JPH075379B2 - Method for manufacturing refractory for molten steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a refractory material for molten steel, and more particularly, a melt loss resistance and heat resistance particularly desirable for use as a break ring or nozzle for continuous casting. The present invention relates to a method for manufacturing a refractory material for molten steel having impact properties.

[Conventional technology]

Conventionally, refractory for molten steel used in the steel industry, for example, boron nitride (BN) for break rings and nozzles for horizontal continuous casting
Sintered bodies and silicon nitride (Si ₃ N ₄ ) sintered bodies have been used. However, the former has a drawback that it is excellent in thermal shock resistance but inferior in abrasion resistance, while the latter is excellent in abrasion resistance but inferior in thermal shock resistance.

For this reason, efforts are being made to combine BN and Si ₃ N ₄ to fill the shortcomings of both. However, Si ₃ N ₄ has a fundamental defect in the refractory for molten steel that when it is immersed in molten steel, it chemically reacts with the molten steel or the impurity components contained in the molten steel to cause melting loss.

Therefore, recently, a refractory has been proposed in which aluminum nitride (AlN) and aluminum oxide (Al ₂ O ₃ ), which are chemically stable against molten steel, are further compounded to improve the melt damage resistance (for example, Japanese Patent Laid-Open Publication No. 2000-242242). JP-A-56-129666, JP-A-60-51669
Issue). However, even in these refractories, Si
_{Since it} contains ₃ N _{4 in an amount} of 10% by weight or more, it has not yet been sufficiently improved in erosion resistance.

On the other hand, it is known that a sintered body obtained by adding yttrium oxide (Y ₂ O ₃ ) to AlN can be used as a high thermal conductivity substrate or a high temperature structural material, but since this sintered body has poor thermal shock resistance, In fact, it cannot be used as a refractory for molten steel.

[Problems to be Solved by the Invention]

Generally, a refractory for molten steel is used in a state of being immersed in molten steel and being stationary or moving in the molten steel, or fixed such as a break ring or a nozzle, and used in a state in which the molten steel flows. Therefore, the refractory for molten steel is not subjected to particularly large mechanical stress or wear force such as sliding between solids, so Si ₃ N ₄ -BN system, Si ₃ N _{4
-BN-AlN-based, Si 3 N 4 -BN-AlN} -Al 2 O 3 system in obtained such high strength and high hardness (wear resistance) is not required. Rather, it is excellent in melting resistance and thermal shock resistance from the viewpoint of preventing dimensional change of metal materials obtained by break rings, nozzles, etc., extending the life of refractory for molten steel, and preventing destruction of refractory due to thermal shock. is important.

Therefore, in the steel industry and the like, there is a strong demand for the development of refractory materials for molten steel that are more excellent in melt damage resistance and thermal shock resistance.

The present inventors have variously studied molten steel refractory not containing Si ₃ N ₄ suitable for applications such as break rings and nozzles for continuous casting, and instead of using Si ₃ N ₄ powder as a starting material, If BN powder, AlN powder and Y ₂ O ₃ powder are made to have a specific ratio and the specific surface area of BN powder is 30 m ² / g or more, it has excellent erosion resistance and is suitable for use as a refractory for molten steel. It has been found that a refractory for molten steel having sufficient strength, abrasion resistance and thermal shock resistance can be produced, and the present invention has been completed.

[Means for Solving the Problems]

That is, the present invention, a specific surface area of 30 m ² / g or more hexagonal BN powder 20 to 70 parts by weight, AlN powder 30 to 80 parts by weight and Y ₂ O ₃ powder 0.1.
After molding the mixed powder containing ~ 8 parts by weight at a pressure of 1000 kg / cm ² or more in a non-oxidizing atmosphere 16
A method for producing a refractory for molten steel, characterized by firing at a temperature of 00 to 2100 ° C.

Hereinafter, the present invention will be described in more detail.

First, the raw material powder used in the present invention will be described. The hexagonal BN powder may be a commercially available one, but its specific surface area is 30 m ²
It must be at least / g. This is one of the major features of the present invention. When the specific surface area is less than 30 m ² / g, the bond strength between particles of the refractory material is reduced, and the strength and the melt damage resistance are reduced.

A particularly preferable hexagonal BN powder is a hexagonal BN powder having a high crystallinity and a specific surface area of 30 m ² / g or more obtained by pulverizing the hexagonal BN powder with an ordinary pulverizer such as an attritor, a ball mill, a vibrating ball mill, and a Raiki machine. Crystalline BN powder. This is because the hexagonal BN powder having high crystallinity is excellent in plastic deformation during preforming, and a high-density preform can be easily obtained.

AlN powder and Y ₂ O ₃ powder may be commercially available products, but both have a purity of 98
% Or more and an average particle diameter of 4 μm or less is desirable. Note that
As the particle size of AlN is equal to or smaller than that of hexagonal BN powder, the density of the sintered body is improved, and as a result, the melt damage resistance is improved.

In the present invention, the blending ratio of the raw material powder is important.

Hexagonal BN powder is 20 to 70 parts by weight. If it is less than 20 parts by weight, the thermal shock resistance of the refractory for molten steel is lowered, while if it exceeds 70 parts by weight, the melt damage resistance to molten steel is lowered and the strength and wear resistance of the refractory are lowered.

AlN powder is 30-80 parts by weight. If it is less than 30 parts by weight, the melting resistance of the refractory for molten steel is lowered, while if it exceeds 80 parts by weight, the thermal shock resistance is lowered.

Y ₂ O ₃ powder is 0.1 to 8 parts by weight. If it is less than 0.1 part by weight, the bond strength between the particles of the refractory material for molten steel becomes weak and sufficient strength cannot be obtained. Moreover, since the density also decreases, the melting resistance decreases. On the other hand, if it exceeds 8 parts by weight, the liquid phase will flow out during sintering, and the melting resistance will be reduced.

Especially when using this refractory for molten steel for break ring and nozzle applications, hexagonal BN powder 40 to 70 parts by weight, AlN powder 3
0 to 60 parts by weight and _{2 to} 4 parts by weight of Y ₂ O ₃ powder are desirable.

Next, the raw material powders are blended in a predetermined ratio and uniformly mixed using a mixer, and the mixed powder is fired by a conventional method such as a hot press method or a normal pressure sintering method.

The molding pressure may be 1000 kg / cm ² or more, preferably 5000
It is more than kg / cm ² . If the pressure is less than 1000 kg / cm ² , the density of the molded product cannot be sufficiently increased, and the strength will be reduced, making it difficult to perform precision processing of the molded product. Only refractories can be obtained. When such a refractory material is immersed in the molten steel, the molten steel easily penetrates into the refractory material through the open pores, so that the melt loss resistance is lowered. A mold molding machine and a cold isostatic molding machine are used for molding the powder.

Firing is performed at a temperature of 1600 to 2100 ° C in a non-oxidizing atmosphere. If the firing temperature is less than 1600 ℃, hexagonal BN particles, AlN particles, Y ₂ O
₃ The bond strength of the particles becomes insufficient and the strength of the refractory material decreases. Also, when the temperature exceeds 2100 ° C, AlN undergoes thermal decomposition and loses its original properties. In particular, in order to obtain a sintered body having excellent melting resistance and high strength, it is preferable to set the firing temperature to 1750 to 2050 ° C.

The firing atmosphere may be vacuum, in an inert gas such as He or Ar, or a non-oxidizing atmosphere selected from H ₂ , N ₂ , NH ₃ gas, etc., but especially AlN, hexagonal BN An N ₂ atmosphere that has the effect of suppressing the decomposition of is desirable. Note that when firing in an oxidizing atmosphere, B ₂ O ₃ is generated in the refractory due to the oxidation of BN, so that the melting resistance, thermal shock resistance, and high temperature strength deteriorate. As the firing device, a Tammann furnace, a high frequency furnace, or a resistance heating furnace is used.

Regarding the density of the refractory for molten steel, the relative density is preferably 70% or more. When the relative density is less than 70%, when the refractory material is immersed in the molten steel, the molten steel easily enters the refractory material through the open pores, so that the melt loss resistance, wear resistance and strength decrease. The relative density can be changed, for example, by the molding pressure.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Example 1 Hexagonal BN powder (manufactured by Denki Kagaku Kogyo KK, grade SP-
1, hexagonal crystal, purity 96%, specific surface area 50m ² / g) 50 parts by weight of Al
N powder (manufactured by Denki Kagaku Kogyo, grade AP-10, purity 9
9%, average particle diameter 2 μm) 47 parts by weight, Y ₂ O ₃ powder (manufactured by Santoku Metal Industry Co., Ltd., high purity yttrium oxide purity 99.9%,
3 parts by weight of an average particle diameter of 4 μm) was added and mixed by a vibrating ball mill to obtain a mixed powder for molding. Next, this mixed powder was cold isostatically pressed at a pressure of 1000 kg / cm ² . The obtained molded body was embedded in a graphite container containing BN powder and fired in a high-frequency furnace at 2000 ° C. for 60 minutes under N ₂ atmosphere. The relative density, bending strength, shear hardness, thermal shock resistance, and amount of melting loss with respect to molten steel of the obtained sintered body were measured. The results are shown in the table.

Example 2 The procedure of Example 1 was repeated except that the molding pressure was 5000 kg / cm ² .

Examples 3 to 5 The same procedure as in Example 1 was carried out except that the raw material powder used in Example 1 was used and the compounding composition shown in the table was changed.

Example 6 Hexagonal BN powder (manufactured by Denki Kagaku Kogyo KK, grade GP, hexagonal, purity 99%, specific surface area 6 m ² / g) was pulverized using an attritor mill until the specific surface area reached 85 m ² / g. Then, hexagonal BN fine powder was obtained. The specific surface area was measured by the BET method. This powder 50
47 parts by weight of the AlN powder used in Example 1 and 3 parts by weight of the Y ₂ O ₃ powder were added and mixed in parts by weight to obtain a raw material powder for molding. After molding this molding raw material powder in the same manner as in Example 1, 2000
Firing was performed at a temperature of ° C for 60 minutes in a N ₂ gas atmosphere.

Example 7 The procedure of Example 6 was repeated except that the firing temperature was set to 1600 ° C.

Example 8 The procedure of Example 6 was repeated except that hexagonal BN powder pulverized to a specific surface area of 35 m ² / g was used.

Comparative Example 1 The procedure of Example 6 was repeated, except that the hexagonal BN powder was used without pulverization.

Comparative Example 2 The procedure of Example 6 was repeated except that the molding pressure was 500 kg / cm ² .

Comparative Examples 3 to 6 Comparative Example 3 was carried out in the same manner as in Example 6 except that the hexagonal BN powder, AlN powder and Y ₂ O ₃ powder were changed to the compounding compositions shown in the table.

Comparative Example 7 The procedure of Example 6 was repeated except that the firing temperature was set to 2200 ° C.

Comparative Example 8 The procedure of Example 6 was repeated except that the firing temperature was 1500 ° C.

Comparative Example 9 The procedure of Example 6 was repeated except that hexagonal BN powder pulverized to a specific surface area of 25 m ² / g was used.

Comparative Example 10 AlN powder 30 was added to 20 parts by weight of the hexagonal BN powder used in Example 1.
20 parts by weight of Al ₂ O ₃ powder (purity 99%, specific surface area 8 m ² / g), and further Si ₃ N ₄ powder (manufactured by Denki Kagaku Kogyo KK, SN-9FW, purity 97%) as a binder. Specific surface area 12 m ² / g) 30
After adding parts by weight and mixing in a vibrating ball mill, this mixed powder was subjected to cold isostatic pressing at a molding pressure of 3000 kg / cm ² . The obtained molded body was fired at 1800 ° C. for 2 hours under N ₂ gas atmosphere.

Comparative Example 11 The same method as in Comparative Example 10 was carried out except that a mixed powder of 30 parts by weight of hexagonal BN powder and 70 parts by weight of Si ₃ N ₄ powder was used.

Each physical property described in the table was measured by the following methods.

(1) Relative density: The volume was obtained from the dimensions of the sintered body, and the density was obtained from the weight, and then the relative density (%) = density (g / cm ³ ).
/ Theoretical density (g / cm ³ ) × 100.

(2) Bending strength at room temperature ... Compliant with JIS R1601.

(3) Shear hardness: in accordance with JIS Z2246.

(4) Thermal shock resistance: After the sample cut out from the sintered body is heated and held at a constant temperature, this sample is rapidly cooled in 20 ° C water and subjected to a rapid shock to measure the bending strength of the sample after the rapid cooling. Then, the critical temperature difference ΔT at which a sharp decrease in bending strength begins to occur was determined as the thermal shock resistance.

(5) Amount of melting loss ... SUS304, SS41 was melted in a MgO crucible at a temperature of 1550 ° C, and the melt was cut out from the sintered body 7 × 20
A sample of × 40 mm was immersed for 30 minutes, and the dimensions of the sample after immersion were measured and calculated by the following formula.

W ₁ : Width of sample before immersion (mm)… 20 mm W ₂ : Width of sample after immersion (mm)… Measurement after immersion T ₁ : Thickness of sample before immersion (mm)… 7 mm T ₂ : Thickness of sample after immersion Length (mm)… Measurement after immersion L: Immersion length (mm)… 30 mm θ: Immersion time (Hr)… 0.5 Hr [Effects of the Invention] The refractory for molten steel obtained by the production method of the present invention has remarkably excellent erosion resistance and thermal shock resistance, and yet has appropriate bending strength and shear hardness (wear resistance). Since it is provided, it is particularly suitable for applications such as break rings for continuous casting and nozzles.

Claims (1)

[Claims] 1. A mixed powder containing 20 to 70 parts by weight of hexagonal boron nitride powder having a specific surface area of 30 m ² / g or more, 30 to 80 parts by weight of aluminum nitride powder and 0.1 to 8 parts by weight of yttrium oxide powder, and 1000 Kg A method for producing a refractory material for molten steel, which comprises firing at a temperature of 1600 to 2100 ° C. in a non-oxidizing atmosphere after or while molding at a pressure of / cm ² or more.