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.