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The degradation mechanism of coatings in MgO-C of the steel pockets
ˡTRAIAIANoura;²SNANI Louafi
ˡLaboratoire of Working of Metallic materials ² Laboratoire of genius of materials Email:traiaianoura@yahoo.fr ; snanilaoufi@yahoo.fr
Metallurgy department and genius of the materials university BADJI Mokhtar-Annaba, B.P.
12, Annaba 23000 - Algeria.
Abstract:
Our work has as an aim the study of the influence of the impregnation, by the slag and the molten metal, on the degradation of the refractory brick linings in MgO-C of the steel pockets of Arcelor-Mittal Annaba.
Introduction:
The refractory material is the major component of heavy industries, such as iron, steel, glass or cement. Due to its high volume of production, but also its constraints to the industrial process, the steel industry is the largest consumer of refractories. The growing demand for steel production increasingly pure and very precise compositions requires the development of high performance refractories. For fifteen years, remarkable advances techniques in
monolithic refractories, competing bricks, have improved the inclusion quality of the steel, the reliability of refractory masonry and extend the life of equipment. Therefore, the trend is a decrease in consumption of refractories per ton of steel produced. The monolithic solution for coating steel has pockets the advantage of being reliable, fast and easy implementation.
Experimentation:
Study metallographic a used brick:
To better understand the mechanisms of degradation of the refractory bricks of the ladle, a closer observation of the refractory damage was done. For this purpose, several pieces of bricks, which had been recovered after the steelworks shake of used refractories were cut and observed.
This observation on photos taken by a high resolution camera aims to see areas that are damaged by slag and molten metal.
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Fig 1: Micrographic results observed areas
Corrosion test:
Corrosion test (self-impregnation crucible)
The observation results showed a pronounced wear, refractories at the slag cord to study this factor, in the laboratory simulations can be conducted using the following protocol
Procedure:
The auto-crucible is formed by coring a hole of 30 mm diameter and 40 mm high in a material pad to be tested 115 x 115 mm2 cross section and 76 mm in height. The hole is filled with a slag of which the composition is representative of the intended application and the whole is stuffs for several hours to a temperature at least equal to the melting point of the slag 1200 ° C). On leaving the oven, a visual check eliminates refractory shades too easily
permeated and corroded and those who have developed significant cracking. This procedure does not allow studying the corrosion over a long period because the amount of milk is so low that, often, it is still more at the end of the test. In addition, this summary does not reproduce experimental conditions existing flow service, including the thermal gradient in the thickness of the wall, the sequential dairy intake and fluid movements. On the other hand, the samples are small enough that the temperature can be considered homogeneous, making it a try quite suitable for the study of microstructural changes related only to the impregnation
A self-crucible has been conducted to better understand the mechanisms bricks impregnation by liquid oxides complementing visual observations by composition measurements and thermochemical calculations.
The methodology and results were then guided the analyzes of a brick levied on a handheld dismantling.
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Fig 2-a: Half crucible after impregnation /2-b: Wear a bubbling lance at the slag
Test of chemical analysis:
This test is intended to determine the chemical composition of different areas of a used refractory brick.
Chemical analysis by XRF:
The purpose of the test:
Chemical analysis is an operation very delicate, we use all means dosage, gravimétrie, volumétrie, colorimétrie, flame photometry etc. to find results comparables, must be employed strictly defined procedures.
Procedure:
• It makes grinding a sample of a refractory brick by a ball mill at the PMA (Mittal Steel) wh0.5 g was mixed with the refractory powder 5g flux and a balance is used to measure the quantities;
• We mix in a graphite crucible or platinum;
• We put the crucible in a furnace at a temperature of 1200 ° C for 10 min;
• After the powder fuses, powder saucer is added in the basket for a pod heated in the oven for a minute or two and the fused powder is poured into the car and cooled the sample to the free area ;
• After cooling there is a round and smooth sample;
• The sample is going on in the machine XRF and we see the results of chemical analysis in the computer that is connected with the thing .en we find a very fine powder;
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Elements top middle low
MgO 69,158% 70,109% 78,158%
CaO 4,209% 3,549% 3,087%
SiO2 3,567% 2,760% 2,334%
Fe 0,605% 0,505% 0,069%
Al2O3 3,381% 2,865% 2,105%
MnO 0,119% 0,09% 0,059%
ZnO 0,012% 0,01% 0,007%
P2O5 0,148% 0,105% 0,065%
TiO2 0,344% 0,163% 0,132%
C 4,043% 6,972% 8,453%
Results and discussion:
1/ The surface of the outer layer (in contact with the slag and the molten metal) of the brick is very uneven and corroded. A thin layer (<1 mm) is encrusted slag. In the first few millimeters of depth, there is a dense network of microcracks perpendicular and parallel to the hot face.
The area is steeped in the dairy and is partially corroded. The grains are not distinguishable from the matrix.
A few centimeters of the surface, parallel cracks are observed in the hot face and metal-filled pores. Areas near the cracks are corroded. 3mm beyond the lips of the crack, the material appears healthy rather the grains are more visible. Finally a very dense network of macrocracks is formed in the rest of the thickness of the brick. Some are filled with liquid metal. Except for the cracks edges the microstructure of the material does not appear to be modified by the slag.
In all cases, the damage method seems to follow the following schedule:
- Embedding of the surface by the slag and metal residues; corroded area saturated in the first millimeters; presence of microcracks and metal infiltration;
- Corroded area in the following centimeters; presence of some macrocracks;
- Safe area beyond the first few centimeters.
Of macrocracks, even present in the healthy zone, parallel to the surface of the slab in contact with the steel bath, were responsible for the breakdown of the samples. It is however not possible to know if these cracks were produced during cooling of the slab or appeared when the refractory lining was demasonned. (The history of the coating has not been communicated by us steelmakers
It is now seeking to correlate the different types of damage observed at identified stress.
To compare the chemical composition of slag from the converter to that of the pocket, it will present the chemical composition of the slag and the chemical composition of the waste bricks.
5 2/ From the Figure 1-b it is noted that the wear of nozzles in the region of the slag, is very important. Indeed, with a diameter of 220mm in the new state, we move to 120mm of the corroded area of the lance and place the refractory coating has completely disappeared to let appear the metal tube used to blow argon. Shows the part which would be in direct contact with the liquid metal, we see that the wear in this part is not quite large compared to the slag zone.
Because the diameter of the spear in this section is to be 190mm 30mm wear.
This test gives us a very clear idea about the influence the aggressiveness of slag on the wear of refractory linings.
3/ The results of the zone of the slag bead show that the percentage of MgO decreases very rapidly 69.15% with the increase of the other oxides of the slag such as CaO, SiO2, Al2O3, Fe2O3, P2O5, Fe. With the decrease of the residual carbon percentage of residual carbon which is 14% to about (4%) in used bricks.
The results of the zone of the wall show that the percentage of magnesia falls relative to the base composition to (70.109%) over about a rather significant decrease in the residual carbon (6%) approximately, with the increase of the other oxides but less low compared to the results of the contact area with the slag.
The sample results the bottom of the wall show that magnesia percentages decrease to
(78.15%) with about increasing the percentage of other oxides over a relatively small decrease in the residual carbon (8%) about, but this change is quite low compared to previous results.
The results show that the percentage of magnesia falls in relation to the chemical
composition of the new brick, which implies that the rate of wear is very large and a large proportion of refractory bricks dissociates into the metal bath with an impregnation very important slag and the liquid metal into the pores of the brick.
Conclusion:
This study allowed us to see the change in the chemical composition and the corrosion of the refractory bricks used for ladle.
The influence of the slag and the liquid metal is very important for the change in the chemical composition of the waste bricks.
In the dairy cord change is very large compared to other coating areas because of the high impregnation and aggressiveness of slag from the liquid metal.
Change in structure due to the impregnation of the slag and the liquid metal into the brick which causes degradation
Key words: Refractory, pocket with steel, impregnation, slag.
Bibliographical references:
1 Eric BLONDDégradation thermomécanique des réfractaires au contact de laitiers sidérurgiques. Thesis of doctorate2003.
2 Eléonore ArfanTenue thermomécanique des dalles d'impact des poches à acier en céramiques réfractaires. Thesis of doctorate2012.
