review of defects in beam blank casting and the measures proposed for their elimination (2)
TRANSCRIPT
REVIEW OF DEFECTS IN BEAM BLANK CASTING AND THE
MEASURES PROPOSED FOR THEIR ELIMINATION
Jorge Madias – metallon, San Nicolas, Argentina
Cristian Genzano – metallon, San Nicolas, Argentina
Marco Oropeza – Gerdau Corsa, Ciudad Sahagun, Mexico
Carlos Moss – Gerdau Corsa, Ciudad Sahagun, Mexico
04/10/2017
Introduction
Surface defects
Internal defects
Conclusions
Content
metallon
Consulting & training company based in San Nicolas, Argentina
Technical assistance
Short courses
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Gerdau Corsa Sahagun
Start-up 2015
Consteel EAF, LF, billet/bloom/beam blank caster, universal mill
1.000.000 tpy crude steel capacity
700.000 tpy rolled products capacity
Introduction
• Background for paper preparation
– Training on defects in beam blanks at Gerdau Corsa Ciudad Sahagun
plant some time after the start-up
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 characterizations
CFD, thermodynamical and thermomechanical modelling
Plant tests
Introduction
Three casting modes
Introduction
Open casting (metering nozzles)
Semi submerged casting
(metering nozzles and funnels)
Submerged casting
(two or one SEN)
Mold
Tubular: small and medium sections
Lower cost
Plates: larger sections
More alternatives to manage water cooling
Slots, holes with 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
Introduction
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 they become filled 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
Surface defects
670 mm wide beam blank
Bleeding in the inner surface of the wing
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, alumina precipitation may occur if it is excessive or it is not in the right point
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, dissolved titanium reacting with silica in the slag
Enhanced 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
Rolled H Beam, Dragon Steel Corporation
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
Surface defects
1050 mm wide beam blank
Longitudinal crack between web and fillet
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
Stahlwerke Thüringen
C 0.08% max
Surface defects
Longitudinal facial cracks
Casting 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 (among other reasons)
Surface defects
Longitudinal facial cracks
Casting 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
Surface defects
Longitudinal facial cracks
Casting speed
Posco experience
As the casting speed increases
Solidifying shell is thinner
Heat flow increases
Strain is larger
Result: More cracks
Surface defects
Longitudinal facial cracks
Secondary cooling
More secondary cooling intensity, more cracking risk
Surface defects
Longitudinal facial cracks
Secondary cooling
Extensive use of mathematical modeling
Jin Yi Iron & Steel: optimization of secondary cooling to avoid these cracks
ANSYS for the thermo mechanical model
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 facial cracks
Summary of plant experiences
Surface defects
Company/Plant Year Corrective actions
JFE Steel Mizushima 1975 Decrease sulphur; increase mold flux viscosity; improve mold alignment
JFE Steel Mizushima 1981 Decrease sulphur; adequate mold flux; minimize mold misalignment; adequate
primary cooling; soft secondary cooling in first segments; better distribution of
sprays in transverse section
Stahlwerke Thüringen 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 properties)
Posco 2002 Avoid 0.12-0.13% C; lower casting speed
Stahlwerke Thüringen 2002 Avoid 0.12% C
Jinyi Iron & Steel 2013 Lower water flow rate in all secondary cooling segments
Maanshan Steel 2014 More secondary water to fillet; less to wing ends and web center. -10%
segment 1; -7% segment 2
Longitudinal facial cracks
Corrective actions
Metallurgy
Low sulphur
Avoid peritectic transformation
Mold flux
High basicity
Even heat transfer
Mold design
To avoid longitudinal cracks in the shoulder
Secondary cooling
Less water, mostly for the first segments
Better transverse distribution
Surface 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
pressure
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 industrial cases
High oxygen
Lack of deoxidation (coordination, slag carry over)
High oxygen and nitrogen
First heat of the sequence
High hydrogen
Moisture in new lining of ladle or tundish
High nitrogen
Ladle with long treatment, when nitrogen is used for stirring
Internal defects
Blowholes
Dragon Steel Corp. case
Casting with metering nozzle
80 kg Al addition during tapping
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 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
Efficient use of roll checker
Equipment for segment
alignment
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
roll gap
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
Solution: optimization of mold taper in wing ends
Internal defects
Study Objective
Observation of breakout boxes Research solidification in 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
Beam blank casting is an established process with a 50 years history
Not free of surface and inner defects
Some of them share features with billet defects; other has more to do with
slab defects
Complex shape induces specific solidification defects
Occurrence of defects requires improvement plans
Defect characterization is important
Modeling is thoroughly used to elucidate formation mechanisms and to
suggest corrective measures
Conclusions
• [email protected] +54 9 336 421 1990
Thank you!
Jorge Madias (1), Cristian Genzano (1), Marco Oropeza (2), Carlos Moss (2)
(1) metallon, San Nicolas, Argentina
(2) Gerdau Corsa, Ciudad Sahagun, Mexico