Lecture 16 - Continuous Casting of Steel Flashcards
1
Q
Integrated Steel Mill
A
- Iron making (conversion of ore to liquid iron)
- Steelmaking (conversion of pig iron to liquid steel)
- Continuous casting (solidification of liquid steel)
- Product rolling (finished shapes)
2
Q
Steel Mini-Mill
A
- Electric arc furnace for scrap melting
- Ladle furnace or vacuum furnace for precision control of chemistry
- Continuous caster (strip or billet) for converting molten steel to solid form)
- Product rolling (finished shapes)
3
Q
Continuous Casting
A
- Tube mold (with lubrication)
- Open pour
- Small cross-section
- Small aspect ratio
4
Q
Continuous Casting Mold
A
- Transforms liquid to a shaped cross-section
- Removes heat to solidify molten steel shell
- Determines productivity (breakouts, casting speed)
- Determines quality (creates surface, affects internal cleanliness and structure)
5
Q
Control of Solidification Structure
A
Larger equiaxed zone with:
- Low superheat: enhance cooling
- Electromagnetic stirring (EMS): break the dendrites
- High alloy content (larger freezing range): enhance undercooling
- Add grain refiners (easier nucleation): enhance nucleation
6
Q
Development of Microstructure
A
Nucleation, growth, final microstructure
7
Q
Nucleation
A
- Undercooling needed to overcome energy barrier to initiate nucleation
- Number of nucleation sites controls # of grains
8
Q
Growth
A
- Competitive growth of columnar grains from walls (certain growth directions preferred)
- Equiaxed grains nucleate in central liquid (constitutional undercooling)
9
Q
Final Macrostructure
A
Depends on competition between columnar vs. equiaxed grains
10
Q
Dendrite Growth
A
- Dendrites start from nucleation sites (branched, 3D, tree-like structures, [100] growth direction preferred in steels)
- Secondary dendrite arm spacing (SDAS) has important effect on material properties (especially yield strength)
11
Q
Segregation
A
- Caused by alloy partitioning during solidification creates enriched liquid
- Caused by fluid flow (from liquid shrinkage, bulging, convection, etc.)
- Worse at higher superheat
- Need superheat < 10C to avoid segregation
- Need some squeezing to match liquid shrinkage (0.3-2 mm/m machine taper, or soft reduction)
12
Q
Microsegregation
A
Small scale (in between dendrites)
13
Q
Macrosegregation
A
- Scale of entire cast section (center to surface)
- Filling in of internal cracks and porosity with enriched liquid
- Cannot be removed once formed
14
Q
Non-equilibrium Solidification (Micro)
A
Higher cooling rates lower the solidus (smaller k value), more segregation
15
Q
Non-equilibrium Solidification (Macro)
A
- Composition variations between regions of casting
- Cannot be removed
