A Review Review of Defects in Beam Blank Casting and the Measures Proposed for their Elimination Jorge Madias (1), Cristian Genzano (1), Marco Oropeza (2), Carlos Moss (2)
(1) metallon, San Nicolas, Argentina (2) Gerdau Corsa, Ciudad Sahagun, Mexico
Content Introduction Surface
defects Internal defects Conclusions
Introduction metallon Consulting
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& training company based in San Nicolas,
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Corsa Sahagun
Start-up
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Introduction
Beam blank casting Mature
process, born in 1968 ≈60 casters installed worldwide Focus
of the paper
Defects
formation mechanism Solutions proposed to decrease their occurrence Changes
in casting system (i.e. SEN design) Mold design Secondary cooling modifications and regulation Strand support Techniques
used to investigate the defects
Metallographic Metallographic
characterizations CFD, thermodynamical and thermomechanical modelling Plant tests
Introduction Three
casting modes Open casting (metering nozzles)
Semi submerged casting (metering nozzles and funnels)
Semi submerged casting (two or one SEN)
Introduction Three
casting modes: advantages and drawbacks dr awbacks
Casting type Open casting
Advantages Advantages Low cost: No need for SEN and casting powder High productivity: possibility of automatic metering nozzle change
Drawbacks Splashing: cold drops Reoxidation: macroinclusions, trapped scum Oil lubrication: Pin holes; risk of cracks due to higher heat transfer Al-killed steel not feasible Higher cost (funnel and casting powder)
Semi submerged Less splashing casting Less reoxidation Less pin-holes More control of heat transfer Submerged No splashing: no cold drops Higher cost (Stopper rod/ slide gate, casting No reoxidation: no macroinclusions SEN and casting powder) Casting of Al-killed steels Lower productivity (limited by SEN life) Higher control of heat transfer (less risk of cracking)
Introduction Mold Tubular:
small and medium sections Plates: larger sections
Possibility of using plate mold in a mold jacket for tubular tubular mold
Introduction Plate
molds
Wide
faces: Holes and spacers Narrow faces: Slots
Introduction Plate
molds
More
alternatives to manage water cooling
Slots,
holes with spacer, spacer, full hole, distance between holes, distance to hot face
More
rigidity of the assembly Stability of transverse geometry of cooling channels Easy achievement of different taper modes Higher cost
Surface defects Pin
holes
Usual
for casting with metering nozzle with oil lubrication May be deleterious for the final product if Concentrated
in a particular zone (“nest”) Deep enough as to not disappear in the reheating furnace Visible if in the first rolling steps the materials has free spreading (it is not contained) somewhere Almost scale-free in the beam blank, but then in the reheating furnace the become filled f illed with scale
Surface defects Pin
holes
Moisture
in oil Moisture pick-up in oil circuit) Too high oil rate Inhomogeneous transverse distribution Too thick oil slot gap (more than 0.5 mm) Partial obstruction of oil slot gap by splashing Sudden variations in steel mold level Use of pulsing bomb Lack of deoxidation Electromagnetic stirring helps in pinhole elimination
Surface defects Bleeding As
in billets, this defect occurs when small strand breakout takes place, healing immediately, without metal loss Annular stress may promote bleeding 670 mm wide beam blank Bleeding in the inner surface of the wing
Surface defects Bleeding Classic
formation mechanism
Surface defects Trapped
scum
Typical
of open casting Due to thorough reoxidation of the liquid steel in contact with air and oxidizing slag Usually a liquid manganese silicate, but if a solid precipitates, viscosity increases and entrapment may occur If
silicon content is too high (due to a low Mn/Si ratio), silica precipitation occurs, If aluminum wire injection in the mold is practiced,
Surface defects Casting
powder entrapment
Similar
to scum entrapment Higher viscosity may occur in this case through Alumina
pick-up Reduction reactions between elements in the steel and oxides in the casting powder Example,
Enhanced
dissolved titanium reacting with silica in the slag
through turbulence
Excessive
electromagnetic stirring Short SEN / funnel submersion
Surface defects Network
cracks
Related
with high copper content in the steel High copper scrap charge Where gap between strand and mold becomes large, grain size increases and if copper content is high, network cracking may occur
Surface defects
Longitudinal facial cracks
Fairly common for beam blanks Formed in the mold Similar to longitudinal cracks in slabs and blooms In rolled product, its metallographic features are Internal
oxidation (as polished, no etching) Decarburization (etching with Nital 2%) Oxygen penetration (hot etching with alkaline sodic chromate)
Influencing factors Chemistry
of the liquid steel Properties of the casting powder Deviations of caster radius caused by mold oscillation Primary cooling: Water flow rate and temperature Secondary cooling: Water flow rate
1050 mm wide beam blank Longitudinal crack between between web and fillet
Surface defects Longitudinal
facial
cracks Steel
chemistry
Sulphur
content Carbon content Peritectic
transformation needs to be avoided POSCO scarfed beam blanks corresponding to 2,000 heats 0.12 – 0.13% carbon the more sensible range
Surface defects Longitudinal Casting
facial cracks
powder
Stahlwerke
Thüringen
Low
viscosity mold flux for small beam blanks at low speed<1 m/min) “Soft” cooling at meniscus level was obtained Lower capacity for infiltration and lubrication compensated by low viscosity JFE
Steel Mitsushima
Different
set of casting conditions Low viscosity gave place to longitudinal cracks
Surface defects Longitudinal Casting
facial cracks
powder
In
cold zones of the meniscus (i.e., close to the SEN), casting powder may reach the limit of its performance and give place to surface cracks
Entrapment of non-molten mold flux; distorted oscillation oscillation marks, small depression Longitudinal facial crack in the web, due to thermal shock by direct contact with the mold
1050 mm wide beam blank
Surface defects Longitudinal Casting
speed
Posco As
facial cracks
experience
the casting speed increases Solidifying shell is thinner Heat flow increases Strain is larger Result: More cracks
Surface defects Longitudinal Secondary More
facial cracks cooling
secondary cooling intensity, intensity, more cracking risk ri sk
Surface defects Longitudinal Secondary
facial cracks cooling
Extensive
use of mathematical modeling Yi Iron & Steel: optimization of secondary cooling Jin Yi to avoid these cracks ANSYS
for the thermo mechanical model MATLAB MATLAB for parameter optimization Maanshan
Steel: thorough modeling of secondary cooling with the same purpose, taking into account all the mechanisms involved in heat transfer
Surface defects Longitudinal Summary
facial cracks of plant experiences
Company/Plant
Year
Corrective actions
JFE Steel Mizushima
1975
Decrease sulphur; increase mold flux viscosity; viscosit y; improve mold alignment
JFE Steel Mizushima
1981
Decrease sulphur; adequate mold flux; minimize mold misalignment; misalignment ; adequate primary cooling; soft secondary cooling in first segments; better distribution of sprays in transverse section
Stahlwerke Stahlwerk e Thüringen Thüringe n
1998
High basicity low viscosity casting powder
JFE Steel Fukuyama
1996
Decrease sulphur; decrease secondary cooling flow rate
Stahlwerke Thüringen
1997
C<0.08% (Mn 0.60 to ensure mechanical mechanical properties)
Posco
2002
Avoid 0.12-0.13% 0.12-0. 13% C; lower casting speed
Stahlwerke Stahlwerk e Thüringen Thüringe n
2002
Avoid 0.12% C
Surface defects Longitudinal Corrective
facial cracks actions
Metallurgy Low
sulphur Avoid peritectic transformation Mold
flux
High
basicity bas icity Even heat transfer Mold
design
To
avoid longitudinal cracks in the shoulder sh oulder
Secondary Less
cooling
water, water, mostly for the first f irst segments Better transverse distribution
Internal defects Blowholes Depending
on the root cause, they may be concentrated in the first heat of the sequence or in some given heat, or all along the sequence Start: close to the beam blank surface, when there is enough gas segregation to the interdendritic spaces End: when somewhere below the meniscus, the ferrostatic pressure is higher than the gas
Internal defects Blowholes Excess
of gases dissolved in the steel (oxygen, nitrogen, hydrogen), enough to produce a bubble Compromise between clogging and blowholes
Internal defects Blowholes Typical High
industrial cases oxygen
Lack
High
oxygen and nitrogen
First
High
of deoxidation (coordination, slag carry over) heat of the sequence
hydrogen
Moisture
High
in new lining of ladle or tundish
nitrogen
Ladle
with long treatment, treatme nt, when nitrogen is used for stirring
Internal defects Blowholes Dragon
Steel Corp. case
Casting
with metering nozzle 80 kg Al addition during tapping t apping 40 kg CaFe to get O<10 ppm Oxygen injection in tundish if temperature too low Thorough study of LMF and caster variables High moisture in tundish repair refractory refr actory material
Internal defects Web
central cracks
Equivalent
to centerline segregation in slabs Not enough support length Insufficient secondary cooling Bulging, and in severe cases, an internal opening in the web Rolled H beam Central
segregation Crack formation Some
countermeasures
Use
of roll checker
Internal defects Inner
crack in wing end
May
promote strand breakout Resemble off-corner cracks in billets and slabs
Internal defects Inner
crack in wing end
JFE
Steel Kurashiki case
Strand
breakout in some heats Improvement plan Study
Objective
Observation of breakout boxes
Research solidification in mold; mold; find cause
Solidification macrostructure
Mechanism of formation of inner cracks in wing end
Sulphur addition test
Measurement of shell thickness in normal operation
Mold temperature
Estimation of heat flow in several parts of the mold
Solution:
optimization of mold taper in wing ends
Internal defects Web
central cracks
JFE
Steel Mitzushima
Caster
12.5 m radius, funnel casting 400 x 460 x 120 mm 287 x 560 x 120 mm Influence of sulphur content and casting speed Solutions Intensive
spray cooling on the web portion Strict maintenance of
Conclusions Beam
blank casting is an established process with a 50 years history fr ee of surface and inner defects It is not free Some of them share features with billet defects; other has more to do with slab defects The complex shape induce specific solidification defects The occurrence of defects requires carrying out improvement plans to make them minimal i s important Defect characterization is Simulations may help to elucidate the formation mechanism and to suggest corrective measures
Jorge Madias (1), Cristian Genzano (1), Marco Oropeza (2), Carlos Moss (2)
(1) metallon, San Nicolas, Argentina Argentina (2) Gerdau Corsa, Ciudad Sahagun, Mexico
