Knowledge About Steel

STEEL AS Engineering MATERIAL

Steel is the backbone of all Industries and the basic ingredient for the growth and development of a country. Traditionally, the fortunes of the steel industry have been linked to the economic cycle of the country. Per capita consumption of steel speaks volumes about the relative position of the country on the development frontier. In India the per capita consumption of steel stands very low (38 kg) in comparison to world average per capita consumption of steel (170 kg), where total production of steel is about 1200 MT. Moreover, steel is completely recyclable and thus environment friendly. Hence a large potential exists in furthering the usage of steel in various segments of industry.

If anyone metal is to be named, which has maximum impact on mankind, it is steel. It is the most important metal today, which enjoys about 95% of total metal output globally. A compound basically made of iron and carbon and some other elements is called Steel. Thus steels are alloys of iron (Fe) and carbon (C). Its mechanical properties (and the electrical or magnetic properties) are influenced by the addition of various elements (Cr,Mn,Ni,Si etc). Carbon is the element that modify the allotropic (crystal structure) changes that iron displays during heating and cooling. These structural changes occur during processing or manufacture of the steel. Different types of steel are produced for particular applications and are manufactured within precise composition limits and processing conditions in order to provide the required microstructure, properties and functionality.Steel as an Engineering Material is of great interest to Architects, Civil and Structural engineers, Designers, Consultants, Metallurgists and Project owners.

Like all metals iron (Pig Iron) is made from iron ore, coke and limestone in blast furnace or by direct reduction processes. Production of steel from pig iron involves processes like steel making, primary and secondary steelmaking, casting and hot rolling. The carbon content of steels is up to 2.0%. The microstructure and atomic structure varies with carbon content and temperature. A wide range of strength levels, from 120 to over 3000 mpa can be obtained with the help of carbon and alloying elements by mechanical working and heat treatment.

Steel enjoys the widest range of applications among the materials known to mankind due to different mechanical properties and product forms. It touches everybody’s life everyday everywhere.

Segments	Typical Applications
Construction	Industrial, Hospital, Institutional Buildings, Exhibition Halls, Stadiums, Railway and Bus stations, offshore structures
Agriculture & Rural areas	Machinery, Storage tanks, Grain bins, Bullock Carts, Meeting Halls
House Hold	Buckets, Scissors, Kitchen cabins, Ovens, white goods, utensils, furniture
Infrastructures	Bridges, Flyovers, Foot Bridges, Culverts and RCC Structures
Oil, Gas & Power	Wells, Platforms and items related to power
Transport	Railways, Buses, Trucks, Luxury Coaches, Cars, two-wheelers, bicycles
Electrical & Electronics	Transformer core, Motors, Transmission Towers

Grades of Steel:

Mild Steel is the commonest grade of steel containing less than 0.25% Carbon. Medium Carbon Steel has carbon content 0.25% to 0.45% and used as input material in the engineering industries. High Carbon Steel refers to the harder steel with carbon content 0.45% to 0.90% and is used to make precision tools and instruments. Alloy Steels/Special Steels including Stainless Steel are produced by adding alloying elements like Mn,W,Ni etc. Stainless Steel contains Cr and Ni with proper proportions. Weathering steels are used for atomoshpheric corrosion resistance purpose.

Finished Steel is divided into two categories like Non Flat products (bars, rods, angles, joist, channels & railways materials) and Flat (plates, hot rolled coils, cold rolled coils/sheets, galvanized plated, galvanized corrugated coils/sheets, tin plates and electrical sheets.)

Hot metal is processed further to get better quality, strength and specific application through different routes like:

• Basic Oxygen Furnace Steel making (BOF) : The purpose is to refine the hot metal produced in the blast furnace, which may be subsequently refined in the secondary steelmaking shop. The main functions are to remove carbon & phosphorus from the hot metal, and to optimize the steel temperature so that any further treatments prior to casting can be performed with minimal reheating or cooling of the steel. 
• Electric Arc Furnace (EAF) : Recycled steel scrap is melted and converted into high quality steel by using high-power electric arcs. The main task is to convert the solid raw materials to liquid crude steel and then refine further in subsequent secondary steelmaking processes. 
• Secondary Steelmaking: It is a critical step in the steel production process between the primary processes (BOF or EAF) and casting. Some elements are added and some have to be removed during secondary steelmaking in order to fine-tune the composition of the steel to meet the specification. . After secondary steelmaking, the ladle of liquid steel is taken, at the required composition, quality, cleanness, time, temperature and at least cost to the casting process.
• Continuous Casting: The molten steel is continuously cast through a tundish into a water-cooled copper mould causing a thin shell to solidify. The solidified shell continues to thicken until the strand is fully solidified. Finally, the strand is cut into desired lengths. Depending on final application cast dimensions may be slab for flat products (plate, strip etc), blooms for structural sections (beams, channels etc) and billets for long products (wire). Hot Rolling: It is the most efficient process of primary forming used for the mass production of steel. The principal effects of hot rolling are the elimination of the cast ingot structure defects and obtaining the required shape, dimensions and surface quality of a product. This makes both semi-finished and finished products. Semi-finished hot-rolled steel products are the starting materials for further hot metal forming processes (flat, long products, seamless tubes, wheels, rings, bars etc) and cold rolling.
  Cold Forming: Hot-rolled products are often subjected to further processing like cold rolling, forming, machining and joining, in order to get desired strength and properties for specific applications.

Mechanical Properties:

Like all metallic materials specific steels are having specific mechanical properties. Steel properties can be divided into two groups:
    a. Structure insensitive
    b. Structure sensitive
Principal structure insensitive properties are elastic modulus, density and some chemical, electrical and thermal characteristics. The structure sensitive properties are wholly dependent upon the past history – whether hot rolled or cold rolled, whether heat treated and if so how. The most important structure sensitive properties are yield strength, tensile strength, ductility fractures toughness, fatigue characteristics, etc.

Elastic Moduli : These are commonly defined in terms of the relationship between stress s,  and strain €, in that region where the curve is linear.

The most frequently used is the modulus in tension :

Young’s Modulus ‘E’ = Tension Stress / Tension Strain

Also used is the Shear Modulus ‘G’ = Shear Stress / Shear Strain

The units of the modulus are the same as those of stress i.e. N/mm2 (ksi). Since the modulus is a structure insensitive property, the normal design value E = 205 kN/mm2 may be used for all steels, regardless of composition, origin, prior history etc.

Yield Strength: Yield point is the transition between reversible and permanent deformation when the material is stressed till yield point stress -strain curve is linear and at the yield point some non-uniform deformation take place as shown in figure. It is in fact that transition from elastic to plastic behavior yield strength is affected by composition and grain size. Some steels do not exhibit a clearly defined and the stress strain as a smooth continuous curve. In such cases, a stress which corresponds to a definite amount of permanent extension (equivalent 2%) is taken as yield stress and is called proof stress.

Tensile Strength: This occurs when material starts to deform locally, a waist or neck is produced at which facture is eventually occur. This is a maximum stress subject to the material before the necking starts. In figure the stress level is shown as (E). 

Ductility: The reduction in area and the total elongation at fracture are used as measure of ductility.

Fracture Toughness: Fracture toughness is a measurement of ductile to brittle transition of material. Brittle fracture always starts at a discontinuity such as a notch an incompletely fused weld or a design discontinuity such as a hole or a corner where stress is locally increased. For a steel of given position, the onset of brittle rather than ductile, behavior is affected both by temperature and rate of loading. The chemical composition of steel and the grain size affect the ductile brittle transition. Practically notch impact test is done in case of steel to assess its relative toughness. The fracture toughness is generally measure in terms of ductile-brittle transition temperature.

Fatigue: Failure by fatigue occurs as a result of reversing or fluctuating stress like brittle fracture, always starts from a stress raiser / discontinuity. Fatigue property has to be taken into consideration while designing a machine, bridge, industrial structure etc.

Specifications: Different classes of steels are available meeting desired requirement of end applications, which are clearly defined in respective specifications. Carbon Steel Fy=240 to 250 MPa is widely used in construction Industry, Micro Alloy Steel with Fy 350MPA and above is also being used for all type of construction and all type of equipment manufacturing, where Fy is yield strength of steel

Joining of Steel: Mostly can be done easily by welding,rivetting and bolting.