Lect-4,Steel

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Steel, manufacturing process, types and applications of Steel:

Steel , manufacturing process, types and applications of Steel Engr.Hassan Saleemi 1

Chemical composition of Steel:

Chemical composition of Steel Alloy of carbon and iron with carbon limits between 0.06% to 2%. Pig iron contains 4-5% carbon as well as many other metallic and non-metallic impurities. To make steel from pig iron a number of other impurities must also be removed along with carbon. 2

Removal of impurities from pig iron:

Removal of impurities from pig iron Many processes are available but they all involve the basic process of “oxidation” to remove the impurities. In blast furnace Carbon was used to remove oxygen by the mechanism of combination of oxygen with Carbon and Carbon monoxide. 3

Removal of impurities from pig iron:

Removal of impurities from pig iron Now to remove excess carbon from pig iron, oxygen must be used. In the molten state carbon present in pig iron combines with oxygen to form carbon monoxide. Oxygen as well as other fluxing elements are used to reduce the level of other impurities in pig iron. 4

Steel production:

Steel production Three types of furnaces can be used for refining pig iron (or scrap steel ) to refined steel Open hearth Furnace (no longer used in US) Basic oxygen Furnace Electric arc Furnace 5

Open hearth process:

Open hearth process One difficulty in the manufacture of steel is its high melting point, about 1370° C (about 2500° F), which prevents the use of ordinary fuels and furnaces. To overcome this difficulty the open-hearth furnace was developed. open-hearth furnaces can reach temperatures as high as 1650° C The furnace is charged with a mixture of pig iron (either molten or cold), scrap steel, iron ore Limestone 6

Open hearth process:

Open hearth process Impurities combine with limestone to produce slag. When the carbon content of the melt reaches the desired level, the steel is tapped through a hole. The molten steel is then sent to casting plant. 7

Basic Oxygen Furnace:

Basic Oxygen Furnace In 2008 it was the most important steel-Refining process from tonnage standpoint. It is a refractory lined vessel. It is charged with pig iron (either molten or cold), scrap steel, iron ore Limestone 8

Basic Oxygen Furnace:

Basic Oxygen Furnace 9

Basic Oxygen Furnace:

Basic Oxygen Furnace After melting the charge, oxygen is supplied to reduce Carbon, Sulfur and Phosphorous. Some furnaces provide oxygen through the top, some from bottom and some from both top and bottom. After pouring the steel out of furnace, nitrogen is blown into the remaining slag pool to coat the refractory lining with a protective coating of slag. This reduces refractory wear. The output of BOF is sometimes routed to secondary refining processes to reduces the impurities further. 10

Electric Arc Furnace:

Electric Arc Furnace EAF can be as small as few tons and as large as 300tons. This process is attractive because it can be shut down between uses. The source of heat in electric arc furnace is an Arc that is established between the melt and the graphite electrodes. 11

Electric Arc Furnace:

Electric Arc Furnace The furnace can be charged with scrap or solid pig iron. Melting is due to the heat produced by electric arc. Refining is done by supplying oxygen into the melt and from the action of refluxing agents. The refined steel is poured out of the furnace and is sent to casting station. This furnace is used to produce Specialty Steel 12

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Open Hearth Furnace Vs EAF and BOF:

Open Hearth Furnace Vs EAF and BOF Open hearth furnaces are long and broad. They can accommodate up to 450 tons of steel. Open hearth furnaces require approximately 10 hours to complete the process. BOF can do this job in 20 minutes. 14

Open Hearth Furnace Vs EAF and BOF:

Open Hearth Furnace Vs EAF and BOF Both BOF and EAF are smaller in size and have capacity less than 350 tons. Both BOF and EAF are more economical than open hearth furnace because of shorter residence time. 15

Casting Process:

Casting Process Ingot steel casting. Continuous steel casting. 16

Ingot steel casting:

Ingot steel casting The molten steel is tapped into refractory lined ladles. The steel is then poured into a series of ingot molds. After the ingots solidify, the ingot molds are stripped and the ingots are placed in soaking pits for heating Following the soak, the ingots are hot rolled in large primary rolling mills to produce slabs, blooms, or billets depending on the final products desired. 17

Ingot steel casting:

Ingot steel casting Soaking Pits The soaking pits are equipped with burners to provide heat when necessary. Primary Rolling Mills Also called a blooming mill, or a slabbing mill, these mills consist of a varying number of rolling stands, from one to twenty, that shape the hot steel into the desired semifinished shape. 18

Continuous Steel Casting:

Continuous Steel Casting The molten steel from the steelmaking operation is cast directly into semifinished shapes (slabs, blooms, and billets). Continuous casting represents a tremendous savings in time, labor, energy, and capital. By casting the steel directly into semifinished shapes, the following steps are eliminated: ingot teeming, stripping, and transfer; soaking pits; and primary rolling. Continuous casting also increases yield and product quality. 19

Classification of Steel:

Classification of Steel The strength and ductility of steel depend on the carbon content. So steel is classified according to the carbon present in it: Mild steel Medium carbon steel High carbon steel 20

Continuous Steel Casting:

Continuous Steel Casting 21

Mild Steel:

Mild Steel Carbon is up to 0.3% Used in all kinds of structural work like buildings Bridges Rails Furnaces Boilers Wires Nuts and rivets Screws etc 22

Medium Carbon Steel:

Medium Carbon Steel Carbon content is between 0.31-0.7% It is used in making High tensile tubes and wires Springs Forging dies Agricultural tools Hammers 23

High Carbon Steel:

High Carbon Steel Carbon content is between 0.71-1.4% Used in making: Taps Drills Files Cutlery Surgical instruments Metal cutting tools 24

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