), the most important Aluminum ore.

Mining of minerals and ore-body:

Mining of minerals and ore-body:

The process or business of extracting ore or minerals from the ground is called mining. It is the selective recovery of minerals and materials from the crust of earth. The term mining industry commonly includes such functions as geological exploration, mineral exploitation by drilling blasting, ore dressing, mineral separation, electrolytic reduction, and smelting and refining.

Mining is broadly divided into three basic methods: opencast and underground. Opencast mining is done on the surface, which involves extraction from a series of successive parallel trenches. Underground mining involves extraction from beneath the surface - from depths as great as some time 10,000 ft (3 km).

The activities of the mining industry begin with geological exploration of economic minerals or ore bodies. Geological exploration is very much complicated, expensive, and highly technical task. After suitable deposits have been found and their worth proved, development, or preparation for mining, is done for exploitation of deposit.

For opencast mining, this involves stripping off overburden, before mineral deposit is approached. Removal of overburden is mostly done by drilling, blasting, lifting the blasted material by excavators on to dumpers and transporting the dumper to a overburden disposal site called overburden dump.

Underground mining include sinking of shafts, driving of adits and other underground openings, and providing for drainage and ventilation before actual mining is done on ore-body. For various development works and for extraction of ore drilling, blasting is necessary. During mining lifting of blasted ore by a machine called load-haul-dump (LHD), transporting them to the surface and filling the void created by exaction by waste material are done.

Associated with mining are many environmental concerns. Large-scale excavation is often necessary to extract a small amount of ore. Ore extraction disrupts the topsoil and can displace local animals and plants, and sometimes native human populations. Runoff can contaminate nearby water sources with pollutants such as the mercury and sodium cyanide used in gold mining. Waste materials and smelters can cause sulfurous dust clouds that result in acid rain. Abandoned strip mines have often been used as unregulated landfills for hazardous wastes.