casting defect in slab

Casting Defects in Slab

Power Point Presentation : Employee Training Centre, Dt: 25/05/2011 (11.00 AM-1230 PM Copyright 2011 Tata Steel Ltd., India. All rights reserved.

Pabitra Palai

Flat Product Technology Group (FPTG)

Tata Steel Ltd. Jamshedpur

Module-1

Outlines

Continuous casting and Definitions

Casting Defects : An overview

Solidification of Steel; Numerical Equations

Role of Constituents on casting characteristics

Defects from Casting Sticker

Thin Shell

Crack, MLF, Hook Formation

Defects in Products

Surface and Sub Surface Defects Segregation

Cracks

Shape Defects in Slab : Causes and Remedial measures

Slide 2

Key Definitions

Continuous Casting ; is the process whereby liq. metal is solidified into a "semi-finished" billet, bloom, or slab for subsequent rolling in the finishing mills. [Sir

Henry Bessmer-1958]

Casting Defect ; Any unwanted deviation from the customers requirements in a cast slab during continuous casting results in a defect. Some defects in the

cast products are tolerable while others can be rectified by additional

processes like scarfing etc. The following are the 2 major defects which are

likely to occur in continuous castings:

1. Defects during Process

2. Defect in products/ Metallurgical defects

Slab Casting; The Huge slab casters solidify sections up to 250 mm thick and 2,600 mm wide at production rates of up to three million tons per year.

Sliver; The elongated line type surface defects, termed FeO sliver and found on rolled coil/sheet. Sliver causes both cosmetic surface imperfection and

forming problem .

Slide 3

Sticker

Thin Shell

Crack

Air entrainment

MLF

Hook Formation

Classification

Internal Defects

Long Bow

Out of shape Camber

Shape Defect Surface defect

Taper

Internal Cracks

Segregations

CLS

Oscillation Mark

Slag spot

Blow, Pin hole

Longitudinal Crack

Transverse Crack

Rhomboidity Trapezoid

Fin

Depressions

Concavity Bulging NMI

Bleed

Lap

Slab Defects

Slide 4

Process Defects Defects in Slab

CASTING DEFECTS

Casting Defect in Slab

Steel Solidification models in Continuous Casting

Slide 6

The solidification models are nonlinear differential equations representing the

conservation of mass, momentum and energy, boundary and initial conditions

which depend on the shape of the slab/ mold, and the cooling system

Fluid flow, Heat transfer, Phase changes, Solid mechanics and

Electromagnetics

One-dimensional heat conduction equation

Where, K is the thermal conductivity (W/m K), c the specific heat (J/kg K), the density (kg/m3), q is the rate of energy generation (W/m3), T the temperature (K), t the time (s), and x the rectangular coordinate (m).

The release of latent heat where L is the latent heat (J/kg), fs the local solid fraction (%)

The fraction of solid in the mushy zone

where Tf is the melting temperature (K), Tl the liquidus temperature (K), and k0 the partition coefficient

Shell Growth

Slide 7

Steel shell growth can be predicted using Ficks law

Slab Length (L) = V x (D/K)2

Where L, slab length in m,

V is the casting speed, m/min

D is shell thickness, m

K is solidification constant depends on steel grade and M/c design

Fig. Temp profile from slab

surface : K. Bruce

Fig. : Calculated and measured solidifying shell thicknesses [R. Pierer ,BHM, 150. Jg (2005)]

Fig.: The shell thickness vs length from meniscus for Al-7wt % Si at different conditions of casting speed, (Q= 4 m3/hr, d = 5 cm) [Y. Rihan etal, OJMSW10-0025].

CE and Ferrite potential

Slide 8

For a multicomponent alloy system effect of various elements is considered in equivalence to carbon by the formula:

C=%[C]+0.04[%Mn]+0.1[%Ni]+0.7[%N]-0.14[%S]-0.4[%Cr]-0.1[%Mo]-0.24[%Ti]-0.7[%Si]

Ferrite potential is defined as the ferrite fraction

Fp=2.5(0.5-C)

Fp>1 -hypoperitectic steel Fp

Effect of Alloying elements

Sulphur (S):

Effects ductility at two temperature, near solidus and below 1200oC.

At Solidus S~0.03% reduces strength and ductility by half.

At 1200oC liquid sulphides at austenitic grain boundaries affect ductility

If Mn:S>60, steel is not embrittled ,as all S is tied to Mn.

Phosphorous(P):

Loss of ductility due to low MP liquid film in the interdendritic region

Responsible for hot tears

Calcium (Ca):

Mitigating influence on cracking sensitivity

Removes S by CaS formation, speeds up final solidification and reduces

Brittle temp range .Act as nuclei on which MnS precipitates

Aluminum(Al):

Precipitation of AlN at grain boundary in the form of a film reduces ductility

drastically

Cr, Ni: Worsen crack sensitivity

Silicon(Si): Improves crack susceptibility

Slide 9

Continuous Casting and Defects

Slide 10

Fig.: Schematic of continuous casting phenomena in mold [B.G. Thomas 03]

Sticker

Thin Shell

Crack

Air entrainment

MLF

Hook Formation

More than 90% of steel production is through continuous casting route

Online prediction of Casting Defect

Slide 11

Fig.: Defect prediction in continuous casting [Matsushita etal; US Patent88]

Fig-1

Sticker in Continuous casting

Sticker

Sticker is a casting abnormality during which solidifying slab

sticks to the mold wall and may lead to b/o if no precaution is

taken.

Restricted flow of slag between mold/strand gap leads to loss

of lubrication.

Sticker Appearance on Slab

Fig.: Ripple mark s on slab surface due to sticker B/O, Steel making conference proceedings, 1993

Sticker Formation Mechanism

Inadequate/Insufficient Lubrication

Slag rim Formation

Mold level fluctuation

Biased Flow

Metal Chemistry

Slag Basicity & Fluidity

Fig.: Sticker formation stages, Steel making conference proceedings, 1993

Fig. Characteristic temperature profile history for sticker breakouts and their detection systems [E. Szekeres1993].

Fig.: Temperature profile at different thermocouple layer during sticker B/out

Sticker Detection through BDS

Impact of Sticker

Loss of Productivity Drastic speed reduction to 0.15m/min Associated damage to machinery due abrupt speed change In case of break out

Poor quality Casting Down grading of slab due to speed change Ripple mark appearance in S/C requires rework Metallurgical changes in casting

Cost : (531 Lakhs/Annum) To reduce the rejection (Last 6 months) from 1.02 % to 0.20 % Cost incurred on a/c of DG, BO, B/D time Failure to supply compliance to customers etc.

Precaution to avoid sticker B/O Casting speed reduced to 0.15 m/min) to heel the slab surface then gradually

revert back to original speed

Local Thin Shell

Local Thin Shell: In the process of continuous casting of steel, sometimes the shell doesnt solidify to the required thickness. When shell thinning is limited to a small area of the strand, it is referred to as Local Thin Shell.

Fig. Schematic of thermal profile in continuous casting mold

Possible reasons :

Biased flow due to SEN port

clogging or alignment can locally

retard shell growth.

Uneven distribution and infiltration

of mould flux between the strand

and the mould wall.

Steel chemistry, Superheat and

solidification behaviour

Mould flux thermal behaviour

Argon flow rate

Casting Speed

Mould condition and life

Temperature distribution in Cu mold

Slide 17

Fig.: Temperature distribution & super heat removal BG Thomas, IST06 & Bai2000

Fig.:Breakout shell thickness profiles and corresponding model predictions, showing thin shell near location of jet impingement on narrow face, relative to steady shell growth down the wide face.27

Solidification and Shrinkage of Steel

Linear expansion coefficient (TLE) & Shrinkage: 1. Phase transformation processes dependent upon the carbon content 2. Large difference in the shrinkage behaviour between the various grades of steel wrt to C

content 3. TLE for Low carbon steel (0.05% C) is calculated to be 213 10-6 K-1, while the TLE of

0.60% carbon steel is shown to be 19.88 10-6 K-1.

[Jhu L.G., IMSM2007]

There are three distinct stages of shrinkage as molten metals solidify: 1. Liquid shrinkage, 2. Liquid-to-solid shrinkage or Solidification Shrinkage 3. Solid shrinkage-Thermal contraction * Shrinkage is decided by composition and Liquidus temperature of steel.

Steel Chemistry & Shrinkage

Slide 19

Fig. : Solidification shrinkage curves in the round billets along the height of mould

Mould Taper

SC 104 grade (Ultra Low Carbon ) reported without any LTS in 23 heats cast with 1.2% taper.

In SA413, % of LTS increased with 1.2% taper.

Mould taper 12CrMoV, X42) Mould taper in

(B72LX, ER70S6)

Mould taper (37Mn5V)

Steel Chemistry and Solidification

Temperatures in bottom region of the mould are higher for IF steel grades because of high liquidus temp. around 1537C which is around 10C higher than peritectic grades. Temperatures of bottom layer T.C.s are around 132C at higher speeds. Temperatures of some T.Cs increased more than others because they are located at the place where hot metal from SEN impinges on the shell.

Fig.: Liquidus Temp Difference in IF & Peritectic grade 15 oC approx.

Thermal Cracks

Slide 22

Fig.: Steel chemistry and surface crack in slab [C.Genzano etal. ISS conference2002]

Look line white lines and in extreme cases looks like sliver . Under SEM it looks as

bas-relief, Presence S on EDAX.

In SA413, % of LTS increased with 1.2% taper.

Temperature variation across Con-cast Mold

Slide 23

Fig. Temperature variation across the Cu mold during continuous casting of Steel

Air Entrainment

Slide 24

Fig.: Optimizing argon gas injection (for a 78-mm bore nozzle with 90 slide gate). Bai.H 2000

Fig.: Oxygen content along the slab centerlines (100-m inclusions).

1. Inclusions, bubbles, slag and other particles during solidification of steel products is a critical quality concern leading to rework or rejection of slab

2. Open-stream pouring produces the worst air entrainment problems 3. Leaks, cracks, inadequate sealing between the nozzle joints /porous nozzle material 4. The internal pressure in the nozzle below atmospheric pressure tends to aspirate outside

air and can be identified by nitrogen pickup and dendritic inclusions from reaction in a high-oxygen environment

Mold Level Fluctuation & Solidification

Slide 25

Fig.: Events during a severe level drop (20 mm for 0.6 second) that lead to a transverse surface depression.BG Thomas96

Fig.: Comparison of predicted and measured top surface liquid levels in steel [Yuan Q, Met. Trans-B04]

Hook Formation

Slide 26

Hook forms when meniscus freezes and overflow

Associated with oscillation mark and plagues ultra low C steel grades

Subsurface hook formation leads to slab surface defect due to entrapment of mold flux and inclusion

laden gas bubbles up to 3 mm from slab s/c.

Hook is of 3 types and shape is affected by thermal distortion and other mechanisms.

Hook formation is greatly affected by steel grade,

super heat, MLF and oscillation conditions

Fig.: Typical hook shape (right) and comparison of hook-shell thickness with shell thickness prediction from CON1D (left).[Shin HJ, TMS04]

SURFACE DEFECTS

Casting defect in steel slab

Surface and Sub surface Defects

Crack Formation Mechanism

Steel with carbon level of 0.08-0.14% is susceptible to cracks due to peritectic reaction at ~1500 C. Peritectic reaction: -Fe + liquid = -Fe Density of -Fe is higher than -Fe so shrinkage will occur during solidification Shrinkage in shell leads to non uniform shell formation. In mold, phase during peritectic reaction makes grain coarse and plasticity of

shell reduces. Difference in thermal shrinkage between and Fe leads to tensile strain/stress

in shell which along with uneven heat transfer in the transverse direction leads to uneven shell thickness in transverse direction.

Above stress is relieved by crack formation.

Remedies of Cracks

The usually adopted strategy is reduction of the stresses by keeping the thickness

of the shell to a minimum. This is achieved by reducing the horizontal heat transfer.

Reduction in heat transfer is achieved by:

Proper Mould Powder selection: -increase thickness of the solid layer of slag-crystallinity of the solid slag layer. Crystalline powder reduces conductivity of

heat-high basicity of powder increases crystallinity and increases softening

point.

Mould Oscillation:-Increase of oscillation frequency of mould results in better larger infiltrated slag thickness, thus less cracks.

Proper Mould Design: -Multi taper mould to accommodate solidification shrinkage thus reducing air gaps-Grooved mould surface: results in reduction of

heat transfer at meniscus.-high basicity of powder increases crystallinity and

increases softening point.

Proper SEN Design:-to reduce mold level fluctuations

Longitudinal Facial Crack (LFC)

Appear in the off-mid portion of slab It occurs on the slab surface parallel to casting direction Length may vary extend up to several centimetres Depth can vary from 3 to 5mm and can go as deep as 15cms! Width from 0.2mm to over 5mm. Occurrence of LFC varies from 5-20% in cast slab depending on grade.

Effect of LFC

LFC visible on slab surface appears as

Light single line and sometimes multiple line type sliver on product

Needs inspection of slabs and repairing, sometime may lead to downgrading

Lowers slab availability

There are reports of sliver from LFC

Effect of LFC

Repair of LFC and Sliver Analysis

Fig. Met Lab Analysis of Sliver from Longitudinal Crack

Fig.: Longitudinal Crack repairing

Transverse Crack

Mechanism Generated by longitudinal tensile strains at the surface in the mould due to high friction or in the sub mould during straitening or bending.

Cause Stress generation in the depressed portion of oscillation mark. Often

found along deep oscillation mark. Uneven variation of heat flux in the longitudinal direction due to

improper mould powder Excessive mold taper. Mold level fluctuation Steel composition

Remedy Proper choice of flux and higher flux consumption. Proper mold taper. Control of mold level fluctuation Proper uniform cooling to avoid the low ductility temperature range.

Fig.: Propagation of transverse crack during hot rolling

Effect of Transverse Crack

Star Crack

Star cracks appear in star pattern on the slab surface. It can be single or in cluster (spider web type). These are visible only after light scarfing i.e., after removal of 2-3mm of surface. Mechanism Preferential concentration of elements such as Cu on the surface of the

strand due to pick up from the mould. Embrittlement of the austenite grain boundary due to liquid copper in

conjunction with tensile stresses.

Cause High Cu levels (>0.15%) Remedies Coating of mould walls Adjust mould alignment

Internal Cracks in Slab

Segregation

Steel Composition

Improper Cooling

Super heat

Mid -Way Cracks

Cause

Surface Re-heating in or below the spray chamber

High Superheat

S & P > 0.02 %

Remedy

Adjust spray system to minimize re-heating

Lower Superheat

Lower P & S

Internal Cracks

Segregation is more at higher superheat

Triple -Point Cracks

Causes

Bulging of Wide Portion of the slabs

Decreasing Mn< 0.9%

Decreasing Mn/S < 30

Remedies

Re-gap rolls

Centre-Line Cracks

Causes

Bulging of Wide Face

Spray Water Intensity

Low roll alignment in the strand

Remedies

Re-gap Rolls

Reduce Casting Speed

Increase Spray Cooling

Internal Cracks

Diagonal Cracks

Causes

Asymmetric cooling in mould and sprays

High Superheat

Predominant in billets

Remedies

Install Corner rolls at the bottom of the mould

Check alignment between mould and roller apron

Look for plugged nozzles

Internal Cracks

SHAPE DEFECTS

Casting defect in steel slab

Definition

It is the distortion of the slab giving rise to concave surface/surfaces. This occurs usually due to inadequate support of the skin against the effects of Ferro static pressure.

Causes

High casting speed

High casting superheat and high mold temperature

Improper mold taper leading to poor cooling

Inadequate roll pressure

High teeming rate and biased flow

Inadequate mold design

Distortion or wear of mold

Fast or uneven cooling in the mold as well as secondary cooling

Bending or straightening of slab at too high or too low temperature

Bulging

Definition

A convex shape that narrows toward another end of the slab. It is generally observed in slab due to online width change and some times due to improper mold design or cooling pattern. Optimum taper in mold is provided in order to compensate the ingot shrinkage.

Causes

Online width changes made by the continuous caster

Remedies

Selection of optimum width change technology

Proper width measurement

Taper

'W1-W2 15mm

Rhomboidity

Definition

Cause

Mold deformation

Different water gap between Cu tube and water jacket for opposite or adjacent faces

Mold water quality

Too high casting temperature

Susceptibility to a rhomboidal shape is enhanced in the case of high carbon content,

peritectic grades and high level of tramp elements ( S,P,Cu,Sn)

Bad centering of casting stream in the mold

Inadequate mold taper

Misalignment in mold / foot rolls area

Rhomboidity is a shape defect mainly appears in slabs and billets when one diagonal of

the rectangular section is longer than other one. This is formed due to asymmetrical

cooling of the strand. Reduction in shell thickness at meniscus level may reduce

rhomboidity due to equal stress development at all 4 sides, It can be reduced, by

reduction in intermittent heating at any face inside the mold . Flushing can improve the

even cooling all the 4 sides

Mold control and changing if necessary

Mold measurement

Reduction in mold water quantity and adequate water gap

Correct alignment of foot rolls

Use of MEMS

Flushing of the strands

Temperature according to casting speed

Increasing relative casting speed

The reason behind the above point a) and b) are to reduce the shell thickness at

meniscus level so, the stress developed at all 4 sides become even, which is the

main reason for rhomboidity, can be reduced. Also , by doing this we can reduce

the intermittent heating at any face inside the mold . Flushing can improve the

even cooling all the 4 sides.

Remedies of Rhomboidity

Long Bow

Long Bow

L

LB

LB 0.01L

Definition

The long bow is a one piece design and occurs when the split roll bearing are

offset from roll to roll they are inline down the whole of the casting bow. The

reduced heat extraction at the bearing position resulted the heat variation in

stripped pattern.

Causes

Uneven temperature distribution across the slab width

Slab surface temperature variation 100 oC approx. which is visible as stripes on the

strand surface.

Peaks in temp coincided with the split roll bearing positions

Reduced water density in the non optimal spray overlap area

Remedies

Monitoring the Slab surface temp variation on the casting bow before entry into straightener

Ideal nozzle positions and spray heights

Adequate heat removal from roll bearing area and intensify the strand surface temp. deviation.

Support at Centre , Both ends and Three points of the slab

Ray Boyle2004

Out of shape

Definition

Causes

S 0.01W

When the axes of both edging rolls are perpendicular to the advancing direction of the slab, the slab tends to ascend on one side becoming tilted. slab geometry

resulted is called as out of shape or out of square cross section which is shown in fig.

1. Also shown a typical out of shape defect in Fig.-2.

The main cause of an out-of-square slab cross-section is the ascension of one side of the slab. slab Distortion takes place either due to temperature gradients,

loading or a combination thereof.

Kokubo et al

Remedies

Improper slab cutting /problem in TCM

Differences in temperature between the top and bottom surfaces of the slab will cause the slab to curl. Since slab weight and contact with the base restrict its

movement, stresses are created.

Load stress. Loads on a slab will create both compressive and tensile stresses within the slab.

Shrinkage and expansion. In addition to curling, environmental temperatures will cause PCC slabs to expand (when hot) and contract (when cool), which

causes change of slab shape

This is generally prevented by using either grooved rolls or tapered rolls with a bottom collar.

To prevent this ascension, the edging roll at this side is tilted at the angle q toward the same direction as the advancing direction of the slab. For a

reversing pass, the edging roll is tilted at the angel q0. This action creates a

component of the roll tangential force that pushes the slab down against the

table rolls.

Trapezoid Shape

Definition

Causes

EDT

TRAPEZOID

'2ED 20mm

Trapezoid shape defect mainly appears in slabs when width at one surface is larger

than other one. This is takes place due to slab withdrawal through curved path by

curvature and straightening of slab with the liquid core. This results difference in

guiding the curved and subsequent straightened slab between the two wide side of

the same slab. As a result the end surface of the slab were inclined relative to each

other rather being parallel which is called as the trapezoid shape. This is also formed

due to asymmetrical cooling of the strand.

Mold deformation to trapezoidal shape in curved mold casting Inadequate mold and machine design Inadequate mold guide and slab acquiring curvature from the mold Misalignment in mold / foot rolls area

Remedies

Proper design of continuous casting machine and mold. The mold having main wide wall disposed on the side of the center of curvature of

the slab, which curvature the slab will assume after leaving the mold.

Proper mold guide through and out of curvature or making the mold wide wall larger than the wall opposite to.

Fins

It is the thin strip of metal protruding approximately at right angle to the surface of the slab. It is caused by molten steel having run into the open cracks in the mold.

Causes Gap between the copper plate in mold High casting superheat Loosened/untight bolt during mold assembly Prolong use of mold Inadequate mold vibration

Remedies Flame cutting. Roll edging. Pressing

Concavity

Causes

It is the distortion of the slab which appears as a concave surface. This defect is usually attributed due to improper spray cooling pattern.

Improper secondary cooling High casting superheat Improper mold shape Inadequate roll pressure and roll surface profile Cooling water quality and volume

Remedies

Avoid the reasons of the defect And flatten the slab by scarfing

Depressions

Definition

Causes

Depression are mainly 2 types. Longitudinal depression is a channel shaped depression on the face of the slab running in the direction of the axis. Where as transverse depression is a localized depression on the slab surface normal to the axis of the slab.

Uneven lubrication Very rapid cooling Low casting superheat Mold level fluctuation

Remedy

Longitudinal depression

Transverse depression Avoid the reasons of the defect

Thank You

DISCUSSIONS

casting defect in slab

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