JPS6411592B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a refractory that connects a tundish and a mold in continuous casting equipment, a so-called joint ring, which has excellent corrosion resistance especially in continuous casting of stainless steel. This article relates to continuous casting refractories that exhibit excellent performance.

Conventionally, silicon nitride or boron nitride refractories have been widely used as refractories that connect the tandem plate and mold in horizontal continuous casting equipment, but in the past, silicon nitride refractories have been developed to improve their thermal shock resistance. This has become strongly desired, and products made by mixing boron nitride and sintering it have come to be provided. Such a sintered body exhibits sufficient thermal shock resistance when casting general carbon steel, but if stainless steel, especially high Cr steel, is to be cast, it may be necessary to use silicon nitride. There was a problem in that the erosion progressed significantly, making it extremely difficult to carry out long-term operation. On the other hand, there is some research into improving this by using boron nitride as the main material, but since it is originally manufactured using a hot press method, there is a problem with manufacturing costs, and the inherent problem of low wear resistance. There are deficiencies, and we have yet to fully overcome them. Therefore, the current situation is that some degree of improvement in thermal shock resistance has been achieved through the combined use of silicon nitride and boron nitride, but this is not the case in continuous casting of stainless steel, especially high Cr steel. It is difficult to avoid melting of the refractories, which leads to deterioration of the surface quality due to damage to the refractories, and in some cases, localized melting damage can damage the refractories and cause breakouts, making stable operation difficult. unable to contribute.

The present invention has been made in view of this situation, and while satisfying the basic characteristics as a refractory for connecting molds for continuous casting, it also has excellent corrosion resistance, especially corrosion damage caused by the Cr component in molten steel. The purpose of this invention is to provide a refractory material that can strongly resist.

The refractory of the present invention that has achieved properties suitable for the above purpose is mainly composed of a stabilized zirconia sintered body containing 20% by weight (hereinafter simply referred to as %) or less of boron nitride. The gist lies in a certain point.

The present inventors conducted various studies on the reasons why silicon nitride sintered bodies are relatively easily eroded by molten stainless steel, and found that under high-temperature conditions exceeding 1500°C, Cr in stainless steel It was discovered that by the reaction between silicon nitride and silicon nitride, silicon nitride undergoes a chemical transformation, changes to a low melting point substance, and is eroded away. Therefore, it was thought that it would be difficult to completely prevent chemical transformation by Cr as long as silicon nitride was used as the base, so it was necessary to develop an alternative sintered body from a completely different perspective. Thinking that this might be the case, we fabricated prototype sintered refractories with various compositions and tested their corrosion resistance in molten stainless steel. As a result, Al ₂ O ₃ , MgO,
It has been found that a sintered body obtained by uniformly dispersing ZrO ₂ etc. in silicon nitride exhibits extremely good erosion resistance against molten stainless steel.
ZrO ₂ was found to be extremely promising. However, ZrO ₂ itself has the disadvantage of transformation expansion, and there are many problems especially when viewed as a raw material for manufacturing the joint rings used in areas where high precision is required, such as between the tundish and the mold. I understand. obey and urge
It has corrosion resistance superior to Al ₂ O ₃ and MgO (particularly corrosion resistance against molten stainless steel), and has excellent high-temperature strength and toughness. Although it was expected that the effect would be large and that the formation of solidified grains inside the refractory would be suppressed and the surface quality of the slab would be improved, the field targeted by the present invention has not yet been addressed. There was an aspect that application was not necessarily easy.

Therefore, we searched for something that would not cause the above-mentioned transformation expansion, and arrived at stabilized zirconia refractory material.
Stabilized zirconia refractory material itself is well known and is generally manufactured by adding CaO or MgO, which have poor thermal shock resistance, to ZrO ₂ , but from an industrial perspective, it has never been used. Various manufacturing methods have been proposed; for example, products manufactured by sintering products obtained by slurry casting, extrusion, or cold pressing are used, and dense and porous materials are used. It can be used regardless of quality. Also, using fused and stabilized zirconia, after crushing and adjusting the particle size, preferably blending them into a densely packed structure, adding a small amount of organic binder, mixing and shaping, drying and baking. The present invention can also be applied to the present invention.

If such stabilized zirconia is used, a refractory with excellent thermal shock resistance and erosion resistance can be obtained, and the advantages of zirconia such as suppressing the formation of solidified shells can be obtained, but the thermal shock resistance has yet to be improved. There is room for Therefore, in the present invention, stabilized zirconia is used as the main ingredient, and boron nitride, which has a high thermal conductivity and a low coefficient of thermal expansion, is blended with the stabilized zirconia to further improve thermal shock resistance and spalling resistance. These effects of boron nitride can also be achieved by adding a small amount of boron nitride, so setting a lower limit is not particularly meaningful from a technical point of view, but it is possible to achieve this by adding 5 to 10% of boron nitride. By doing so, the above effects will become noticeable. On the other hand, for the upper limit, 20
If it exceeds 20%, the strength of the refractory will decrease and solidified shells will be more likely to form, so it should be kept below 20%.

Since the present invention is configured as described above, even when continuous casting is performed for stainless steel, especially steel containing 10% or more of Cr, the present invention has good thermal shock resistance and melting resistance as a refractory. It has a high degree of durability and spalling resistance, and it has become possible to safely complete continuous casting without causing accidents such as breakouts.

Next, examples of the present invention will be shown.

A ring-shaped sintered body of 165φ×150φ×20t (mm) made of stabilized zirconia containing 10% boron nitride and 70% CaO was placed between the tundish nozzle and the mold. Stainless steel (SUS304) 4.5 tons
When it was poured at 1570℃ and continuously cast at a drawing speed of 1.3m/min, approximately 30m of it was able to be completely cast. During this period, no cracks were observed in the sintered body, and no melting damage occurred, so the surface quality of the slab was extremely good. For comparison, similar continuous casting was performed using a ring made of silicon nitride sintered body and a ring made of boron nitride sintered body, and breakout occurred at 10 m in the case of the former, while breakout occurred in the case of the latter. At 20m, the operation was forced to stop due to increased pulling resistance. When we investigated the reason for the increase in pulling resistance, we found that
The boron nitride refractory was locally worn away by solidified grain, and molten steel was inserted between the refractory (joint ring) and the mold.

Claims (1)

[Claims] 1. A refractory for connecting the tundish and mold of horizontal continuous casting equipment, which is for continuous casting characterized by being mainly composed of a stabilized zirconia sintered body containing 20% by weight or less of boron nitride. Refractories for mold connections.