JPS624353B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a casting nozzle for casting steel, and more particularly to a casting nozzle mainly made of silicon nitride.

As a nozzle for casting steel, for example, a tundish nozzle is an important member for adjusting the amount of molten steel flowing from the tundish to the mold, and is a key point in casting technology. In other words, the nozzle that connects the tundish and the mold, together with the effect of the heat-insulating powder inside the mold, not only prevents oxidation of the molten steel and retains its heat, but also prevents the molten steel flow from being disturbed and slag from getting involved during pouring, thereby improving the quality of the cast product. to make it good.

The casting efficiency in casting steel etc. is mainly determined by the life of the casting nozzle, and in particular, it is a necessary condition to control the amount of molten steel constant that the nozzle diameter does not change over a long period of time. It is necessary to use refractory material. Refractories for nozzles must have excellent corrosion resistance and thermal shock resistance, be dense, and have low thermal conductivity. Conventionally, refractories made of high silicic acid, zirconia, or zircon have been used for casting nozzles. It's here.

However, high silicic acid products are relatively soft and have poor corrosion resistance, and when used as immersion nozzles in particular, the parts that come into contact with molten steel are severely corroded, causing problems such as melting damage and enlargement of the nozzle diameter. Furthermore, although zirconia and zircon materials are excellent against corrosion damage, they have the problem of adhesion of aluminum and its oxides mixed in molten steel, causing the nozzle diameter to shrink and become clogged. .

In addition, fused silica, high alumina, and graphite materials have been developed, but fused silica materials tend to clog the nozzle due to alumina adhesion and have a manganese content of 1.2 Melting loss occurred when the molten steel was as high as ~1.5% by weight, which was not satisfactory. In addition, high alumina materials have poor thermal shock resistance, and also have the disadvantage of reacting with oxides of manganese, iron, aluminum, etc. in molten steel to form an altered layer, which then peels off. be. In addition, graphite has poor corrosion resistance and high thermal conductivity, which causes a drop in nozzle temperature, which, together with the adhesion of oxides, tends to cause nozzle clogging.

Silicon nitride has corrosion resistance against various molten metals,
It has excellent thermal shock resistance, is difficult to adhere to metals and their oxides, and has lower thermal conductivity than graphite or high alumina, so it has good heat retention properties.In order to use it as a casting nozzle, There are examples of sintering and molding with a large amount of sintered material such as quartz added.
Since the porosity was about 20%, there was a problem that molten steel could seep into it, and the compressive strength was low at 1000 kg/cm ² at most.

The present inventor has conducted various studies on sintering accelerators for silicon nitride, and found that if magnesium oxide and lanthanum group oxides are added and sintered, a dense and high-strength molded body can be obtained. The present invention was completed based on the knowledge that excellent corrosion resistance and thermal shock resistance are not impaired.

The object of the present invention is to provide a casting nozzle that does not have the drawbacks of the conventional ones, and uses a powder mixture containing silicon nitride as a main component and magnesium oxide and lanthanum group oxides as sintering accelerators. This is a casting nozzle that is preformed and sintered using a conventional method, so that it does not suffer from corrosion, erosion, or blockage.

In the casting nozzle of the present invention, a powder mixture containing 70 to 99% by weight of silicon nitride and the balance of magnesium oxide and lanthanum group oxides each of 0.5% by weight or more is processed by an isostatic pressing method, a slurry casting method, etc. 1600~ in non-oxidizing atmosphere after preforming
Obtained by normal pressure sintering at 1800℃.

The reasons for these limitations will be explained below.

If the silicon nitride content is less than 70% by weight in the powder mixture formulation, the corrosion resistance and thermal shock resistance of silicon nitride will be significantly impaired due to too much sintering accelerator, leading to enlargement of the nozzle diameter and melting damage. If the amount exceeds 99% by weight, the amount of sintering accelerator is too small, making it impossible to obtain a dense molded body with an apparent porosity of 1.5% or less. Furthermore, if either the sintering accelerator, magnesium oxide or lanthanum group oxide, is less than 0.5% by weight, the thermal shock resistance will sharply decrease. It should be noted that magnesium oxide and lanthanum group oxides, which are sintering accelerators, do not necessarily need to be added as oxides from the time of blending the raw materials.
It may be added in the form of carbonate or nitrate, which converts into oxide during the sintering process.

Further, even if one or more kinds of lanthanum group oxides are selected and used, the effect of promoting sintering does not change.

Next, methods for making a preform in the shape of a casting nozzle from this powder mixture include the isostatic press method and the slurry casting method. For example, in the case of the isostatic press method, the powder mixture is
After adding water and an organic binder such as CMC, methylcellulose, or poval and thoroughly kneading the mixture, fill a rubber mold with the kneaded product or its granules, and press it into a rubber mold using an isostatic press. By applying hydrostatic pressure, a preform with a uniform texture is obtained.

When sintering the preform, the sintering atmosphere should be a non-oxidizing atmosphere such as nitrogen or argon to prevent oxidation of silicon nitride, and the sintering temperature should be adjusted to ensure rapid sintering progress. It is necessary to keep the temperature above 1600°C and below 1800°C to prevent thermal decomposition of silicon nitride.

Hereinafter, the present invention will be explained according to examples.

Example 1 A powder mixture consisting of 70% by weight of silicon nitride, 15% by weight of magnesium oxide, and 15% by weight of cerium oxide
An appropriate amount of a 1.5% CMC aqueous solution was added, mixed well, granulated, filled into a rubber mold of the desired shape, and molded using an isostatic press method. After drying, it was heated and sintered at 1700°C for 1 hour in a nitrogen atmosphere to produce a casting nozzle. This molded body has an apparent porosity of 0.2% and compressive strength
At 30,000Kg/ ^cm2 , no cracks or "su" were observed.

Example 2 A suitable amount of 1.5% poval aqueous solution was added to a powder mixture consisting of 80% by weight of silicon nitride, 0.5% by weight of magnesium oxide, and 19.5% by weight of lanthanum oxide to create a slurry, which was then cast into a plaster mold of the desired shape. After demolding and drying, the mold was heated and sintered at 1600°C for 2 hours in an argon atmosphere to produce a casting nozzle. This molded body has an apparent porosity of 0.4% and a compressive strength of 28,700Kg/cm ²
So cracks and "su" were not allowed.

Example 3 To a powder mixture consisting of 90% by weight of silicon nitride, 19.5% by weight of magnesium oxide, and 0.5% by weight of praseodymium oxide, 6% by weight of methylcellulose and an appropriate amount of water were added, thoroughly kneaded, and granulated into the desired shape. It was filled into a rubber mold and preformed using an isostatic press method. After drying, this was filled with silicon nitride powder in a graphite container, and heated and sintered at 1800°C for 30 minutes in a nitrogen atmosphere to produce a casting nozzle. This molded product had an apparent porosity of 0.8%, a compressive strength of 26,300 Kg/cm ² , and no cracks or "stains" were observed at all.

Example 4 A suitable amount of 1.5% poval aqueous solution was added to a powder mixture consisting of 99% by weight of silicon nitride, 0.5% by weight of magnesium oxide, and 0.5% by weight of cerium oxide to create a slurry, which was then cast into a plaster mold of the desired shape. After demolding and drying, the mold was heated and sintered at 1700°C for 1 hour in a nitrogen atmosphere to produce a casting nozzle. This molded body has an apparent porosity of 1.4% and a compressive strength of 22,700Kg/ ^cm2 .
Kratsk and “su” were not allowed at all.

When the casting nozzles obtained in Examples 1, 2, 3, and 4 were used as tundish casting nozzles for continuous casting of aluminum killed steel, conventional casting nozzles were used several times and the casting amount was several tons. However, the change in nozzle diameter is extremely small, and it has been found that the nozzle diameter can withstand several times the number of uses and casting amounts ranging from several tens of tons to several hundred tons. In particular, with the immersion nozzle, the erosion caused by contact with the slag line was significantly reduced compared to the conventional nozzle.

Claims (1)

[Claims] 1. Preform a powder mixture containing 70 to 99% by weight of silicon nitride and the balance of magnesium oxide and lanthanum group oxides each of 0.5% by weight or more, and heat it at a temperature of 1600 to 1800°C in a non-oxidizing atmosphere. A silicon nitride casting nozzle with an apparent porosity of 1.5% or less, which is made by pressure sintering.