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Flashcards in HSLA Steels Deck (30)
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1
Q

Why did new metals (advanced metals) arrive?

A
  • 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.
2
Q

Typical application of advanced metals?

A

Automotive

3
Q

Examples of advanced metals?

A
  • High strength steel sheeting
  • Super plastic aluminium
  • Smart materials
  • Rapidly cooled materials
  • Stainless steel alloys
4
Q

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

A

1970 - 7%

2000 - 40%

5
Q

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

A

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

6
Q

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

A
  • Microalloying
  • Dual-phase (ferrite-martensite microstructure)
  • Precipitation hardening processing (to control solid solubility and grain size)
7
Q

What process is now possible to follow the controlled rolling?

A

Continuous annealing

8
Q

What is continuous annealing?

A

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

9
Q

What is the difference between controlled rolling and conventional rolling?

A

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.

10
Q

What are the advantages of HSLA steels?

A
  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
11
Q

Disadvantages of HSLA steels?

A
  1. Cost of steel (relative to mild steel)
  2. Limited width of strip
  3. Difficult to galvanize
  4. Limited weldability
12
Q

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

A

Yes/no

13
Q

What is the strength range for HSLA steels?

A

300 - 1000 MPa

14
Q

What happens to the mechanical properties for comercial applications?

A

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

15
Q

What is an example of an application of HSLA steels?

A

The front side member of modern cars

16
Q

What should the car be able to absorb?

A

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.

17
Q

How is this energy absorption achieved?

A

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

18
Q

What are the two ways to assess impact?

A
  • 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
19
Q

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

A

Strength, corrosion resistance and formability.

20
Q

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

A

Aluminium alloys and composites.

21
Q

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

A

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

22
Q

What is the equation for slenderness ratio?

A

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

23
Q

What is the equation for radius of gyration?

A

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

24
Q

What is the equation for max buckling load?

A

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

25
Q

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

A
  • Formability
  • Joining
  • Paintability
  • Corrosion Resistance
26
Q

What is the problem with HSLA steels and zinc coating?

A

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

27
Q

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

A

Electrodeposition and polymer coating/laminating

28
Q

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

A

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

today - 100% coated

29
Q

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

A

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

30
Q

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

A

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.