GB1596819A - Method of prolonging durable life of aod furnace refractory linings - Google Patents

Description

(54) METHOD OF PROLONGING DURABLE LIFE OF AOD FURNACE REFRACTORY LININGS (71) We, QUIGLEY COMPANY, INC. a Corporation organized and existing under the laws of the State of New York, United States of America, of 235 East 42nd Street, New York, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is a method for prolonging the durable life of the refractory lining of an AOD furnace, a type of bottom-blowing converter for steel making, by effectively reducing its loss on fusing.More specifically, the invention relates to a method for increasing the durability of a magnesia refractory lining of an AOD furnace by adjusting the concentration of magnesium oxide in the slag during the operation of the furnace, thereby reducing the dissolution of the magnesium oxide of the lining into the slag.

Generally, steel-making furnaces are constructed such that an iron container is lined with a refractory lining material, and molten iron is placed in it to produce steel. The lining is replaced when it is markedly lost by the effect of the molten metal and the slag.

Replacement of the lining is performed by stopping the operation for a fairly long period of time, and the cost and labor for the replacement operation are enormous. Moreover, steel cannot be produced during the replacement period, which is a great disadvantage.

The AOD furnace is usually lined with a magnesia-type refractory lining material, which is a basic refractory. The furnace is adapted to strongly remove impurities such as carbon from molten steel by blowing a gas such as oxygen, argon or nitrogen into the molten steel from the bottom of the furnace. In steel making by the AOD furnace, reactions such as decarburization are especially vigorous, and the heat generated by the reaction makes the temperature of the molten steel in the furnace extremely high. This results in the marked fusing loss of the refractory lining of the furnace, particularly its inside wall corresponding to the slag line at the steel bath surface. The furnace whose refractory lining has been markedly lost by fusing must be relined.According to the conventional technique, spray repairing is, for example, performed in order to increase the durability of the lining, and the lining material is replaced after the furnace has been operated through about 80 to 150 cycles.

In general steel-making furnaces, various methods such as spray repairing are used to increase the durability of the lining. As one of such methods, it is known from Japanese Patent Publication No. 12327/67 (Patent No.

505,698), corresponding to U.S. Patent No.

3,288,592, that the durable life of a magnesiatype lining material in the operation of a furnace lined with it is prolonged by adding a slag-forming agent such as active magnesium oxide or active dolomitic lime to adjust the concentration of magnesium oxide in the slag to 4 to 12 percent by weight and thereby inhibit the dissolution of the magnesium oxide of the lining material into the slag. However, since the AOD furnace is operated at a higher temperature than in ordinary steel-making operations, the inhibiting effect is not so appreciable when the concentration of magnesium oxide in the slag is 12 percent by weight or less.

In ordinary steel-making operations, the flowability of the slag becomes poor and refining is difficult when the concentration of magnesium oxide in the slag exceeds 12 percent by weight. Furthermore, a slag in a converter process has a total iron content of as high as about 17 to 23 percent by weight and a low magnesium oxide content.

The present invention is a method for prolonging the durable life of the refractory lining of an AOD furnace characterized in that in the operation of an AOD furnace lined with a magnesia-type refractory lining, a substance containing magnesium oxide is added to adjust the concentration of magnesium oxide in the slag to 13 to 25 percent by weight to reduce the dissolution of the magnesium oxide of the lining material into the slag.

The drawing is a longitudinal sectional view schematically showing the structure of an AOD furnace.

The magnesium oxide-containing substance used in this invention includes, for example, magnesia (magnesium oxide), magnesium carbonate, dolomite, lightly calcined dolomite obtained by burning dolomite to release carbon dioxide, dolomite clinker, magnesia-dolomite brick scraps, talc and forsterite. Better results are obtained by using these magnesium oxidecontaining substances together with quicklime or other substances as the slag-forming agent, or by molding such a combination together with a binder.

The concentration of magnesium oxide in the slag is adjusted to 13 to 25 percent by weight by adding the magnesium oxide-containing substance. If the concentration is less than 13 percent by weight, the effect of inhibiting the dissolution of magnesium oxide present in the refractory lining into the slag is small, and if it exceeds 25 percent by weight, the flowability of the slag becomes extremely poor and refining becomes difficult.

To illustrate the present invention, an AOD furnace having a capacity of 100 metric tons of molten steel was lined with magnesia-dolomite bricks. The running tests were performed using two kinds of stainless steel, SUS 304 and SUS 430, and the slag-forming agents shown in Table 1. The indicated amounts of the slagforming agents were standard amounts for a charge. About 2 metric tons of the magnesium oxide-containing substance was fed in the early stage of blowing the reaction gas, with the remainder of the substance and the quicklime, in about 2 metric ton increments, being fed subsequently. Deoxidization was performed using a silicon alloy.

Table 1 Slag-forming Agent Kg Added Magnesia Lightly dolomite Calcined Brick Run Quicklime Dolomite Dolomite Scraps 1 8,000 - - - 2 6,500 1,500 - 1,000 3 4,500 - 5,000 4 5,500 - - 3,500 Each run was performed through one lifetime cycle of the furnace, i.e., in the same furnace until the lining was lost by fusion and replaced by a new one.

Table 2 Experimental Results Number Slag Composition, of Cycles Average Weight Percent in the Run Furnace MgO CaO SiO2 A12O3 1 93 6.12 51.87 33.16 4.19 2 139 10.85 49.31 31.85 4.75 3 295 13.86 46.70 32.27 2.28 4 352 20.14 41.86 30.35 3.17 Experimental results from the testing are shown in Table 2, in which the indicated number of cycles in the furnace is the number of charges which were fed during operation through one lifetime cycle of the furnace.

As shown in Table 2, the number of cycles in the furnace strikingly increased when the concentration of magnesium oxide in the slag reached at least 13 percent by weight.

In Run 1, a magnesium oxide-containing substance was not added. The magnesium oxide in the slag thus resulted from the dissolving of magnesium oxide contained in the refractory lining material of the furnace and the sprayed material. In Run 2, the magnesium oxide concentration of the slag was about 11 percent by weight; the number of cycles in the furnace increased in this case, but the effect was not so appreciable. When the magnesium oxide concentration of the slag exceeded 25 percent by weight, the flowability of the slag was poor and refining was difficult.

As stated hereinabove, the present invention consists in markedly reducing the dissolution of magnesium oxide of a magnesia-type refractory lining material, especially of the slag line portion, into the slag and thereby strikingly increasing the durability of an AOD furnace, by adding a magnesium oxide-containing substance to adjust the concentration of magnesium oxide in the slag to 13 to 25 percent by weight in the operation of an AOD furnace lined with the refractory lining. This greatly contributes to steel making in AOD furnaces.

WHAT WE CLAIM IS: 1. A method for prolonging the durable life of the lining of an AOD furnace lines with a magnesia-type refractory, which comprises adding during the steel-making cycle of said furnace sufficient magnesium oxide-containing substance to adjust the magnesium oxide content of the slag formed during said cycle to from 13 to 25 percent by weight of said slag.

2. A method according to claim 1, wherein said substance is magnesia, dolomite, lightly calcined dolomite or magnesia-dolomite brick scraps.

3. A method according to claim 1 or 2, wherein said substance is added together with quicklime.

4. A method according to claim 3, wherein said substance and said quicklime are molded together with a binder.

5. A method as claimed in claim 1 for prolonging the durable life of the lining of an AOD furnace lined with a magnesia-type refractory, substantially as herein described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. obtained by burning dolomite to release carbon dioxide, dolomite clinker, magnesia-dolomite brick scraps, talc and forsterite. Better results are obtained by using these magnesium oxidecontaining substances together with quicklime or other substances as the slag-forming agent, or by molding such a combination together with a binder. The concentration of magnesium oxide in the slag is adjusted to 13 to 25 percent by weight by adding the magnesium oxide-containing substance. If the concentration is less than 13 percent by weight, the effect of inhibiting the dissolution of magnesium oxide present in the refractory lining into the slag is small, and if it exceeds 25 percent by weight, the flowability of the slag becomes extremely poor and refining becomes difficult. To illustrate the present invention, an AOD furnace having a capacity of 100 metric tons of molten steel was lined with magnesia-dolomite bricks. The running tests were performed using two kinds of stainless steel, SUS 304 and SUS 430, and the slag-forming agents shown in Table 1. The indicated amounts of the slagforming agents were standard amounts for a charge. About 2 metric tons of the magnesium oxide-containing substance was fed in the early stage of blowing the reaction gas, with the remainder of the substance and the quicklime, in about 2 metric ton increments, being fed subsequently. Deoxidization was performed using a silicon alloy. Table 1 Slag-forming Agent Kg Added Magnesia Lightly dolomite Calcined Brick Run Quicklime Dolomite Dolomite Scraps 1 8,000 - - - 2 6,500 1,500 - 1,000 3 4,500 - 5,000 4 5,500 - - 3,500 Each run was performed through one lifetime cycle of the furnace, i.e., in the same furnace until the lining was lost by fusion and replaced by a new one. Table 2 Experimental Results Number Slag Composition, of Cycles Average Weight Percent in the Run Furnace MgO CaO SiO2 A12O3

1 93 6.12 51.87 33.16 4.19

2 139 10.85 49.31 31.85 4.75

3 295 13.86 46.70 32.27 2.28

4 352 20.14 41.86 30.35 3.17 Experimental results from the testing are shown in Table 2, in which the indicated number of cycles in the furnace is the number of charges which were fed during operation through one lifetime cycle of the furnace.

As shown in Table 2, the number of cycles in the furnace strikingly increased when the concentration of magnesium oxide in the slag reached at least 13 percent by weight.

In Run 1, a magnesium oxide-containing substance was not added. The magnesium oxide in the slag thus resulted from the dissolving of magnesium oxide contained in the refractory lining material of the furnace and the sprayed material. In Run 2, the magnesium oxide concentration of the slag was about 11 percent by weight; the number of cycles in the furnace increased in this case, but the effect was not so appreciable. When the magnesium oxide concentration of the slag exceeded 25 percent by weight, the flowability of the slag was poor and refining was difficult.

As stated hereinabove, the present invention consists in markedly reducing the dissolution of magnesium oxide of a magnesia-type refractory lining material, especially of the slag line portion, into the slag and thereby strikingly increasing the durability of an AOD furnace, by adding a magnesium oxide-containing substance to adjust the concentration of magnesium oxide in the slag to 13 to 25 percent by weight in the operation of an AOD furnace lined with the refractory lining. This greatly contributes to steel making in AOD furnaces.

WHAT WE CLAIM IS: 1. A method for prolonging the durable life of the lining of an AOD furnace lines with a magnesia-type refractory, which comprises adding during the steel-making cycle of said furnace sufficient magnesium oxide-containing substance to adjust the magnesium oxide content of the slag formed during said cycle to from 13 to 25 percent by weight of said slag.

2. A method according to claim 1, wherein said substance is magnesia, dolomite, lightly calcined dolomite or magnesia-dolomite brick scraps.

3. A method according to claim 1 or 2, wherein said substance is added together with quicklime.

4. A method according to claim 3, wherein said substance and said quicklime are molded together with a binder.

5. A method as claimed in claim 1 for prolonging the durable life of the lining of an AOD furnace lined with a magnesia-type refractory, substantially as herein described.