REPORT on STEEL INGOT CASTING INDUSTRY …...2003/03/07 · At some steel plant, T.O. increases from 20ppm at ladle to 40ppm at final product, which indicates serious contamination

IMF Research Project: Investigation of Steel Cleanliness during Ingot Teeming March 7, 2003

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REPORT on STEEL INGOT CASTING INDUSTRY SURVEY Lifeng Zhang Formerly Research Scientist at the Department of Mechanical and Industrial Engineering at the University of Illinois at Urbana-Champaign, now professor at the Department of Materials Science and Engineering Norwegian University of Science and Technology (NTNU) 7491 Trondheim, Norway. Tel: +47-73 55 12 00 Fax: +47-73 55 02 03 Email: [email protected], [email protected] B.G. Thomas Professor at the Department of Mechanical and Industrial Engineering at the University of Illinois at Urbana-Champaign University of Illinois at Urbana-Champaign Tel: 217-333-6919, Fax: 217-244-6534 Email: [email protected] Background: Around 15 copies of the survey were delivered at IMF Winter meeting (Jan. 16, 2003, Pittsburgh) and around 40 copies were sent by Shawn Holmberg (Ellwood Quality Steels) on Feb.1, 2003. Including the response that arrived March 7, six replies have been received. This report summarizes those survey replies. The total annual tonnage of bottom-poured ingots where cleanliness is a concern in these steel plants is about 700,000 tons. Rejections due to inclusions range from 0.2 - 5%. The cost of rejected product is $900-3600/ton (depending on grade). Assuming an average rejection fraction of 1%, and average cost of $1500/ton, then roughly $10,000,000 per year are lost by these companies by inclusion defects. Section II: Process Configuration for Cleanliness Investigation 1. Steelmaking and Steel Refining The steel with most interest for the cleanliness investigation is mainly with 0.32-0.50% [C]), 60-150ppm [N], <40ppm T.O., 0.008-0.025% [Al] (corresponding dissolved oxygen 3-8ppm), 0.23-1.0% [Si]. At some steel plant, T.O. increases from 20ppm at ladle to 40ppm at final product, which indicates serious contamination during teeming process. A quick calculation of reoxidation (assuming all of the air in the runner system and mold becomes entrained in the steel) gives a maximum oxygen pickup of 43ppm. Thus, the assumption of reoxidation defects being negligible appears to be questionable, at least at some plants. Ladle capacity ranges from 25-70 tons. Steel refining is by different process such as vacuum degassing, AOD, LF, ladle-to-ladle stream degasser. The deoxidizer is grade and process-


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dependent, mainly by Al or Si-Fe. At ladles, samples are taken during steel refining and before teeming mainly for chemical composition analysis but not for the investigation of inclusion morphology, composition, amount or size distribution (owing to difficulties with air contamination and other problems with sample collection). 2. Ingot Teeming (Bottom Pouring Process) Before teeming, no argon is purged into the ingot molds at any responding plant. The trumpet is packed with silica sand to prevent brick movement, the space between trumpet brick sleeves and cast iron mold is filled with dolomite. The ladle is simply lowered down over the trumpet funnel. At some steel plants, argon is injected at the connection between ladle and trumpet with a flow rate of 80-100 cfh. At other steel plants, instead of injecting argon between ladle and trumpet, a fiber shroud is placed between collector nozzle and trumpet. For ladle opening, slide gate is used. Free open percentage varies from 90% - 99%, to a low of only 50%. The lower percentages are a concern because it is well known that during continuous casting, ladle lance opening induces serious reoxidation, increasing T.O. oxygen in the tundish to 10ppm higher than that for free opening. [1] The method to detect ladle slag during teeming (to prevent slag carryover into the trumpet) is visual detection, and the standard practice used currently is to have enough metal so no slag is poured into the trumpet. 3. Ingot Casting / Mold Filling All of the teeming process is by bottom pouring. The ingot size for this cleanliness study was recommended to be: 13,700 lbs weight, 28” square section, and 63” height. No company reported ingot taper, but ingot taper affects fluid flow pattern and inclusion motion, thus has effect on entrapment of inclusions. There are 5-8 ingots in a cluster. Teeming rate (filling rate) is dependent on steel grade, and ingot geometry and size, typically - 4.0 to 13.0 in/min, and the typical filling time is 13-18 minutes. Mold powder is added by a suspended bag (1.4-5kg) with a suspension height: 0-half or 2/3 height of the mold. Thickness of mold powder is dependent upon mold size, typically 2” to 2½”. No steel plants measure the composition of mold flux after casting, however, by comparing it with its original composition, the inclusions removal to top slag can be roughly evaluated. Filling rate and turbulence, possibility to entrap mold powder, rate of rise, shrouding, refractories erosion, slag entrapment by vortex near the ladle nozzle at the end of teeming, are recommended topics of interest. The main compositions of refractory, such as ladle lining and slag, well block, filler sand, trumpet, runner and mold flux, are shown in Table I. Some of the refractory contains high SiO2, which normally reoxidizes molten steel serious. [2] The compositions of these refractory are needed in order to locate the sources of entrapped exogenous inclusions.


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Table I. Main compositions of refractory

Refracory Composition Ladle lining Mainly MgO+CaO >95%;

Some >60% Al2O3, 7% SiO2 Ladle slag >60% MgO+CaO, 5-20% Al2O3, 15-27 SiO2 Well block >90% Al2O3 Well block sand >5% MgO, >10% Al2O3, >25% SiO2, >30% Cr2O3, >15% Fe2O3 Inner nozzle >90% Al2O3 Slide gate plate >85% Al2O3, >5% SiO2;

or >95% MgO Collector nozzle >80% Al2O3, 4-13% SiO2 Trumpet >40% Al2O3, >55% SiO2;

or >60% Al2O3, >30% SiO2 Runner spider brick 35-65% Al2O3, 30-60% SiO2; Runner filler materials >95% CaO+MgO

or 100% Cr2O3 or 45-60% Al2O3, 35-50% SiO2

Mold flux 5-25% CaO, ~15% Al2O3, 15-25% SiO2, 5-10% Na2O+K2O, ~25% C Mold insulating /exothermic board

30-35% Al2O3, 45-65% SiO2

Mold hot-top compound 25-40% Al2O3, 5-10% SiO2, 3-8% Fe2O3 Section III: Main Defects Affecting Ingots From the replies of the survey, 10-25% of defects sources are estimated to be related to ladle sand/packing sand entrapment, 25-50 from mold flux entrainment, 0-5% from runner erosion, 0-35% related to other exogenous inclusions. Beside the above exogenous materials, estimates are 0-15% related to alumina inclusion (deoxidation inclusion), 0-20% from air absorption, 0-5% from reoxidation reactions with slag and refractory (eg. SiO2 in lining and slag), and 0-10% of the defects are still not clear about their source. Clearly, exogenous defects are the greatest problem. The morphology, formation mechanism, estimated sources of the main defects from exogenous materials are listed in Table II. The sources range from:

- Entrapped Nozzle Sand; - Entrapped Mold Flux - Other entrapped refractory such as ladle well block, ladle inner nozzle, ladle stirring

block, upgate or runner brick - Refractory and slag type inclusions

The percentage of these sources should be investigated by industrial trials in this project.

Table II. Main defects affecting ingot from exogenous materials Internal Defect Exogenous Inclusion Estimated Entrapped Nozzle Sand: nozzle sand that is placed inside the well block prior to


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sources teeming. Simple mechanism of the defect formation

Prior experiments have shown that nozzle sand will sinter to the well block and to itself. Investigations done on used well blocks have shown large ridges and valleys where sand can lay and be sintered to the well block by the ferro-static head pressure and liquid steel temperature. During the teeming operation, this nozzle sand agglomeration can be eroded by the steel flow. The agglomeration then travels through the runner system and is entrapped in the solidifying shell of the ingot during teeming.

Pictures

Left figure: Photomacrograph displaying an isolated view of the porosity associated with the defect Right figure: Photomicrograph revealing the interior morphology of the defect exposed by opening the sample. The entire surface is engulfed with extraneous nonmetallic material (Backscatter imaging)

Backscattered electron image of an inclusion, light gray phase (composition as right figure) is chromite, dark gray phase is silica. Source is slidegate borefill sand.


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Appearance of defect on machined surface of sample (upper left) and cross-sectional view of this defect (upper right), light gray material is chromite (lower left) and darker gray material is silica (lower right).


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SEM detection of the large agglomeration of exogenous materials. The source is well block sand (Cr2O3 32.9%, SiO2 27.6%, Fe2O3 18.6, Al2O3 11.8%, MgO 7.1%, C 0.6%).

Backscattered electron image of an inclusion in steel sample, light gray phase (composition as right figure) is chromite, dark gray phase is silica. Source is slidegate borefill sand because borefill sand is mixture of chromite and silica.

Internal Defect Entrapped Mold Flux (i.e. Mold Slag) Estimated sources

Mold flux

Simple mechanism of the defect formation

During initial teeming, the teeming rate is at a maximum to flush the system with steel and ensure that the system does not freeze off. During this time violent mixing can occur in smaller diameter mold sizes resulting in entrainment of mold flux into the liquid steel.


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Pictures

SEM photomicrograph of inclusions from mold flux ( SiO2 29.0-36.0%, CaO 1.0-5.0%, MaO 2.0% max, Al2O3 15.0-21.0%, Fe2O3 5.0-11.0, MnO 1.0%, Na2O 4.0-6.0%, K2O 2.0%, F 0.5%, C 23.0-26.0%)

EDS scan of a large inclusion in Steel sample, containing Si, Ca, Al, Na. Likely source is mold flux.

EDS scan of a large inclusion in Steel sample, containing Si, Ca, Al, Na. Likely source is mold flux.


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Internal Defect Entrapped Well Block Estimated sources

The well block is a high alumina refractory used at the bottom of all ladles. This block is the exit point for the steel out of the ladle and into the slide gate system.


This block is in service for 65+ heats. During this time refractory erosion will occur. The eroded material will then exit the ladle, pass through the runner system and become entrapped on the solidifying shell of the ingot.

Pictures

SEM photomicrograph of large particles containing high Al and Mg content, coming from the refractory of ladle well block (Al2O3 91%, MgO 6%).

Internal Defect Entrapped Inner Nozzle Estimated sources

This is the nozzle adjacent to the well block.


The inner nozzle is in service for 12+ heats. During this time refractory erosion will occur. The eroded material will then exit the ladle, pass through the runner system and become entrapped on the solidifying shell of the ingot.

Pictures

SEM photomicrograph of large particles containing high Al and Mg content, coming from the refractory of ladle inner nozzles (Al2O3 91%, MgO 6%).


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Internal Defect Entrapped Stirring Block Estimated sources

This is the block through which argon stirring is done.


The stirring block is in service for 50 heats. During this time refractory erosion will occur. The eroded material will then exit the ladle, pass through the runner system and become entrapped on the solidifing shell of the ingot.

Pictures

SEM photomicrograph of large particles containing high Al and Mg content, coming from the refractory of Stirring Block (Al2O3 91%, MgO 6%).

Internal Defect Inclusions Estimated sources

Refractory-Type inclusions


Refractories erosion creating sharp angled inclusions that are going into the melt. Often, those inclusions cluster with others (especially alumina) or cluster with slag at metal-slag interface and get reentrapped.

Pictures

Spinel : Al2O3 75% - MgO 25%

Al2O3 50% - CaO 40% - SiO2 10%


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Slag-Type inclusions


Slag is entrapped while stirring during refining operations or gets trap in vortex at the end of teeming while low steel level in present in the ladle

Picture


Reoxidation inclusion


Steel reacting with air while teeming or stirring in previous refining operations

Pictures

Al2O3 30% - CaO 50% - SiO2 20%

MgO 100%

SiO2 100%

SiO2 60% - MnO 35% - Al2O3 5%


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Internal Defect

Refractory brick, upgate, or hollow ware.

Estimated sources

EDS scan of a large inclusion network (around 1mm in length) in Steel sample, containing Si, Al and small amount of Mn and Fe. More likely source is refractory brick, upgate, or hollow ware.

Pictures

Internal Defect Fireclay refractory particle Estimated sources

Backscattered electron image of an inclusion, mainly containing Al, Si and Cr in steel sample. Source is possibly fireclay refractory.

Pictures

Internal Defect Entrapped refractory, such as upgate or runner brick Estimated sources

SEM scan of a inclusion in a steel sample, containing Si 59%, Ca 17%, Mn 19%, Al 8%, Na 4%, Cr 2%. This nonmetallics is most likely refractory, such as upgate or runner brick.


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Pictures

Internal Defect

Entrapped dolomite

Estimated sources

Backscattered electron image and EDS scan of a dolomite inclusion mainly containing Ca and Mg

Pictures

Surface Defect Rippled Surface (Looks similar to scales on a reptile) Estimated sources

Absence of or insufficient amount of teeming flux.


The absence of or insufficient amount of teeming flux causes the steel to lap over itself forming a scale like skin. This can also be caused by poor flux properities orimproper flux addition practices.

Surface Defect Slag and Mold entrapment in steel shell Estimated sources

Mold Flux


Presence of inclusions entrap in the steel solidifying shell approximately at the bottom third of the ingot mold, in the first 2 inches of steel which has solidified at the very beginning of the teeming process


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Section IV. Research Projects To conduct a successful research project, which will involve both computational models and experiments, it is important to have a clear understanding of the question(s) that we want answered. The work needed to answer each question may be quite different, so only a few can be investigated. 1. Potential Objectives The importance of the following potential questions that could be addressed with this project are averaged over all respondents are put in order or preference, based on the following rating scheme: 4.0 = very important and very interesting 3.0 = important and interesting 2.0 = interesting 1.0 = not directly relevant but would still support (for scientific understanding) 0.0 = not relevant and would not support 3.5 Q21) How does fill rate affect inclusion removal? 3.5 Q1) Where do the majority of inclusions come from in the final product?

(Mold slag? Reoxidation products? Sand pickup?, refractory erosion? Hydrogen?) 3.2 Q2) What fraction of inclusions entering the mold end up trapped in the final product? 3.2 Q16) How should the in-gate refractory geometry into the ingot bottom be shaped to best

deliver molten steel into the mold with minimal air reoxidation? 3.0 Q14) If an exogenous particle enters the mold, where would it end up (what fraction is

entrapped in the flux layer, steel meniscus, side walls, bottom, interior, etc?) 3.0 Q3) Where do inclusions end up in the final product? (Are they distributed more

towards the top?, the bottom inverse segregation zone? 3.0 Q19) What is the best time / height to add mold flux? (At what height up the ingot

should the mold powder bags be suspended to obtain the least reoxidation and minimum slag entrainment, as a function of ingot size, inlet flow rate, etc.?)

2.8 Q7) Where do exogenous inclusions entering the trumpet (eg from ladle packing sand) end up? (ie. what fraction attach to runner walls, float into the slag layer, become trapped in final product?, etc.)

2.8 Q17) How high is the “spout” of liquid squirting into the mold bottom from the ingate, as a function of ingate design and filling rate?

2.8 Q20) How does ingot wall taper promote inclusion removal? What is the optimal taper? 2.8 Q22) After filling, how many inclusions are removed to the top slag? How important is

the “hot top” to extending this inclusion removal time? 2.7 Q12) What are guidelines for runner system design to maximize inclusion removal from

contact with the walls? 2.7 Q15) How does liquid filling rate into the ingot bottom vary with runner system design? 2.5 Q11) What are guidelines for runner system design to avoid refractory erosion? 2.5 Q18) How much mold powder should be added? (based on thermal insulation? Based on

inclusion removal? Based on melting rate? Based on pouring rate?


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2.3 Q10) What is the theoretical maximum benefit of purging the system with argon before pouring? (ie. How much reoxidation occurs in the absence of gas shrouding?)

2.0 Q5) What is the morphology of inclusions trapped in typical bottom-poured ingots? 2.0 Q9) How much improvement can be obtained with argon gas injection during pouring? 1.8 Q4) What is the size distribution of inclusions trapped in typical bottom-poured ingots? 1.8 Q6) What is the composition of inclusions trapped in typical bottom-poured ingots? 1.8 Q13) What are guidelines for runner system design to minimize air entrainment? 1.3 Q8) How much benefit would occur from changing from simple dolomite packing sand

(around the trumpet top) to steel shot? Most of the interests (with score > 3.0) are focusing on the sources of exogenous inclusions in ingot and the inclusions motion and entrapment, such as 1). How does fill rate affect inclusion removal? 2). Where do the majority of inclusions come from in the final product? (Mold slag?

Reoxidation products? Sand pickup?, refractory erosion? Hydrogen?) 3) What fraction and where do exogenous inclusions entering the trumpet end up? 4). If an exogenous particle enters the mold, what fraction and where would it end up Less interests (with score <2.3) are for argon protection, inclusion size distribution and some side research such as “How much benefit would occur from changing from simple dolomite packing sand (around the trumpet top) to steel shot?”. 2. Computational Modeling Studies Each potential project will involve computational modeling. The average survey score for each modeling project follows, in order of preference. In addition, the questions that can be answered by each modeling projects are listed under them. 3.6 2) Trumpet / Runner / Gate / Ingot Mold Fluid filling study (transient) Repeat #1, but perform transient calculations, including the condition of the

advancing free surface. With more accurate, but expensive models, emphasis would be on quantitative prediction (matching measurements) to determine the absolute fraction of inclusions experiencing different fates. Differences between start – cast and steady state could be compared. Fewer parametric studies could be performed, owing to the higher cost per run. Emphasis would be on quantitative determination of where inclusions end up (matching measurements) – determining fractions of inclusions to different fates (trapped in runners vs. entering mold).

Questions that can be answered: Filling rate :

3.5 part of Q21) How does fill rate affect inclusion removal? 2.8 Q17) How high is the “spout” of liquid squirting into the mold

bottom from the ingate, as a function of ingate design and filling rate? 2.7 Q15) How does liquid filling rate into the ingot bottom vary with

runner system design? Runner:


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2.7 Q12) What are guidelines for runner system design to maximize inclusion removal from contact with the walls?

1.8 Q13) What are guidelines for runner system design to minimize air entrainment?

Trumpet: 2.8 Q7) Where do exogenous inclusions entering the trumpet (eg from

ladle packing sand) end up? (ie. what fraction attach to runner walls, float into the slag layer, become trapped in final product?, etc.)

In-gate: 3.2 Q16) How should the in-gate refractory geometry into the ingot

bottom be shaped to best deliver molten steel into the mold with minimal air reoxidation?

Mold 3.0 part of Q14) If an exogenous particle enters the mold, where would it

end up (what fraction is entrapped in the flux layer, steel meniscus, side walls, bottom, interior, etc?)

2.8 part of Q20) How does ingot wall taper promote inclusion removal? What is the optimal taper?

6) Exogenous particle generation / removal 3.4 6b) Measure the initial and final well block shape to determine the extent of erosion

and deposition (sintering) of particles to the refractory walls during teeming, in order to investigate the susceptibility of different refractory compositions and geometries.

3.4 6c) Repeat this study for the runner and other components. 3.2 6a) Model (steady and transient) the effect of well-block geometry, ferrostatic

pressure, and steel flow rate on temperature, thermal stresses, and possible fracture / erosion of the refractory material.

Questions that can be answered: 2.7 Q15) How does liquid filling rate into the ingot bottom vary with

runner system design? 2.5 Q11) What are guidelines for runner system design to avoid

refractory erosion?

3.0 4) Ingot flow / solidification study Transient flow and solidification would be performed throughout ingot filling and

solidification, assuming a given inflow rate profile (with time) and corresponding level rising velocity. Inclusion trajectories within the liquid cavity would be computed, and their capture in the solidifiying shell or removal from the top surface would be computed, using entrapment criteria based on particle size. Emphasis would be on predicting inclusion distributions in the product: both size and location. Later parametric studies could compare ingot size and shape (eg. wall taper) effects on these results.

Questions that can be answered: 3.5 part of Q21) How does fill rate affect inclusion removal?


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3.2 Q2) What fraction of inclusions entering the mold end up trapped in the final product?

3.0 Q14) If an exogenous particle enters the mold, where would it end up (what fraction is entrapped in the flux layer, steel meniscus, side walls, bottom, interior, etc?)

3.0 Q3) Where do inclusions end up in the final product? (Are they distributed more towards the top?, the bottom inverse segregation zone?

3.0 part of Q19) What is the best time / height to add mold flux? (At what height up the ingot should the mold powder bags be suspended to obtain the least reoxidation and minimum slag entrainment, as a function of ingot size, inlet flow rate, etc.?)

2.8 Q22) After filling, how many inclusions are removed to the top slag? How important is the “hot top” to extending this inclusion removal time?

2.8 Q20) How does ingot wall taper promote inclusion removal? What is the optimal taper?

2.8 5) Powder flux melting study Heat transfer in top liquid and powder layers will be modeled to investigate optimal

placement of mold flux bags suspended in molds in order to maintain optimal surface cover and melting of the powder (in fine powder or granulated form)

Questions that can be answered: 3.0 Q19) What is the best time / height to add mold flux? (At what

height up the ingot should the mold powder bags be suspended to obtain the least reoxidation and minimum slag entrainment, as a function of ingot size, inlet flow rate, etc.?)

2.5 Q18) How much mold powder should be added? (based on thermal insulation? Based on inclusion removal? Based on melting rate? Based on pouring rate?

2.6 1) Trumpet / Runner / Gate / Ingot Mold Fluid filling study (steady): Steady 2-D computational models of turbulent flow in the entire bottom poured ingot

system would be developed using steady-state models (K-ε models assuming typical conditions during steady filling). Later enhancements would extend the simulation to 3D. Inclusion particle trajectories would be computed, assuming a range of different particle sizes, densities, (eg sand vs alumina) and drag (spheres vs. clusters / dendrites). Flow patterns would be analyzed for recirculation zones (assuming these to be detrimental?). Statistics would be collected on the particles to determine their destinations. Emphasis would be on parametric studies to compare different geometries and inclusions on a relative basis.

Questions that can be answered: Filling rate :

3.5 part of Q21) How does fill rate affect inclusion removal?


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2.7 Q15) How does liquid filling rate into the ingot bottom vary with runner system design?

Trumpet: 2.8 Q7) Where do exogenous inclusions entering the trumpet (eg from

ladle packing sand) end up? (ie. what fraction attach to runner walls, float into the slag layer, become trapped in final product?, etc.)

Runner: 2.7 Q12) What are guidelines for runner system design to maximize

inclusion removal from contact with the walls? Mold

3.0 part of Q14) If an exogenous particle enters the mold, where would it end up (what fraction is entrapped in the flux layer, steel meniscus, side walls, bottom, interior, etc?)

2.4 3) Mold Filling: in-gate design Repeat #2, but focus on the details at the in-gate entering the ingot. The effect of in-

gate geometry on initial shape of the free surface spouting into the ingot cavity would be studied. Emphasis would be on producing the most stable free surface shape to avoid reoxidation and slag entrainment, and the height where flux bags could be safely be suspended. Questions that can be answered: 3.2 Q16) How should the in-gate refractory geometry into the ingot



1.8 Q13) What are guidelines for runner system design to minimize air entrainment?

3. Experimental Studies Each potential project will involve experiment studies. The ideal experiments simultaneously measure as many aspects of the same process as possible. They also change something in the process which produces a measurable change in cleanliness outcome in the final product, while holding everything else constant. 3.1. The experiments and measurements (The number in [ ] at the left side of each item is the number of repliers who can do these experiments and measurements): [3] 1) Put a different tracer ceramic (eg. BaO, La2O3, Sr2O3, etc.) into each type of refractory (ladle, trumpet, ladle sand, runners). [2] 2) If the mold and ladle slag cannot be distinguished, then tracer should be added to them also.


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[3] 3) Measure runner refractory geometry and surface layer composition before and after casting (to check for erosion or deposition of inclusions)

By 1),2),3)., the following questions can be answered: 3.5 Q1) Where do the majority of inclusions come from in the final product?

(Mold slag? Reoxidation products? Sand pickup?, refractory erosion? Hydrogen?)

3.2 Q2) What fraction of inclusions entering the mold end up trapped in the final product?


2.8 Q7) Where do exogenous inclusions entering the trumpet (eg from ladle packing sand) end up? (ie. what fraction attach to runner walls, float into the slag layer, become trapped in final product?, etc.)

[2] 4) Conduct experimental casting(s), (eg. in pairs); holding everything constant except for the variable of interest (see below)

Experimental studies of interest: Trial ingots should be cast at the plant with all conditions held constant except for: 3.4 5) different heights of mold flux bags suspended within mold 2.8 8) Different flux types ( fine powder vs granulated powder)

The following questions can be answered by the above 2 questions: 3.0 Q19) What is the best time / height to add mold flux? (At what

height up the ingot should the mold powder bags be suspended to obtain the least reoxidation and minimum slag entrainment, as a function of ingot size, inlet flow rate, etc.?)

2.5 Q18) How much mold powder should be added? (based on thermal insulation? Based on inclusion removal? Based on melting rate? Based on pouring rate?

2.8 4) Different in-gate geometries

The following questions can be answered: 3.5 Q21) How does fill rate affect inclusion removal? 3.2 Q16) How should the in-gate refractory geometry into the ingot



2.6 2) different runner shapes

The following questions can be answered: 2.7 Q15) How does liquid filling rate into the ingot bottom vary with

runner system design? 2.7 Q12) What are guidelines for runner system design to maximize

inclusion removal from contact with the walls?


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2.5 Q11) What are guidelines for runner system design to avoid refractory erosion?

2.4 7) Purging system with argon or not

The following questions can be answered: 2.3 Q10) What is the theoretical maximum benefit of purging the

system with argon before pouring? (ie. How much reoxidation occurs in the absence of gas shrouding?)

2.2 1) Different runner refractories

The following questions can be answered: 2.5 Q11) What are guidelines for runner system design to avoid

refractory erosion? 2.0 6) Different ingot wall tapers

The following questions can be answered: 2.8 Q20) How does ingot wall taper promote inclusion removal?

What is the optimal taper? 1.4 3) Different runner lengths

The following questions can be answered: 2.7 Q12) What are guidelines for runner system design to maximize

inclusion removal from contact with the walls? 2.5 Q11) What are guidelines for runner system design to avoid

refractory erosion? Several of these cases could be accomplished together or combined simply by altering ingots in a single cluster.

3.2. Steel and Slag Sampling and Inclusions Analysis (The number in [ ] at the left side of each item is the number of repliers who can collect these samples or can do the inclusions analysis): Collect steel samples at following places and analyze them for inclusions: [3] 1) ladle samples [4] 2) as cast trumpet and runner metal [3] 3) top and bottom trimmed from the ingot [1] 4) molten steel samples from near the top of the solidifying ingot at different times after

filling is complete [3] 5) the as-cast (solid) ingot [4] 6) the final formed product for inclusions Collect slag samples and analyze their compositions: [3] 1) ladle slag [4] 2) Initial mold power


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[2] 3) Intermediate mold slag sample (eg. 3-10 samples as time permits) [3] 4) Final mold slag Use the following methods to perform the measurements [4] 1) Individual inclusion analysis (using Microscope observation, SEM, SIMS, Auger,

etc.) [4] 1.a) composition (particularly looking for the tracer oxides to identify the inclusion

source) [4] 1.b) morphology (round / clustered / dendritic and center vs. exterior to determine

whether the inclusion grew onto a nucleus particle with a different origin. [4] 2) Total Oxygen, N, C, Al, Si, Mn, Ti S, P measurement Determine inclusion quantity and size distribution by: [0] 1) volumetric analysis, (such as X-rays, acid extraction, slimes tests, etc.) [2] 2) surface analysis taken on sections, using sulfur prints, etc. [3] In addition, complete blueprints should be provided in order to define the geometry for

flow (including the ladle, trumpet / runner / gating / mold) [4] Also, the complete process conditions should be recorded, (see Section II for details).

By steel and slag sampling and inclusions analysis, the following questions can be answered: 3.2 Q2) What fraction of inclusions entering the mold end up trapped in the final

product? 3.0 Q3) Where do inclusions end up in the final product? (Are they distributed

more towards the top?, the bottom inverse segregation zone? 2.8 Q7) Where do exogenous inclusions entering the trumpet (eg from ladle packing

sand) end up? (ie. what fraction attach to runner walls, float into the slag layer, become trapped in final product?, etc.)

2.0 Q5) What is the morphology of inclusions trapped in typical bottom-poured ingots?

1.8 Q4) What is the size distribution of inclusions trapped in typical bottom-poured ingots?

1.8 Q6) What is the composition of inclusions trapped in typical bottom-poured ingots?

Section V: How to help? Four of the survey repliers are willing to do industrial experiment to support this project (Elwood, Timken, Sorel, and A. Finkl & Sons), and five can provide samples and measurement for typical condition and conduct trials. Four of them can provide operation conditions (complete record), measurement of steel cleanliness (inclusions), material property data (eg. thermal conductivity, density, etc. of refractories, slags, etc.), three can provide research reports, and two of them can provide relevant papers published by your company.


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Section VI: Recommendation on the First Step of Modeling Studies and Industrial Trials Although reoxidation problems, air entrainment during lance-open ladles and trumpet joints, and other issues are still a concern, the greatest defect source is exogenous inclusions, which should be the focus of the remainder of this study. Within this category, a variety of sources should be investigated, ranging from very large (>500 micron) sand / brick particles to entrapped slag (10-5000 microns) 1. Modeling Studies: Significant interest was found for virtually all of the proposed projects. The most popular project was transient filling, but the project that most directly addresses the questions of greatest importance is the ingot flow / solidification study. It would appear that both projects should be addressed. The difficulty is choosing which to tackle first. 1) The first step of transient is a steady state. Thus, steady 2D and / or 3D (depending on

geometry) computational models of turbulent flow in the entire bottom poured ingot system (starting from ladle bottom well, through trumpet, runners, and ingot mold) would be developed using steady-state models (K-ε models assuming typical conditions during steady filling). Inclusion particle trajectories would be computed, assuming a range of different particle sizes, densities, (eg sand vs mold flux) and drag (spheres vs. irregular shapes). Statistics would be collected on the particles to determine their destinations. This project would be done for the chosen trumpet / runner geometry system.

2) Transient heat transfer and solidification simulation, starting from filling of mold would

be performed for a few different ingot sizes. Inclusion trajectories within the liquid cavity would be computed, and their capture in the solidified shell or removal from the top surface would be computed, using entrapment criteria based on particle size. Emphasis would be on predicting inclusion distributions in the product: fraction entrapped and location, as a function of inclusion type and size.

By these two step simulation, the following questions can be started to be addressed:

3.5 Q21) How does fill rate affect inclusion removal? 3.2 Q2) What fraction of inclusions entering the mold end up trapped in the final

product? 3.0 Q14) If an exogenous particle enters the mold, where would it end up (what

fraction is entrapped in the flux layer, steel meniscus, side walls, bottom, interior, etc?)


2.8 Q22) After filling, how many inclusions are removed to the top slag? How important is the “hot top” to extending this inclusion removal time?

2 Industrial trial (need to be discussed in detail by the committee members)


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The top 3 projects appear to be: 1) Put a different tracer ceramic (eg. BaO, La2O3, Sr2O3, etc.) into each type of refractory

(ladle, trumpet, ladle sand, runners, and into the mold slag, if its composition is similar). Measurements recommended for this include analysis of the steel product for inclusions, (especially if the final product test can detect inclusions in a large volume). In addition, the initial and final mold slag composition should be measured, (to see if particles entering the mold ended up there). In addition, liquid steel samples should be taken from the mold (to find evidence of the refractory tracers or mold flux). Sorel has volunteered that they can obtain such liquid samples.

2) Measure runner refractory geometry (before and after casting) and surface layer composition before and after casting (to check for erosion or deposition of inclusions). Ideally, this project should be done at the same time as the first project.

3) Conduct experimental casting(s), (eg. in pairs); holding everything constant except for the variable of interest: 3.1) different heights of mold flux bags suspended within mold; 3.2) different flux types ( fine powder vs granulated powder); 3.3) different in-gate geometries; different runner shapes. 3.4) Different mold filling rates (this is a new project that appears warranted based on

the strong response to this project objective.) These experiments also should be performed at the same time as the first 2 projects, perhaps simply by changing the conditions of different ingots in the cluster. Other projects of group interest should be considered also.

For each of these projects, detailed sampling plans (steel sample, slag samples, refractory samples) and analysis methods (Microscope observation, SEM, SIMS, Auger, composition analysis, acid extraction etc.) need to be discussed and decided. This will likely depend on the capabilities of the steel plant(s) selected. We recommend experiments at more than one plant. We await further opinions and recommendations regarding the modeling study and decisions regarding the plant experiments.


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Section VII: Suggested Industrial Trial Sampling Methods The following suggests some sampling methods that could be used for the industrial trials. Naturally, other methods (such as ultrasonic inspection of the final product to isolate rare large inclusions) are recommended as well. 1). Ladle sampling One slag sample, and if possible, also one steel sample, from the ladle after steel refining. 2). Mold Sampling (molten steel) - One steel and one slag sample as soon as safe after ingot teeming - More intermediate samples if possible (eg. Every 30 minutes) - One steel and one slag sample near the end of solidification (i.e., after 1-2 hours depending on

ingot size) The slag sampler can be a steel spoon. The following schematic of the molten steel sampler features a Cu lid with < 1mm thickness (or other lower-melting metal or even cardboard lid) designed to prevent slag entrainment during the sampling process.


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Example molten steel sampler

The molten steel samples are recommended to be taken 100mm or more beneath the top slag, and in the center of the ingot as shown below. The depth and time of sampling (from start of teaming) must be recorded. Immersion time depends on time to melt lid and must be removed straight up (to avoid slag entrapment on removal). Water quench to remove the sample as usual.

80mm

30mm

50mm

2000mm

Steel

<1mm thickness Cu


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Schematic of molten steel sampling in mold The molten steel sample is then machined (by lathe) as below. Machining chips (filings) can be used to analyze for Al, Si, Mn, Mg, Cr, C. The thin bar (φ5mm×15mm or as according to LECO specifications etc.) is for T.O. and nitrogen analysis. The larger sample (φ20mm×20mm) is for microscope observation, and some typical inclusions will be analyzed by SEM and other methods.

Schematic of machining of molten steel sample 3). Sampling of the rough-hot-deformed product (forged or rolled). For example, from an ingot with 1420mm×580mm section area and 3320mm height, samples could have been taken at the head, middle, and end parts of the rolled ingot as shown below.

>100mm

Sampler

Ingot


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Sampling a rolled ingot

The samples could be machined as shown in the figure below. Machined chips (or filings) can be used to analyze for Al, Si, Mn, Mg, Cr, C. A small (5×5×10mm3) bar (for LECO or equivalent). could be cut from each 20mm×20mm ×20mm sample for T.O. and nitrogen analysis. The remainder of the each samples will be for microscope observation, including analysis of typical inclusions by SEM and other methods.

Machining of rolled ingot samples

4). Large samples for acid extraction We recommend collection of samples for acid extraction (for non-stainless steel) to obtain the rare large three dimensional exogenous inclusions from a larger volume. The method is easy to

Head Middle End


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extract Al2O3 and SiO2 based exogenous inclusions, so refractories based on Al2O3 are recommended. This acid extraction dissolves FeO, CaO, MgO and MnO. Because exogenous inclusions (such as shown in Table II) are rare, the samples for acid extraction should be as large as possible, such as φ50mm×150mm for molten steel samples, and 45mm×45mm×150mm for solidified steel samples (the size used in previous studies at BaoSteel and Wisco Steel). 5). Runner sampling Runner sampling is shown below, both the refractory erosion and inclusions in steel in runner will be analyzed. We recommend several samples at different places along the runners. Examples of the runner steel sample, inclusion, and the interface between steel and runner refractory are also shown below [3].

Schematic of runner sampling An example of runner sample

Al2O3 inclusions in a runner steel sample


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Interface between steel and runner lining refractory

6). Finally, temperature measurements during the process are needed to determine superheat

during solidification for the solidification modeling. References: 1. K.P. Hughes, C.T. Schade and M.A. Shepherd, "Improvement in the Internal Quality of

Continuously Cast Slabs at Lukens Steel," I & Smaker, Vol. 22 (6), 1995, 35-41. 2. L. Zhang and B.G. Thomas, "State of the Art in Evaluation and Control of Steel

Cleanliness," ISIJ Inter., Vol. 43 (3), 2003, in press. 3. X. Zhang and K. Cai, "Project Report: Investigation of Inclusion Behavior of 16MnR

Steel at WISCO," Report, 1996.

Needle configuration: Matrix:

Average:

Solidified steel

Solidified steel

Interface

Lining

Documents

REPORT on STEEL INGOT CASTING INDUSTRY …...2003/03/07 · At some steel plant, T.O. increases from 20ppm at ladle to 40ppm at final product, which indicates serious contamination