HSLA Steels

Question 1

Why did new metals (advanced metals) arrive?

Answer 1

• We started to understand microstructures of solid materials and the properties of defects in solid state.
• Advances in processing materials: production of raw materials and manufacturing processes.
• The ability to observe, measure and control chemical composition.

Question 2

Typical application of advanced metals?

Answer 2

Automotive

Question 3

Examples of advanced metals?

Answer 3

• High strength steel sheeting
• Super plastic aluminium
• Smart materials
• Rapidly cooled materials
• Stainless steel alloys

Question 4

How much has the use of advanced materials in cars increased by between 1970 and 2000?

Answer 4

1970 - 7%

2000 - 40%

Question 5

How does the controlled rolling process of low carbon steel play a key roll in the development of high strength strip steel?

Answer 5

The controlled rolling process prevents the undesirable recrystallisation phenomena of the grains during the hot rolling process.

Question 6

What are some techniques used in controlled rolling to inhibit the recrystallisation?

Answer 6

• Microalloying
• Dual-phase (ferrite-martensite microstructure)
• Precipitation hardening processing (to control solid solubility and grain size)

Question 7

What process is now possible to follow the controlled rolling?

Answer 7

Continuous annealing

Question 8

What is continuous annealing?

Answer 8

Slow cooling of metal from an elevated temp, to soften, add toughness, remove stresses and increase ductility.

Question 9

What is the difference between controlled rolling and conventional rolling?

Answer 9

In conventional rolling there is a need for an intermediate heat treatment stage to control the grain structure of the hot rolled strip before undergoing batch annealing and cold rolling processes.

Question 10

What are the advantages of HSLA steels?

Answer 10

1. Light gauge
2. Formability
3. High Strength
4. Good surface finish
5. Low coefficient of friction
6. Consistency of properties
7. Tight tolerance
8. Coil weight flexibility

Question 11

Disadvantages of HSLA steels?

Answer 11

1. Cost of steel (relative to mild steel)
2. Limited width of strip
3. Difficult to galvanize
4. Limited weldability

Question 12

Diagrams showing the scatter in mechanical properties for batch annealed vs continuously annealed HSLA steels?

Answer 12

Yes/no

Question 13

What is the strength range for HSLA steels?

Answer 13

300 - 1000 MPa

Question 14

What happens to the mechanical properties for comercial applications?

Answer 14

Lower mechanical properties are given to compromise the various design requirements.

Question 15

What is an example of an application of HSLA steels?

Answer 15

The front side member of modern cars

Question 16

What should the car be able to absorb?

Answer 16

A head on impact (collision) energy at a min speed of 35mph to avoid serious injury and shock/brain damage to the driver and passengers.

Question 17

How is this energy absorption achieved?

Answer 17

By carefully designed progressive deformation mechanisms in the front side members of the car.

Question 18

What are the two ways to assess impact?

Answer 18

• dynamic loading analysis using advanced numerical analysis and vast computing time
• the only certain way to assess impact is experiment, which is also very costly

Question 19

What are important factors in the design of the front side member of a car?

Answer 19

Strength, corrosion resistance and formability.

Question 20

What are other competing materials for the front side member of a car?

Answer 20

Aluminium alloys and composites.

Question 21

What is the equation for buckling stress/critical stress in a steel column?

Answer 21

Critical stress = (pi^2 * Et)/ (Lambda)^2
where
Et = tangent modulus of elasticity
Lambda = slenderness ratio

Question 22

What is the equation for slenderness ratio?

Answer 22

Slenderness ratio (lambda) = length (l) / radius of gyration (k)

Question 23

What is the equation for radius of gyration?

Answer 23

k = sqrt(I/Ac)
I : second moment of area
Ac = cross sectional area

Question 24

What is the equation for max buckling load?

Answer 24

Pmax = sqrt( critical stress * yield strength) * Ac

Question 25

Why is zinc-iron and zinc-nickel coating of strip steel now desirable?

Answer 25

• Formability
• Joining
• Paintability
• Corrosion Resistance

Question 26

What is the problem with HSLA steels and zinc coating?

Answer 26

Zinc coating by hot dipping is not suited for these sheets because it causes damage to the microstructure.

Question 27

What are other better ways to coat HSLA steels with zinc?

Answer 27

Electrodeposition and polymer coating/laminating

Question 28

What percentage of steel in cars was coated in the 90’s vs today?

Answer 28

90’s - 30% of sheet and plate steel coated for cars

today - 100% coated

Question 29

Why is the development of new coating on strip steel still intense?

Answer 29

Because they have the objective of eliminating the painting of products in the final stage.

Question 30

What is a recent development in coating of HSLA steel strips?

Answer 30

Involves combining heat treatment to produce a martensitic structure followed by Zn-Fe coating. These can then be hot formed to produce very high strength and corrosion resistant parts.