Iron And Steel Manufacturing Processes
Steel making is a complicated processes involving many stages and yielding thousands of by-products. Steel can be produced either from steel scrap or iron. The process used for the manufacturing of iron& steel has a bearing on the cost of production & the quality of the steel produced.
The Iron Making process
The first step in the steel making process is producing iron from iron ore. The process is called the iron-making process. Three different methods can be used for the production of iron from Iron ore.
1. Coke-Ovens -Sinter-Blast Furnace (BF) Route
2. Direct Reduction Technique
3. Smelting Reduction Technologies
1. Coke-oven-Sinter-BF route
Coke-oven-Sinter-BF route is used by integrated steel plants that produce steel from iron ore. In this process, the iron ore is first subjected to a sintering process. Sintering is the technology for agglomeration of iron ore fines into useful ‘Blast Furnace’ burden material. Simultaneously coke is prepared in coke ovens from coal.
The treated iron from the sintering plant is fed in to the Blast furnace along with coke. The blast furnace basically does the function of converting iron oxide in to liquid iron. Coke is used as a reducing agent as well as fuel. The output from the furnace is the pure iron in molten stage, which is called hot metal.
The hot metal output from the Blast furnace is either directly fed in to the Basic Oxygen Furnace or used for producing ‘pig iron’.
Pig iron- definition- Liquid iron from blast furnace solidified in moving metal moulds of a "Pig Casting Machine". Named long ago when molten iron was poured through a trench in the ground to flow into shallow earthen moulds and arrangement looked like newborn pig sucklings.
2. Direct Reduction Technique
The Direct reduction technique is an alternative to the Blast furnace technique. Iron ore is first crushed and reduction agents remove the impurities and oxygen. The reduction agents used in this process are coal and gas. Reducing gases used here are carbon monoxide, hydrogen and their mixture, which are generally found in natural gas. This produces 97.0 percent pure iron, which is called, solid Sponge Iron or direct reduced iron or hot briquetted iron. It is used as substitute for steel scrap in Electric Arc Furnaces.
3. Smelting Reduction Technique
Smelting reduction technique is the third technique used for the production of iron from iron ore. Here the hot metal (liquid iron) is produced from iron ore in two steps. Ores are partly reduced in the first step and then final reduction and melting, takes place in the second stage. Different processes are used for smelting. COREX technique is the latest process used for smelting. Here coal is directly used in a melter gesifier as an energy carrier & reducing agent, thereby eliminating the need for a Blast Furnace, Sinter Plant & Coke Ovens. The other smelting techniques are Hismelt, Ausmelt, Romelt, Plasmasmelt and The Direct Iron Smelting Reduction (DIOS)
The Steel Making Process
The second stage in iron and steel making is the steel making process. Generally two types of techniques are adopted for steel making. They are the Basic Oxygen Furnace (BOF) type and Electric arc furnace (EAF) type.
Basic Oxygen Furnace Technique (BOF)
The Basic oxygen furnace is used for producing steel from the refined iron. Hot metal from the Blast furnace, which is a pear shaped furnace or reduced iron from the smelting plant is fed in to the Basic Oxygen Furnace (BOF) after pre-treatment to remove undesired elements like sulfur, silicon or phosphorous. Molten Steel in liquid form is obtained. Impurities go out in gaseous form and as slag. The liquid steel produced here is called crude steel.
Electric Arc Furnace (EAF)
Steel scrap or Pig iron or Sponge iron is used as the raw material in this process. Here the raw material is melted using heat generated with the aid of an electric arc produced by graphite electrodes. Here again the output is crude steel. The crude steel produced from the above said processes are either used for producing ingots or fed in the continuous casting process cycle. Ingot-definition- Molten steel cast in vertical cast iron moulds intended for rolling after heating in two stages i.e. in a primary mill and then in a finishing mill.
Crude Steel To Finished Steel
In the Continuous Steel Casting process, liquid steel is directly cast into semi-finished products, which eliminates the need for primary rolling of ingots. The crude steel or liquid steel is poured into a reciprocating refractory-lined receptacle, called a Tundish. Below the Tundish are water-cooled copper moulds of desired size. The steel solidified in the moulds is slowly pulled out to produce an "endless" strand, which is gas-cut to desired lengths. This steel is called semi-finished steel. The semi-finished steel is fed in to re-rolling mills to get finished steel products.
Finished steel products are classified in to two types:
1. Finished Carbon Steel
As per the International Iron & Steel Institute (IISI), Finished Carbon steel is defined as steel that has properties made up mostly of the element carbon, and which relies on the carbon content for structure.
Finished Carbon steel can be further classified in to three types:
a) Long products
b) Flat products and
c) Other carbon steel products.
Long products are Bars, rods, channels, angles and other structural materials. They are used in the construction and engineering industry and, to some extent, in the manufacturing sector.
Flat products consist mainly of sheets, coils and plates. They may be hot-rolled, cold-rolled or galvanized and find use mainly in automobile, shipbuilding and the white goods/consumer durables sectors.
2. Finished Alloy Steel
Alloy steel is defined as a material containing Iron, carbon (less than 2.0 percent), Silicon, Manganese plus Alloy elements like Chromium, Vanadium, Molybdenium, Tungsten, Nickel, Lead, Naboium, Copper etc.
Alloy steels can be further classified into two categories:
a) Stainless Steel And
b) Alloy Steels.
Stainless steel is alloy steel that contains more than 10.0 percent chromium, with or without other alloying elements. Stainless steel has special properties like high level of corrosion resistance and the ability to maintain its strength at high temperatures.
Influence of Elements in Steel