Iron is a base metal extracted from iron ore. Pure iron has melting point of 1530 Degree Centigrade and density of 7.86 gm/cc.
Iron making is the process of Reduction of iron ore using the relevant reducing agent (Reductant).
It is the hot, liquid, metallic iron product obtained upon reduction of iron ore (normally in Blast Furnace or in Corex Furnace). It contains about 93-94% Iron (Fe) and other elements/impurities like Carbon (4%), Silicon (~1%), Manganese (+1%) Sulphur, Phosphorus etc. Hot metal is the primary input for production of steel in the Integrated Steel Plants.
A product in solid (lumpy) form obtained upon solidification of Hot Metal in Pig Casting Machine. It is called Pig or Pig Iron because of its typical humpy shape. It is produced in 2 broad categories/grades:
Direct Reduced Iron (DRI): Solid metallic iron product obtained upon Direct Reduction of high grade iron ore in solid state itself without being converted into liquid form like that in Blast Furnace.
Sponge Iron (SI): DRI is also known as Sponge Iron because of its spongy micro structure.
Hot Briquetted Iron (HBI) : At times the DRI/SI coming out from the furnace is converted into a bigger compact mass i.e. Briquettes for ease in transportation/charging in steel making furnace, which are known as Hot Briquetted Iron (HBI).
STEEL:
Steel is an iron based alloy containing Carbon, Silicon, Manganese etc.
STEEL MAKING:
Steel making is the process of selective oxidation of impurities present in the charge material (Hot metal/Scrap/DRI) in the presence of suitable fluxes in the Steel Melting Shops (SMS).
LIQUID STEEL:
The immediate hot molten steel product from Steel Melting Shop (LD Converter/Electric Arc Furnace/Electric Induction Furnace/Energy Optimising Furnace). It is further cast into ingots/Semis. The by-product from SMS is called SMS slag.
INGOT STEEL (INGOTS):
PENCIL INGOTS: small ingots in Kgs produced in mini-steel plants.
Intermediate solid steel products obtained by Hot rolling/Forging of ingots (in conventional process) or by Continuous casting of liquid steel are known as Semis. These are called so since they are intended for further rolling/forging to produce finished steel products.
Various types of semis are as under:
Products obtained upon hot rolling/forging of Semi-finished steel (blooms/billets/slabs). These cover 2 broad categories of products, namely Long Products and Flat Products:
Finished steel products produced normally by hot rolling/ forging of Bloom/billets/pencil ingots into useable shape/sizes. These are normally supplied in straight length/ cut length except Wire rods which are supplied in irregularly wound coils.
Different types of long products are:
Finished steel flat products are produced from slabs/thin slabs in rolling mills using flat rolls. These are supplied in Hot Rolled (HR), Cold Rolled (CR) or in Coated condition depending upon the requirement.
Different types of flat products are:
HOT ROLLED (HR) FLAT PRODUCTS are produced by re-rolling of slabs/thin slabs at high temperature (above 1000 Degree C) in Plate Mills (which produce plates) or in Hot Strip Mills (which produce strips). Hot Rolled Strips are cut into straight length to produce HR Sheets or Thin Plates.
COLD ROLLED (CR) STRIPS are produced by cold rolling of HR Strips in Cold Rolling Mills (normally at room temperature). CR Strips are cut to produce CR sheet. CR Strips/sheets are characterised by lower thickness, better/bright finish, closer dimensional tolerance and specific mechanical/metallurgical properties. They are directly used in automobiles (cars/ scooters, motorcycles etc.), white goods, consumer durable etc. or for production of coated sheet products.
Cold Rolled Sheets/Strips are supplied in as rolled condition (CRFH- Cold Rolled Full Hard) or in closed annealed (CRCA -Cold Rolled Close Annealed) condition or in closed annealed & skin passed/temper passed condition, depending upon the requirement of the end users.
Specific variety of Cold Rolled Sheets/ Strips with specific chemical composition used in Tin Mills are known as Tin Mill Black Plate (TMBP).
COATED PRODUCTS: There are cold rolled products coated with metals or organic chemicals as under:
The term is internationally used to mean the 1st solid steel product upon solidification of liquid steel. In other words, it includes Ingots (in conventional mills) and Semis (in modern mills with continuous casting facility).
According to International Iron & Steel Institute (IISI), for statistical purpose, crude steel also includes liquid steel which goes into production of steel castings.
The term is used to designate various types of solid steel products, which are sold to outside customers for further processing or for direct use/consumption. Therefore, it includes ingots and/or semis and/or finished steel products. (Liquid steel is normally not traded).
STEEL AS PER COMPOSITION
Steel which is produced with intended amount of one or more alloying elements in specified proportions to impart specific physical, mechanical, metallurgical and electrical properties.
Common alloying elements are manganese, silicon, nickel, lead, copper, chromium, tungsten, molybdenum, niobium, vanadium etc. Some of the common examples of alloy steels are:
Commonly used grades of stainless steels (SS) are:
These steels by definition do not contain any alloying element in specified proportions (i.e. beyond those normally present in commercially produced steel in industry). Non- alloy steel is divided into 3 categories namely
Non-alloy steel constitutes approx. 90% of total steel production, of which, mild steel takes the lion's share.
Steel, in production of which special care has to be taken so as to attain the special/desired properties, such as, cleanliness, surface qualities and mechanical/ metallurgical properties.
In layman's language, all steel other than mild steel fall under the category of special steel. But metallurgically, even mild steel/low carbon steel i.e containing less than 0.25%/0.30% carbon, may still fall under the category of special steel if any special properties is specified in the steel. Examples are DD/ EDD steel, Forging Quality steel, Free Cutting steel etc.
CLASSIFICATIONS OF STEEL BASED ON END USE:
In terms of uses, steels are often classified as Structural steels, Construction steel, Deep Drawing Steel, Forging quality, Rail steel and the like.
IRON ORE:
Definition: A naturally occurring mineral from which iron (Fe) metal is extracted in various forms viz Hot metal/ DRI etc.
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 approximately 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.
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
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.
Natural Pellet: It is a term coined by producers like NMDC to designate sized iron ore used directly in Sponge Iron production.
Blue Dust: Blue Dust is the name given to naturally occurring, extremely friable, high grade Haematite Iron Ore powder.
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. Processes adopted to upgrade ore is called Beneficiation.
Indian ores are : Indian ore is generally rich in iron (Fe) content but the Alumina content is very high which call for special adjustments/techniques for production of iron/steel at the cost of productivity and fuel consumption and hence money.
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.
COAL:
Definition: Coal is a naturally occurring combustible rock containing 70% (by Vol) carbonaceous material including moisture.
Classification based on level of Maturity: Depending upon the level of maturity/metamorphism, coal is classified under 3 main categories namely, Lignite/Brown Coal, Bituminous Coal, Anthracite Coal.
Grouping based on Property: Coals are grouped according to particular properties as defined by their Rank (which is a measure of degree of maturity/metamorphism), Type (Vitrinite, Liptinite and Inertinite which are the 3 main groups of materials that constitute coal) and Grade (depending on Impurities and Calorific Value).
Use of Coal: Natural coal in general is too dense and/or fragile and has limited use as a fuel/reductant in metallurgical plants like Blast Furnace. However, some specific varieties of natural coal (crushed and screened in specified size ranges) find direct application in other metallurgical operations (such as Corex Plant, Coal Dust Injection/Pulverised Coal Injection in Blast Furnace etc.).
Coking/Non-coking Coal: Based on coking property, coals are broadly classified into two categories namely, Coking Coal and Non-coking Coal. Steam coal used for steam/power generation falls under the broad group of Non-coking coal.
COKING COAL:
Definition: Coking coals are those varieties of coal which on heating in the absence of air (process known as Carbonisation) undergo transformation into plastic state, swell and then re-solidify to give a Cake. On quenching the cake results in a strong and porous mass called coke.
Primary/Medium/Semi/Week Coking Coal: Coking coal is divided into 3 sub-categories namely, Primary Coking Coal (Low ash, low Volatile, High Coking property) Medium Coking Coal (low ash, medium volatile, low caking index) and Semi/ Weak Coking Coal: (low ash, high volatile, very low caking index).
Characteristics of Coking coal for BF Coke: Coking coal for production of BF Coke (which is the right type of fuel/reductant needed for a BF) is characterised by certain specific properties in terms of appropriate composition (viz low Ash (10% max), Volatile Matter (20-26%), and very low sulphur and phosphorous content, appropriate Rank of coal (1- 1.3), good rheological properties, wide range of fluidity , low inert content etc.
Indian Coking Coal: Indian Coking Coal found in Gondwana belt (Bihar &West Bengal region) has very high ash (17% or more) and poor rank and other properties, which results in lower productivity and higher coke consumption in blast furnace. Assam coking coals though, are low in ash have very high sulphur which limits their use in iron making in blast furnace.
Washing of Coal: Since ash content in Indian coal is very high, washing is resorted to lower the ash content to some extent. However, Indian coals are notorious with respect to its Washability because the ash / inerts are fairly and finely distributed in the coal matrix thereby rendering washing difficult.
Blending of Coal: Because of limited availability of good quality coking coal, the Indian Steel plants use a optimal Blend of the 3 or more varieties of coking coal to compensate for the lack of individual coals with the necessary properties. Another important consideration in selecting a coal blend is that it should not exert a high coke oven wall pressure and should contract sufficiently to allow the cake/coke to be pushed out from the oven.
COKE:
Coke is the residual solid product obtained upon carbonisation of coking coal. Depending upon property, coke is known as Hard Coke, Soft Coke and Metallurgical Coke.
Metallurgical Coke : Not all coke can be used in metallurgical operations for which good quality coke made from specific blend of coking coal is essential. Such coke is classified as Met. Coke.
Blast Furnace (BF) Coke : The term is used to refer to such Met Coke which is used for iron making in BF. BF coke fulfills 3 main functions in the blast furnace operation:
BF Coke is characterised by the following parameters:
These characteristics depends not only on the coal properties but also on the coking technology/parameters as well as pre-carbonisation & post carbonisation techniques adopted thereof.
| Grade | Ash Content (%) |
|---|---|
| Steel – I |
Up to 15 |
| Steel- II |
Exceeding 15 and up to 18 |
| Washery. I |
Exceeding 18 and up to 21 |
| Washery. II |
Exceeding 21 and up to 24 |
| Washery. III |
Exceeding 24 and up to 28 |
| Washery. IV | Exceeding 28 and up to 35 |
Adverse effects of Ash: Ash has highly adverse effect on the productivity of BF and on consumption of coke in the BF. An increase in ash content by 1% over a critical limit results in increase in coke consumption by about 45% and decrease in BF productivity about 3-6%.
Indian Integrated Steel Plants normally use high ash coke produced inhouse, at the cost of productivity, energy consumption etc. The Mini Blast Furnace units however use mainly imported low ash Met coke from China and other sources.
NON-COKING COAL (NCC):
These are coal of poor coking properties i.e. does not soften and form cake like coking coal during carbonization in the coke oven. Such coals with relatively lower ash and higher fixed carbon are used in metallurgical applications viz. COREX technology based iron (pig iron) plants, Coal based DRI Plant etc, while those with higher ash are normally used in thermal Power Plants as steam coal. So far, NCCs were classified into A, B, C, D, E, F & G grades depending upon its heat value which is a function of carbon and volatile matter and ash content in the coal.
| Grade | Useful Heat Value (UHV) (Kcal/Kg) UHV= 8900-138(A+M) | Corresponding Ash% + Moisture % at (60% RH & 40OC) | Gross Calorific Value GCV (Kcal/ Kg) (at 5% moisture level) |
|---|---|---|---|
| A | Exceeding 6200 | Not exceeding 19.5 | Exceeding 6454 |
| B | Exceeding 5600 but not exceeding 6200 | 19.6 to 23.8 | Exceeding 6049 but not exceeding 6454 |
| C | Exceeding 4940 but not exceeding 5600 | 23.9 to 28.6 | Exceeding 5597 but not exceeding. 6049 |
| D | Exceeding 4200 but not exceeding 4940 | 28.7 to 34.0 | Exceeding 5089 but not Exceeding 5597 |
| E | Exceeding 3360 but not exceeding 4200 | 34.1 to 40.0 | Exceeding 4324 but not exceeding 5089 |
| F | Exceeding 2400 but not exceeding 3360 | 40.1 to 47.0 | Exceeding 3865 but not exceeding. 4324 |
| G | Exceeding 1300 but not exceeding 2400 | 47.1 to 55.0 | Exceeding 3113 but not exceeding 3865 |
Recently, a new classification of NCC has been adopted as per which NCC adopted as is classified as under:
| Grade of Coal |
Range of Gross Calorific
Value
In Kilo Calories |
Grade of Coal |
Range of Gross Calorific
Value
In Kilo Calories |
|---|---|---|---|
|
G1 |
Greater than 7000 |
G2 |
Greater than 6700 and Less than 7000 |
|
G3 |
Greater than 6400 and Less than 6700 |
G4 |
Greater than 6100 and Less than 6400 |
|
G5 |
Greater than 5800 and Less than 6100 |
G6 |
Greater than 5500 and Less than 5800 |
|
G7 |
Greater than 5200 and Less than 5500 |
G8 |
Greater than 4900 and Less than 5200 |
|
G9 |
Greater than 4600 and Less than 4900 |
G10 |
Greater than 4300 and Less than 4600 |
|
G11 |
Greater than 4000 and Less than 4300 |
G12 |
Greater than 3700 and Less than 4000 |
|
G13 |
Greater than 3400 and Less than 3700 |
G14 |
Greater than 3100 and Less than 3400 |
|
G15 |
Greater than 2800 and Less than 3100 |
G16 |
Greater than 2500 and Less than 2800 |
|
G17 |
Greater than 2200 and Less than 2500 |
COKE OVEN/ COKE OVEN BATTERY:
Coking Coal is converted into Coke in coke ovens which are silica refractory lined ovens/ chambers. Coke Oven battery comprises of a large number of ovens, 50-70 in tandem. Such batteries are normally attached with By-product plant where in valuable constituents are recovered from the volatile /gaseous content of coal driven out during carbonisation. Accordingly, such coke ovens are known as By-product coke oven battery vis-a-vis Non-recovery type coke ovens, also known as Bee-hive type coke ovens.
COKING TIME: Coking time is defined as the time required for conversion of coal to coke in the coke oven which varies in the range of 1520 hrs.
YIELD OF DIFFERENT VARIETIES OF COKE: Typical yield from one tonne of dry coal charge to coke is 75%. Depending upon size ranges, coke is classified into the following categories: -
YIELD OF DIFFERENT VARIETIES OF COKE:
| Category | Yield | Use |
|---|---|---|
|
BF Coke (25/40-80mm) |
85% |
Blast Furnace |
|
Nut Coke (15-25mm) |
5% |
Sinter Plant /Ferro Alloy/Pig Iron Industry |
|
Coke breeze (0-15mm) |
10% |
Sinter Plant /Cement Industry |
Coal Dust Injection (CDI)/ Pulverised Coal Injection (PCI) : These are technologies wherein pulverised/ granulated/ dust coal is injected into the blast furnace through the tuyers along with the Blast to replace part of the coke requirement.
(v) TERMS RELATED TO ‘TECHNO-ECONOMIC EFFICIENCY PARAMETERS’:
These are parameters which are normally used to judge the operational efficiency/effectiveness of iron & steel making processes in the steel plants. Most commonly used parameters are:
A comparative indicative picture of above efficiency parameters in conventional integrated steel plants vis.-a-vis. the international level (during 2015-16) are given below
| Parameter | International Standards | SAIL | RINL |
TATA Jamshedpur |
JSW Bellary |
|---|---|---|---|---|---|
| BF Productivity (t/m3/day) | 2.5-3.5 |
1.58
(1.15-1.68) |
1.45 (1.1-1.5) | 2.41 (1.4-2.6) |
2.64
(2.5-2.7) |
| BF Coke Rate (kg/thm) | 275-350 |
489
(464-499) |
543 (532-571) | 380 (353-545) | 398-490 |
| BF PCI/CDI Rate (kg/thm) | 200-225 |
58
(21-125) |
5 (10-2) | 168 (40-191) | 85.0-135.3 |
| LD Productivity (heats/conv/yr) | 10000-12000 |
315
(261-372) |
6915 | 10795 (9732-12523) | 9097-10754 |
| Specific Energy Consumption (Gcal/tcs) | 4.5-5 |
6.51
(6.42-6.70) |
6.4 | 5.77 | 6.41 |
| Specific CO2 Emission (t/tcs) | 1.8-1.9 | 2.6 | 2.8 | 2.26 | 2.61 |
| Water Consumption (m3/tcs) | 2-2.5 | 3.51 | 4.68 | 3.98 | 2.8 |
FLUXES: Limestone, Dolomite, etc. used in Iron/ Steel making which react with the undesirable gangue material/ impurities and removed ash slag.
FERRO ALLOYS: Master alloys used for de-gassing/ de-oxidising/alloying in steel making. Common varieties are ferro silicon, ferro manganese, silico manganese, ferro chrome, ferro nickel etc.
REFRACTORIES: Heat resistant bricks/ shapes/ monolithic mass used for construction/ lining of reaction vessels/ furnaces. Common varieties are Silica, Magnesite, Dolomite, Alumina, Fire-clay, Mag-carbon, Mag-chrome etc.
STEEL MELTING SCRAP: Steel waste/scrap not usable as such in its existing form which is further re-melted to produce liquid steel to produce various products. Depending on their form/type, they are classified as Heavy Melting Scrap, Light Melting Scrap, Turnings/borings etc.
RE-ROLLABLE SCRAP: Cuttings/end cuttings, Used steel products like used rails etc which could be directly used for re-rolling up to reheating (without resorting to re-melting) into finished products for specified applications. Ship breaking generates substantial quantity of re-rollable steel scrap.
HOT ROLLING: Rolling of Steel at above the re-crystallisation temperature of steel (normally above 1000 C) to produce Hot Rolled Long products/Flat Products from semis. Ingots are also hot rolled to get semis. At times blooms are also hot rolled to produce Billets. Rolling Mills used for hot rolling are known as Hot Rolling Mills.
COLD ROLLING: Rolling of steel (normally flat products) below the recrystallisation temperature of steel (normally at Room Temperature) to produce cold rolled sheets /strips /coils. Mills used for the purpose are called Cold Rolling Mills.
2Hi/4 Hi/6 Hi/20 Hi Mills : Rolling Mills are classified as 2-High / 2 Hi, 4 Hi and so on depending on Number of Rolls used in the arrangement/configuration of rolls in single stand. For example, a 2 Hi mill consist of 2 rolls one above the other known as Upper roll and the Lower roll. In a 4 Hi mill, there are 4 rolls in a stand—2 upper rolls one above the other and 2 lower rolls one above the other