Construction Materials Structural Steel - Construction Materials & Management

Introduction

Steel is an iron-carbon alloy whose properties depend largely on the percentage of carbon and the presence of other elements. In terms of carbon content, steel occupies an intermediate position between cast iron and wrought iron. Typical ranges are:

• Cast iron: carbon about 2-4%, brittle, good compressive strength, free graphite present.
• Wrought iron: carbon not exceeding about 0.15%, very ductile, tough, fibrous structure.
• Steel: carbon usually in the range 0.25-1.5%, balanced strength and ductility.

If the carbon content exceeds approximately 1.5% the excess carbon tends to remain as free graphite in the solidified metal; when free graphite is present the alloy is classed as cast iron rather than steel. When no free graphite is present the material is classed as steel.

Steels are commonly classified by carbon content:

• Very low-carbon (dead) steel: less than about 0.10% carbon that is fully deoxidised; free from blow-holes.
• Mild (low-carbon) steel: about 0.10-0.25% carbon; widely used in structural work.
• Medium-carbon steel: about 0.25-0.60% carbon.
• High-carbon (hard) steel: about 0.60-1.10% carbon.

Uses of steel

Steel is a versatile construction and engineering material. Common uses include:

• Structural members: beams, columns, trusses and frames in buildings and industrial structures.
• Reinforcement: bars for reinforced concrete (mild steel bars, ribbed/ deformed bars, TMT bars, CTD bars).
• Fabricated sections: angles, channels, I-sections, T-sections used for secondary framing and supports.
• Plates and sheets: floor plates, ship plating, roofing (corrugated sheets) and cladding.
• Railway rails, bridges and heavy engineering components.
• Pipes and tubes for water, gas and structural use.
• Fasteners, bolts, rivets and machine parts (requiring varied hardness and toughness).
• Wires and welded wire fabric for fencing and reinforcement mesh.

Properties: Mild Steel Vs Hard Steel

Mild steel and hard (high-carbon) steel differ in their mechanical behaviour and typical applications.

Defects in steel

Steel may show several defects that develop during melting, solidification or subsequent processing. Common defects and their causes are:

• Cavities / blow-holes: gas entrapped in molten metal forms bubbles which become cavities on solidification. Causes include dissolved gases, poor de-gassing or turbulence during pouring. Remedies include improved melting practice, vacuum treatment or fluxes to remove gases.
• Cold shortness: cracking when worked cold due to excessive phosphorus content. Phosphorus causes brittleness at low temperatures. Remedy is control of phosphorus in raw materials and suitable refining.
• Red shortness: cracking when worked hot due to excessive sulphur. Sulphur forms low-melting compounds that weaken the grain boundaries at high temperatures. Remedies include desulphurisation and adding manganese which binds sulphur.
• Segregation: some constituents solidify earlier and separate from the main mass, producing chemical inhomogeneity and banding. Causes include slow cooling and inadequate stirring; remedies include controlled solidification, homogenisation heat treatments and remelting techniques.

Steel manufacturing processes

Several processes are used to produce steel from iron and scrap. Major processes encountered in industry and in historical development are listed below with brief descriptions of the principal methods:

• Bessemer process: molten pig iron is blown with air in a converter to oxidise impurities (carbon, silicon, manganese). It was historically important for mass steel production; modern steelmaking has largely replaced it.
• Cementation process: an early process for making high-carbon steel by heating wrought iron in contact with a carbonaceous material so carbon diffuses into the iron (surface carburising).
• Crucible steel process: small batches of steel melted in closed crucibles to produce high-quality, homogeneous steel for specialised purposes.
• Duplex process: a two-stage or combined method that employs sequential refining steps to obtain specific composition or cleanliness. (Used where a single refining stage is insufficient to meet required quality.)
• Electric process (Electric Arc Furnace, EAF): steel is produced by melting scrap and/or direct reduced iron using electrical energy; allows precise composition control and is flexible for small to medium batches.
• LD Process (Linz-Donawitz or Basic Oxygen Process): oxygen is blown into molten iron to remove carbon and other impurities by oxidation; widely used today for large-scale steelmaking because of speed and efficiency.
• Open-hearth process (Siemens-Martin): a regenerative furnace that produced steel by melting pig iron with scrap and refining over a relatively long period; historically important but largely superseded by basic oxygen and electric furnaces.

Mechanical treatment of steel

After steel is cast into ingots or continuously cast into billets, it is subjected to mechanical treatments to produce marketable shapes and to improve mechanical properties. The purposes are to obtain desired cross-sections, refine grain structure and improve density. Common operations are:

• Drawing: cold or hot operation to reduce cross-section and increase length proportionally; used to produce wires and small-diameter rods.
• Forging: repeated blows under a hammer or press to shape metal and to close internal defects. Forging refines grain flow, increases density and improves strength; used for bolts, crankshafts, connecting rods and heavy components.
• Pressing: slow forming operation using presses; produces uniform products without shock and is suitable for mass production of elements requiring precise shapes.
• Rolling: passing metal between rollers to reduce thickness and form sections; rolling mills produce flats, plates, angles, channels, I-sections, rails and sheets.

Factors affecting physical properties of steel

The physical and mechanical properties of steel-such as strength, ductility, hardness and elasticity-are controlled primarily by:

• Carbon content: carbon increases hardness and strength but reduces ductility. Selection of carbon percentage is therefore a trade-off between strength and workability. Mild steel (≈0.10-0.25% C) is commonly used for structural work.
• Presence of impurities and alloying elements: small quantities of elements such as silicon, phosphorus, sulphur and manganese influence properties significantly. Typical effects are listed below.

• Silicon: up to about 0.3-0.4% silicon increases elasticity and strength without seriously reducing ductility; silicon is often a deliberate alloying element.
• Sulphur: even small amounts of sulphur reduce strength, ductility, malleability and weldability when present in hot metal; sulphur causes hot shortness.
• Phosphorus: increases strength and hardness slightly but reduces shock resistance, ductility and toughness; excessive phosphorus causes cold shortness.
• Manganese: combines with sulphur to reduce harmful effects and can strengthen steel, but if manganese exceeds about 1.5% the steel may become brittle.
• Desirable limits: foundry and structural specifications set maximum desirable limits for these impurities; control is achieved in raw-material selection and refining.

Heat treatment processes

Heat treatment is the controlled heating and cooling of solid steel to change its microstructure and thereby its mechanical and physical properties. Heat treatment is used for the following purposes:

• To alter magnetic and electrical properties.
• To change the microstructure (grain size, phase distribution).
• To increase resistance to wear, heat and corrosion (by suitable alloying and surface treatments).
• To increase surface or core hardness.
• To improve workability for forming or machining.
• To vary strength and toughness for specific applications.

The principal heat-treatment processes with brief descriptions are:

• Annealing → softness, ductility
• Normalising → grain refinement
• Hardening → high hardness (martensite)
• Tempering → toughness restoration
• Nitriding → surface hardness without quenching

Market form of steel

Steel is supplied in a wide range of standard market forms to suit different construction and engineering needs. Common market forms include:

• Angle sections
• Channel sections
• Corrugated sheets
• Expanded metal
• T-sections
• I-sections (beams)
• Plates
• Ribbed (HYSD) bars
• Round bars
• Square bars
• Flat bars
• Ribbed mild steel bars
• Thermo-mechanically treated (TMT) bars
• Cold twisted deformed (CTD) bars
• Welded wire fabrics (WWF)

A working knowledge of carbon ranges, common impurities, production processes, mechanical treatments and heat-treatment options allows engineers to select the appropriate grade and form of steel for structural, reinforcement and engineering applications. Proper control of composition and processing ensures the desired balance of strength, ductility, toughness and durability for each use.