Why did new metals (advanced metals) arrive?
- 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.
Typical application of advanced metals?
Examples of advanced metals?
- High strength steel sheeting
- Super plastic aluminium
- Smart materials
- Rapidly cooled materials
- Stainless steel alloys
How much has the use of advanced materials in cars increased by between 1970 and 2000?
1970 - 7%
2000 - 40%
How does the controlled rolling process of low carbon steel play a key roll in the development of high strength strip steel?
The controlled rolling process prevents the undesirable recrystallisation phenomena of the grains during the hot rolling process.
What are some techniques used in controlled rolling to inhibit the recrystallisation?
- Dual-phase (ferrite-martensite microstructure)
- Precipitation hardening processing (to control solid solubility and grain size)
What process is now possible to follow the controlled rolling?
What is continuous annealing?
Slow cooling of metal from an elevated temp, to soften, add toughness, remove stresses and increase ductility.
What is the difference between controlled rolling and conventional rolling?
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.
What are the advantages of HSLA steels?
- Light gauge
- High Strength
- Good surface finish
- Low coefficient of friction
- Consistency of properties
- Tight tolerance
- Coil weight flexibility
Disadvantages of HSLA steels?
- Cost of steel (relative to mild steel)
- Limited width of strip
- Difficult to galvanize
- Limited weldability
Diagrams showing the scatter in mechanical properties for batch annealed vs continuously annealed HSLA steels?
What is the strength range for HSLA steels?
300 - 1000 MPa
What happens to the mechanical properties for comercial applications?
Lower mechanical properties are given to compromise the various design requirements.
What is an example of an application of HSLA steels?
The front side member of modern cars
What should the car be able to absorb?
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.
How is this energy absorption achieved?
By carefully designed progressive deformation mechanisms in the front side members of the car.
What are the two ways to assess impact?
- 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
What are important factors in the design of the front side member of a car?
Strength, corrosion resistance and formability.
What are other competing materials for the front side member of a car?
Aluminium alloys and composites.
What is the equation for buckling stress/critical stress in a steel column?
Critical stress = (pi^2 * Et)/ (Lambda)^2
Et = tangent modulus of elasticity
Lambda = slenderness ratio
What is the equation for slenderness ratio?
Slenderness ratio (lambda) = length (l) / radius of gyration (k)
What is the equation for radius of gyration?
k = sqrt(I/Ac)
I : second moment of area
Ac = cross sectional area
What is the equation for max buckling load?
Pmax = sqrt( critical stress * yield strength) * Ac
Why is zinc-iron and zinc-nickel coating of strip steel now desirable?
- Corrosion Resistance
What is the problem with HSLA steels and zinc coating?
Zinc coating by hot dipping is not suited for these sheets because it causes damage to the microstructure.
What are other better ways to coat HSLA steels with zinc?
Electrodeposition and polymer coating/laminating
What percentage of steel in cars was coated in the 90’s vs today?
90’s - 30% of sheet and plate steel coated for cars
today - 100% coated
Why is the development of new coating on strip steel still intense?
Because they have the objective of eliminating the painting of products in the final stage.
What is a recent development in coating of HSLA steel strips?
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.