3. Special steels
page 12

High strength low alloy steels
Some types of steel are specifically for use in fabricating large structures. They are called ‘high-strength low alloy’ or HSLA steels. These steels are:
  • much stronger and tougher than ordinary carbon steels
  • ductile
  • highly formable
  • weldable
  • highly resistant to corrosion - which is important since the structure may be in place for a long time.
Photo of crane
Picture 3.1 Cranes can be made taller because of HSLA steel.
HSLA steel cars, trucks and bridges
The increased strength of HSLA steels means that structures can be built that contain less steel and are therefore lighter than they otherwise would be. This is an important feature for cars and trucks because it leads to fuel economy and to less damage to road surfaces. It is also important in the design of bridges since it means that the centre spans can be longer and need fewer supporting beams.
HSLA steel TV masts
An interesting advantage of HSLA steels is seen in the construction of tall television transmitter masts. The extra strength of the steel means that the sections making up the mast can be thinner and more stable because they offer less resistance to the wind.
The durability of HSLA steels
The extra strength of HSLA steel makes it more durable and gives it a longer useful life span. It is therefore selected for use in street lighting poles, oil storage tanks and earth moving equipment.
The steel angel
One type of HSLA steel is called COR-TEN because it is particularly resistant to corrosion. It can be exposed to the atmosphere in its ‘bare’ condition which eliminates the need and cost of painting and re-painting.

You will see a famous application of corrosion resistant steel if you drive up the A1 near Gateshead. High above the road is the ‘Angel of the North’, a 54 metre wide sculpture. The techniques used in its construction owe more to the skills used in shipbuilding than in traditional sculpture.

Photo of street
Picture 3.2 Street lamps and car bodies benefit from the strength and durability of HSLA steel.
Why HSLA steels are strong
The starting point in achieving the desirable properties of structural steels is to get the composition of the alloy right. The extra strength can be achieved through various combinations of alloying elements which means that there can be some choice in the other properties that the steel will have.

A typical HSLA steel will contain about 0.15% carbon, 1.65% manganese and low levels (under 0.035%) of phosphorous and sulphur and additions of other elements:

Element Percentage added Effect on properties
Copper 0.2 – 1.5 Improves atmospheric corrosion resistance
Nickel At least half of the copper content Benefits surface quality
Niobium 0.02 Increased tensile strength and yield point
Nitrogen 0.003 – 0.012 Contributes to strength and can improve weldability
Vanadium Up to 0.12 Increases strength without reducing weldability

Chromium, molybdenum, silicon and zirconium may also be added.

Microstructure of HSLA steels
The strength of all steels, including HSLA steels, comes from their microstructure. Strength is increased by:

This is usually achieved by rolling the steel at low hot rolling temperatures (see page 9).

Question 3-1.
a) HSLA steels have good formability. What does this mean?

b) What element might have been added to the steel used to construct the ‘Angel of the North’ to help resist corrosion?

Summary                   Close
  • HSLA steels are used in construction
  • they have small amounts of alloying metals added
  • the strength of the steel comes from its microstructure