Manufacturing of Steel

Four Steel-Making Processes

1. The Bessemer Process

2. The Oxygen-Furnace Method

3. The Open-Hearth Process

4. The Electric-Furnace Process

Heat Treatments for Steel

• Normalizing consists of heating the steel to a

temperature of about 815ºC or higher,

depending on the type of metal and cooling

several hundred degrees slowly in air. This

process increases uniformity of structure.

• Annealing consists of heating the steel to a

temperature slightly lower than for normalizing

and cooling it several hundred degrees very

slowly, usually in a furnace. It is used to toughen

and reduce brittleness in steel.

Heat Treatments for Steel (Contd)

• Quenching consists of cooling steel very rapidlyin oil, water or brine from a temperature of about815ºC. Quenching increases hardness andstrength, but reduces ductility and toughness.

• Tempering consists of reheating the quenchedsteel to a temperature of 150 to 650ºC andcooling in air to reduce the residual stresses andincrease ductility. Heating to the lowertemperature range produces greater hardness,strength and wear resistance, while higher heatproduces greater toughness.

Forming Processes of Metals

• Hot working breaks up coarse grains andincreases density by closing tiny air holes andforcing the grains closer together.

• Cold working elongates grain in the direction ofthe steel elongation, increases strength andhardness, and decreases ductility. Cold workingresults in more accurately finished products,because there is no cooling shrinkage to beestimated.

Rolling

• Rolling consists of compressing and shaping

an ingot into a useful shape by squeezing it

through a succession of rollers. The piece

being rolled becomes longer and wider as it is

compressed. Variety of cross sections used in

construction can be rolled in long pieces by

means of specially shaped rollers.

Extrusion

• Extrusion consist of forcing a billet of hot, plastic

steel through a die of the desired shape to

produce a continuous length of material of

reduced cross-section in the shape of that die.

The resulting product has the shape of a rolled

product. Most intricate shapes can be formed by

extrusion than by rolling, and the surface is of

higher quality. An extrusion is made in one

operation rather than repetitive operations as in

the case of rolling.

Stainless Steel Extrusions

Steel Products

Structural Steel products consist of the following:

• Standard steel section

• Steel bars reinforcement

• Steel mesh reinforcement

• Alloy steel

• Sheet steel

Standard Steel Section

These consists of hot rolled steel sections,

shapes and plates not less than 3mm thick

necessary to hold a structural frame in

place. Each type of steel used must

conform to various specification as given

in Codes of Practice or Singapore

Standards or British Standards.

Steel Bars Reinforcement

• The steel often comes in standard lengths to suit

the sizes of the delivery vehicle or handling

methods, but the smaller rods can be delivered

in continuous rolls or coils under certain

circumstances. The required lengths are

normally stated on the bending schedule.

Steel Mesh Reinforcement

• Steel mesh reinforcement is manufactured

from plain round, indented or other

deformed rods.The term ‘wire’ is generally

used to denote hand-drawn mild steel and

comes in sizes 5, 6, 7, 8, 10 and 12mm

diameter.

Alloy Steel

• Alloy steel contains substantial quantities of alloy

elements to provide special qualities such as

high strength, corrosion resistance, or heat

resistance. In fact with 14% chromium, it is the

well known ‘Stainless Steel’.

• Alloy steels are usually designated by the

element or elements from which the alloy

derives its particular characteristics.

Sheet Steel

• Steel sheet is available in mild steel, highstrength steel and in stainless steel. The steel isavailable uncoated and with factory appliedcorrosion resistance and decorative pre-finishes,disperse with the need for costly and lesseffective shop and site treatment.

• Sheet pre-finished with paint, PVC coating andlaminates are produced in a wide range ofcolours and textures.

Mechanical Test for Steel

Mechanical tests for steel include:

– Tension

– Bending

– Hardness

– Impact

For structural steel, the tension or tensile test is

the most important.

Tensile Test

Protection of Steel

• Rusting is oxidation or combining of the iron withoxygen, which occurs in the presence ofmoisture.

• It proceeds more rapidly where there isnoticeable dampness, but it occurs in any airwith a relative humidity higher than 70 percent.The rust is formed from the solid metal, reducingits size so that the member becomes weakerand loses any decorative finish it might have.

Methods of Limiting Corrosion

Some general methods of preventing or

limiting corrosion are outlined below:

• Production of corrosion-resistant alloys

• Anodic metal coatings

• Chemical treatments

• Protective paintings

Protective Painting

The protection provided by the paint is due to:

• the presence of constituents calledinhibitive pigments which suppress theanodic and cathodic reactions.

• The slowing down of the movement ofions through the coat. Some cathodicprotection is provided by pigmentedpaints.

Painting of Steel

• Painting of steel prevents rust. However painting

is expensive and paint must be replaced

periodically at additional cost.

SPECIAL PURPOSE COATING SYSTEMS

• Intumescent Paint are examples of special

purpose coating systems. They can provide fire

ratings for exposed steel for up to three hours.

• Hot-Dip Galvanizing - There are several

reasons for selecting galvanizing as a coating

system. For light fabrication and some medium

structural applications, galvanizing can be the

lowest cost coating system.

Intumescent Paint

• Solve constraints between design and fire-

resistance.

• Smooth and pleasant feature in various colours

provide both fire-protection and decorative

functions.

Hot-Dip Galvanizing

• Hot-dip galvanizing is a special form of cathodic

protection in which sacrificial zinc is chemically

applied onto the bare steel to be protected.

Mill Certificate

• Name of Mill, Country

• Steel Standard, Grade

• Heat Number

• Tensile, elongation, chemical properties

• Other delivery conditions agreed with supplier

• Check material delivery against mill certificate

Delivery Condition

• Material

- standard, grade

• Dimension

- shape, length

• Tolerance

- straightness of beam

- flatness of plate

• Other delivery conditions

- witness testing

Material Testing

• Inspection and Testing ensures compliance withstandards and specifications

• Acceptance by lot or cast

• Test Unit

- same form

- same grade

- same quality

Material Storage

• Avoid ponding

• Avoid support settlement

• Avoid member distortion

• Avoid rain

• Avoid corrosion

• Allow inspection, stock-keeping, handling

• Observe safety, stability of stacking

Steelwork Specification

• National Productivity and Quality Specifications

(NPQS)

- common spec for building works

- performance based

• Chapters on Structural Steelwork

- C5-10 Structural Steelwork

- C5-20 Protective works for Structural Steelwork

• Project Specific Data (PSD)

Steel Fabrication

• From material take-off to final erection and

handover

• Includes material planning, cutting, fitting,

welding, protective coating and assembly

• Best done in covered workshop with proper

equipment and working environment

Steel Erection

• Simple lifting operation using mobile crane

• Heavy lifting operation of entire structure using

strand jack system

• Improper handling can cause additional

distortion to delivery condition

Steelwork Quality Assurance

• To ensure steelwork quality:

- employ qualified welder

- engage accredited fabricator

- appoint independent bodies

* accredited inspection body for inspection

* accredited laboratory for testing (NDT)

Importance of Weldability

• Generally all steel can be welded

• Different procedures will be required, if hydrogencracking is to avoided

- absoprtion of hydrogen into the weld causing

brittleness

• Can be avoided by using:

- clean joints

- baked electrodes

- slow cooling

Fire Resistance

• Reduction of strength and stiffness of steel

structure in elevated temperature leading to

possible failure and collapse

• Fire resistance design is required for steel

structure to maintain structural safety in fire

condition.

Fire Resistance Design

• Fire resistance design assesses the ability of a

steel structure to withstand the effect of fire and

provides designs to maintain structural safety.

• Three issues involved in fire protection include:

- life safety

- protection of the structure, and

- fire suppression

Reinforcing Steel

• Reinforced concrete is the combination ofconcrete and steel.

• Concrete is strong in compression but lessstrong in tension. Steel is equally strong in bothtension and compression.

• Hence steel is reinforced into concrete in orderto take up the tensile stress produced in astructure.

Reinforcing Bar

• Steel bars for reinforced concrete are termed

“reinforcing bar” and are commonly referred to

as “rebar”.

• There are only two grades of steel involved,

namely mild steel and high yield steel. The

term mild steel refers to low carbon steel having

nominal carbon and manganese contents.

Stress- Strain Relationship

STAINLESS STEEL

• Stainless steel which are known as high-alloy

steels, have chromium and nickel in their chief

alloying elements.

• They contain 16 – 28 percent chromium and

may contain up to 22 percent nickel.

• Stainless steel are available is rolled, extruded

and drawn forms.

Classification of Stainless Steel

Stainless steel is classified as:

1. Martensitic - contain about 13 per cent

chromium

2. Ferritic - contain usually about 17 per cent

chromium

3. Austenitic contain usually 16 – 19 percent

chromium together with 6 – 14 percent nickel.

Advantages of Stainless Steel

1. Stainless steel is compatible with non-metalsand the metal, or washing from it, will not stainadjacent materials.

2. Thermal movement is low and stainless steelis also resistant to high temperatures.

3. It is very hard and strong and has good, mirrorlike, appearance.

4. It is corrosion resistant.

Advantages of Stainless Steel (Contd)

4. In service, the surface of stainless steel does

not hold dirt readily, and light accumulations of

non-greasy dirt may often be washed off with

plain water.

5. Stainless steel does not have magnetic

properties.

6. They have good weldability.