Week 3: Steel manufacturing process

4 qualities of why we use steel

Strong: Withstand substantial forces before failure → High strength to weight ratio

Stiff: Resists deformation when forces are applied + wind loads

Fabricated: Built in factory, ready to be used on site → reduce waste + quality control

Ductile: Can undergo significant deformation before failure → More steel reinforcement in columns, slabs, beams = High ductility

Strength VS Stiffness

Strength: Tells how much stress a material can handle before yielding or failing

Stiffness: Tells how much the material can resist deformation under loading.

Process of making steel

1. 1. Iron making: Iron found in Earth’s crust as iron ore, transported to blast furnace site

2. 2. Steelmaking: Turn iron into steel with more suitable carbon content. Uses two methods: BOS and EAF

3. 3. Secondary steelmaking: Treating molten steel produced in both BOS and EAF to refine steel to achieve desired properties

4. 4. Continuous castings: Molten steel is cast into cooled mold, solidifying thin steel shell. The shell strand then gets cut into desired length

5. 5. Primary forming: Steel is shaped into useful products, often through hot rolling. Hot rolling uses roller to shape steel and improve its surface quality and internal structure

6. 6. Fabrication: Steel is prepared for its final application → gives steel its final shape and properties (shaping, coating, surface treatment, machining, heat treatment)

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BOS (Basic oxygen steel making) VS EAF (Electric blast furnace)

BOS: Produces steel from molten iron using oxygen, generally resulting in higher embodied carbon due to coal intensive iron production

EAF: Produces steel using electricity, often from recycled scrap, generally resulting in lower embodied carbon

Strength limit condition

S*= Demand

Ru= reduced design resistance (capacity)

Demand must be lesser than capacity

Capacity reduction factor for steel= 0.9

How does tensile strength test work and how is it recorded

A material (e.g steel) is placed into a testing machine and pulled until failure. During the test, the machine records the applied force and elongation. Then the tensile strength is the maximum pressure observed before failure occurs

• - These info are used to understand the material’s behavior under tension

3 Behaviors of material under tension

a) Brittle: Fractures with little to no deformation

b) Ductile: Undergoes deformation before failure

c) Completely ductile

Stress and strain equation and what each one means

Stress is the pressure applied to the material → basically how concentrated the pull from tensile force is over the cross-sectional area. It measures the force per unit area

Stress = Applied axial force/cross-sectional area [Pa]

Strain is the change in length as a percentage of original length

Strain = Final length - original length/original length

Explain different stages of stress-strain curve

Elastic region: The beginning stage where stress and strain increase approximately proportional to one another. The material returns back to normal length/shape when load is removed.

Yield strength: Maximum stress the material can undergo before it permanently deforms, cannot go back to original shape → This is the stage where we want to avoid. From this point onwards, the material is in plastic region

Strain hardening: After yielding, the material can still withstand increasing stress as it continues to deform plastically

Ultimate strength: Maximum stress the material can handle before failure/breaking

Necking: After reaching maximum tensile force, deformation becomes concentrated in one area, making cross section thinner as it reaches towards failure

• - At this stage the curve slopes down because force is acting over a smaller area of the cross section, making stress value reduced.

Fracture: Eventually, that thin cross section would eventually break

Young’s Modulus (E)

Measures how stiff a material is → How much material can resist stretching or compression when forced is applied.

E= stress/strain → which this is the gradient of the stress-strain curve. So, the steeper the slope, the stiffer the material is

High E: Stiffer material, resists bending

Low E: Flexible material, large deformation and bends more easily

Explain the difference between these two graphs and the property of the material

Graph 1: Material has High strength and high ductility. This means the material is able to withstand high load, and continues to deform its shape until failure, which is why the graph has high strain value and high stress value

Graph 2: The graph shows that the material break at very less strain value, meaning that the material is stiff so it experienced very little deformation before it breaks. However, it’s strong in strength as it’s able to carry large amount of load (high stress value on graph)

5 important properties of steel

Ductility: Ability to deform under tensile load without breaking

Brittleness: Material failure with little deformation

Elasticity: Ability to return to original shape and size when unloaded

Plasticity: Ability to deform permanently without breaking

Stiffness: Resistance to deformation, related to Young’s modulus E

Equilibrium

A structure is in equilibrium when all forces or moment acting on it are balanced. So sum of Fy, Fx, and M would equal to zero

Support connection types

Roller support: Free to rotate, moves along the surface upon which the roller rest, but resists vertical movement

• - This support provides on vertical force acting perpendicular to the surface (Fy)

• = Fy ONLY

Pinned support: Can resist bother vertical and horizontal force but not a moment.

• - This support allows beam to rotate, but prevents it from moving horizontally and vertically

• = Fy + Fx

Fixed support: Resists vertical, horizontal, and moment

• - This means the structure only need one fixed support to be stable

• = Fy + Fx + M

Why does stress rise during strain hardening

Plastic deformation makes dislocation in the material increasingly difficult to move, meaning more stress is required to continue deforming the material

Why does stress decrease during necking

Stress decreases because the material’s load bearing capacity decrease as its cross-sectional area rapidly reduce during necking

What permanent load (G) and what is live load (Q)

Permanent or dead load is the self-weight of the structure, they are permanent. Whereas live load are occupancy load such as people and furniture