What are the main types of pouring methods in ingot casting?
Top pouring (downhill casting) and bottom pouring (uphill casting).
How does superheating differ between top and bottom pouring?
Top pouring typically involves 30-50°C superheating, while bottom pouring uses 40-70°C.
Describe the typical macro-structure of an ingot.
It includes columnar grains near the mold walls and equiaxial grains towards the center.
How does the cooling rate affect the ingot structure?
Faster cooling rates promote a longer columnar zone, while slower rates favor equiaxial grain formation.
What are common defects in ingot casting?
Segregation, gas porosity, shrinkage porosity, oxides and inclusions, and crack formation.
What is micro segregation in ingot casting?
It's the interdendritic segregation of elements, which can be recovered with high-temperature soaking.
How can macro segregation be reduced in ingots?
By promoting an early transition from columnar to equiaxial grains.
Why are ingot molds preheated, and to what temperature?
Preheating to approximately 80°C prevents thermal shock and sticking of the steel to the mold.
What is the purpose of using a hot top in ingot casting?
Hot tops help the last liquid to solidify and compensate for shrinkage during solidification.
What is the life expectancy of a typical ingot mold?
An ingot mold can last for 20-200 casting cycles.
What are the challenges associated with ingot casting compared to continuous casting?
Ingot casting involves complex thermal management and defect control, whereas continuous casting generally has stronger cooling and uniform structure formation.
Of what material are ingot molds typically made?
cast iron
What is the role of casting powder in ingot casting?
It forms a protective slag and assists in the last liquid solidification at the hot top.
What are the main components of a continuous casting system?
Ladle, tundish, mold, electromagnetic stirrer, guide rolls, and secondary cooling system.
What is the function of the tundish in continuous casting?
Pour different casting lines
facilitate further inclusion removal
Act as a reservoir
Act as a buffer
What role does secondary cooling play in continuous casting?
It controls the solidification of the steel and prevents surface defects.
What are the typical dimensions for billets produced in continuous casting?
Billet sizes range from 100 to 160 mm with lengths of 5.3 – 7.5 m.
How is productivity calculated?
How is heat extracted from the molten steel in the mold?
Through direct water cooling and by conduction through the mold walls.
What is the purpose of the dummy bar in continuous casting?
It initiates the casting process and forms the initial solidified shell.
How does mold oscillation benefit the casting process?
It prevents sticking of the solidifying shell to the mold and helps in the formation of a uniform shell.
What factors influence the formation of surface cracks in continuous casting?
Thermal stresses, solidification shrinkage, and friction between the strand and the mold.
What is the significance of the solidification front in continuous casting?
It is where the liquid steel transforms into solid, and its control is critical for quality.
What are the common defects associated with continuous casting?
Defects include internal and surface cracks, segregation, and porosity.
How is the solidification rate controlled in continuous casting?
By adjusting the casting speed and cooling intensity in the secondary cooling zone.
What is the role of electromagnetic stirring in continuous casting?
To improve internal quality by promoting homogeneity in the solidifying steel.
What material are the casting molds usually made of in continous casting?
Watercooled Copper
How does secondary cooling practically look like?
Cooling through water spraying.
What is the metallurgical length?
The length until of the beam to the point of full solidification
Which wear mechanisms apply to the rolls in continous casting?
tribo-oxidative sliding wear
Thermomechanical Fatigue
What is the dominant error source in continous casting?
Misalignment of the rolls after the tuyer and consequtive bulging
What are the defect categories in contimous casting?
Surface Defects
Internal Defects
Geometric defects
What causes longitudinal cracks in continuous casting and why?
Critical chemical composition
highT brittleness due to inaapropriate Mn/S ratio
peritectic steel have thermal deformations on the shell
High casting speeds
high metal T and resulting a thin shell
Insufficient primary cooling
insufficient support below mould
elevated strain on the shell
What are the characteristics of longitudinal cracks?
facial and corner cracks
May cause breaks
formed in mold or first part of secondary cooling
can be removed with surface grinding
How do cracks evolve?
through tensile strains and brittle materials
How are transverse cracks formed in continuous casting and why?
Chritical chemical composition
hot brittleness
high thermal gradient on the faces
thermal strains on the shell
Straightening at the wrong temperature
tensile strains on the surface
too deep oscillation marks
strain intensification
What role does chemical composition play in crack formation?
Certain compositions, like peritectic steels, are more prone to cracking due to specific solidification characteristics.
How do oscillation marks occur?
They occur if the oscillation of the mold (to ensure uniform solidification) is unpropperly conducted.
What are the reasons for surface makro inclusions?
Slag with low fluidity
slag composition and to low fluidity
refractory erosion
Excessive variation in the steel level in the mold (turbulence)
What are pin/blowholes and how do they form?
Pin/Blowholes are porosities formed close to the surface.
They form due to the dendritic growth with too many present gasses (O2, N2, CO) and can be avoided with increasing the ferrostatic pressure
It can also be caused by a too high sulfur concentration
How can center seggregation be reduced?
Favour the columnar to equiaxial transition (=non dendritic growth) then pour with low superheat.
How does center porosity form?
The solidification front emerges from the edges and shrinkage appears, when the fronts meet in the middle
How does V seggregation form?
How can V-seggregation be avoided?
soft reduction (small reducing of diameter)
strong cooling
Electro magnetic stirring (prevention of distinct solidification front by breakin dendrites)
What defects can be prevented by Electro magnetic stirring?
Surface Slags
pinholes
blowholes
subsurface slags
breakouts
internal cracks
columnar structure
centre line seggregation
centre porosity
v seggregation
How are internal cracks formed?
bulging missalignment straightening
thermal strains
High S and P content, low Mn/S ratio
Why do diagonal cracks form?
non uniform heat transfer
inadequate mold taper
poor water spray distribution
misalignment between mould and support rolls
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