JP2003126946A - Pretreatment method in case of long time continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To establish a technique by which a stable long time continuous casting can be executed over the whole period of the casting without a special heating device and a high molten steel overheat temperature, and without the dispersion of quality of the cast slab and operational troubles. SOLUTION: In the case that the long time continuous casting is executed, the treatment by a ladle at a secondary refining stage prior to the continuous casting is performed for a time longer than that required for making the temperature of the refractory in the internal wall of the ladle to be almost stabilized, and making the heat radiation amount from the ladle to be stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pretreatment method for continuous casting for a long time, and particularly to a new proposal for a secondary refining method suitable for stable casting for a long time.

[0002]

2. Description of the Related Art Special steels such as stainless steels and high alloy steels are often cast at a low speed from the viewpoint of maintaining quality, and there is a problem that continuous casting requires a long time. That is, in recent years, there has been an increasing demand for the function and quality of steel materials, and along with this, improvement in the quality of cast slabs at the steelmaking stage is required. In order to meet such demands, a continuous casting method has been proposed in which the casting speed is reduced for the purpose of suppressing the intrusion of inclusions and reducing the cross-sectional defects such as center segregation during continuous casting. However, when the casting speed is reduced and the casting time is extended, the molten steel temperature of the ladle residual steel gradually decreases, and as a result, the degree of superheat of the molten steel also decreases.

Generally, it is desirable that the molten steel superheat degree (difference between the tundish molten steel temperature and the solidification temperature) during continuous casting is high in order to promote the floating of inclusions, but if it is too high, normal solidification is achieved. This will cause operational problems such as breakouts due to the hindrance of shell growth. On the other hand, when equiaxed crystals are desired as a solidification structure, it is desirable that the molten steel superheat degree is low, and there is an advantage that the molten steel temperature can be lowered in terms of refining cost. But,
If it is too low, there is a problem that casting cannot be continued or the slab surface defects increase due to the floating separation of inclusions being hindered, so it is important to maintain an appropriate temperature range throughout casting. .

In response to such a problem, when casting a steel type which is conventionally forced to perform low speed casting as described above,
Measures have been taken such as increasing the temperature of molten steel at the end of refining to ensure the degree of superheating of molten steel at the end of casting. But,
In such a method, the difference in the degree of superheat of molten steel between the initial stage and the final stage of casting becomes large, which often causes variations in the quality of cast slabs and causes operating problems. As a solution, if the casting time is long or narrow control of the degree of superheating of molten steel is required, the temperature of molten steel is increased by using electrical energy such as induction heating or plasma torch in the tundish. A means for compensating for the molten steel temperature drop at the end of casting has been studied (for example, JP-A-1-237064).

[0005]

Regarding the above-mentioned prior art for performing long-time continuous casting, the tundish needs to be modified, special equipment such as a molten steel heating device in the tundish is required, or There is a problem that when the superheated degree of molten steel is increased, the lining life of the ladle and the tundish is shortened and the cost is increased. Further, these methods also have a problem that variations in slab quality inevitably occur due to a decrease in thermal efficiency and a remarkable change (drop) in molten steel temperature from the initial stage to the final stage of casting.

Therefore, an object of the present invention is to provide a stable long-term casting over the entire casting period, without the need for a special heating device or a high degree of molten steel superheat, variation in slab quality, and operational troubles. We are proposing a technology that enables continuous casting in time.

As a result of diligent studies aimed at achieving the above-mentioned object, the inventors have found that the invention according to the following gist constitution can advantageously solve the above-mentioned problems that the prior art has. Invented the invention. That is, in the present invention, when performing continuous casting for a long time, the treatment in the secondary refining stage in the ladle prior to this continuous casting is performed for a time period during which the temperature of the refractory of the ladle inner wall is substantially stabilized. Is.

In the present invention, the time required for continuous casting with one charge is preferably about 70 minutes / ch or more.

Further, it is preferable that the time for the temperature of the refractory material on the inner wall of the ladle to stabilize, that is, the time for the amount of heat radiated from the ladle to become steady is about 40 minutes or more.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention, while studying a solution to the above-mentioned problems of the prior art,
The heat capacity (heat content) of the ladle has a large effect on the temperature drop at the end of casting during long-term continuous casting.
Moreover, it has been found that the heat content has a great influence on the secondary refining condition which is the process in the pre-process of continuous casting. In general, it is known in the field of steelmaking that the heat content of a ladle affects the temperature drop of molten steel in a ladle. A method of increasing the heat content is used by heating the oil with a combustion burner or the like, or by reducing the number of ladle in operation and increasing the usage ratio. In this respect, the inventors also take measures such as sufficient preheating with a burner or increasing the heat content in advance by casting in a general-purpose steel type, especially in the case where casting is required for a long time. However, in some cases, the temperature of the molten steel fell too low at the end of casting, which made it impossible to continue casting.

Therefore, the inventors examined the secondary refining time performed prior to continuous casting in the study of various factors affecting the temperature drop in the final stage of casting. As a result, it was found that this secondary refining time had a strong influence on the decrease in molten steel temperature at the end of casting. The time for secondary refining is
Generally, stirring for ensuring the uniformity of molten steel temperature and composition, floating separation of non-metallic inclusions floating in molten steel, vacuum removal of gas components such as C, N, H, O in molten steel, or temperature rise It is determined by the time required to bring the composition, temperature, cleanliness, etc. of molten steel to the desired range. Therefore, the secondary refining time varies greatly depending on the specifications required for the molten steel and the conditions of the molten steel to be subjected to the secondary refining treatment.The secondary refining time is improved by various improvements for the purpose of reducing refining costs and refractory costs. Has been shortened. However, when the secondary refining time is observed from the viewpoint of the thermal energy of the ladle, especially the refractory (lining), the amount of heat supplied from the molten steel to the ladle by heating the burner is as shown in Fig. 1. It is overwhelmingly larger than the amount of heat that can be supplied, and considering it only when using a ladle, it is enough heat even if it compensates for the variation in heat content due to the difference in the number of times this ladle is used continuously. It turned out that the amount could be supplied.

Therefore, the inventors tried to change the amount of heat supplied from the molten steel to the ladle by the secondary refining time. That is, an experiment was carried out in which the secondary refining time was intentionally shortened or extended, and the change in molten steel temperature at the end of casting during long-time casting was investigated. As a result, with a short secondary refining time charge, the molten steel temperature significantly decreased even in a ladle that was continuously used, and conversely, even in a ladle that was not continuously used with a long secondary refining time charge, As a result, the decrease in molten steel temperature was small. It was also found that even if this secondary refining time was extended more than necessary and the secondary refining time was lengthened, the molten steel temperature decrease rate was gradually saturated.

From these facts, the inventors of the present invention carried out the treatment of the secondary refining stage by the ladle until the heat transfer from the molten steel to the ladle reached a steady state, and the temperature of the refractory on the inner wall of the ladle was changed. It has been found that, if it is continued for more than a stabilizing time, even if continuous casting is performed for a long time, a remarkable decrease in molten steel temperature at the final stage of casting can be suppressed. The time required for secondary refining in the present invention is affected by steel type, heat size, ladle dimension, type of ladle refractory, casting temperature, casting time, secondary refining treatment content, and therefore is unique Although it cannot be specified, it is possible to predict in advance by heat transfer calculation based on the dimensions and thermal conductivity data of the materials that make up each facility, and accumulate operation data centered on the calculation prediction time. Can set a specific time in actual operation.

The refractory material on the inner wall of the ladle is made up of refractory layers called work linings, which are permanently attached in order from the side close to the iron skin of the ladle. Among them, the permanent tension is composed of a refractory made of a material having a high heat insulating property, although the fire resistance and the melting resistance are relatively low. On the other hand, the work lining is
It has much higher erosion resistance, spalling resistance, etc. than permanent tension, but has low heat insulation (that is, high thermal conductivity). The heat that has flowed into the refractory from the molten steel in the ladle through the work lining with high heat conduction is blocked by the highly heat-insulating permanent layer, and is accumulated in the work lining. Then, due to the difference between the temperature of the boundary between the work lining and the permanent upholstery and the temperature of the iron skin, it gradually moves in the permanent upholstery layer and shifts to the iron skin, and finally from the iron skin to the atmosphere or vacuum degassing treatment. It diffuses into the tank. The temperature of the ladle iron crust does not change much because it is cooled by the atmosphere or atmosphere. Therefore, the heat flux radiated from the ladle through the iron shell depends almost on the temperature at the boundary between the permanent tension and the work lining. The fact that the temperature inside the ladle inner wall refractory reaches a steady state is nothing but the stabilization of the above heat flux. Therefore, whether the temperature inside the ladle inner wall refractory reaches a steady state or not. The most reasonable way of understanding is to monitor the temperature at the boundary between the permanent tension and the work lining. Therefore, in the present invention, to grasp the thermal state of the refractory of the inner wall of the ladle, whether by the heat transfer calculation or the actual measurement by a thermocouple or the like, the temperature at or near the boundary between the permanent tension and the work lining is measured. Is practical and preferred.

For example, the secondary refining time for stabilizing the heat radiation from the ladle and stabilizing the temperature of the refractory on the inner wall of the ladle (the temperature at the boundary between the permanent tension and the work lining) is as follows:
As shown in Fig. 1, it is about 40 minutes / ch or more, and if secondary refining is performed beyond this time, as shown in Fig. 2, even when the number of continuous use of the ladle is 3 or more, 3 Even when it is less than the number of times, it is possible to obtain the result that the temperature drop during casting is 10 ° C. or less.

In the present invention, long-term continuous casting is a case of continuously casting for 70 minutes / ch or more. In this case, as shown in FIG. At 20 minutes (conventional example), the continuous casting time: the superheat degree of molten steel begins to drop sharply at the boundary of 70 minutes, while continuous casting is performed under the conditions compatible with the present invention (secondary refining time 60
Min), it was found that even in the final stage of casting, the degree of superheat of molten steel was within the proper range of 35 to 55 ° C.

[0017]

EXAMPLES The experiments conducted to verify the effects of the present invention will be described below. In this experiment, martensitic medium carbon stainless steel was used as a raw material, and this applied material was subjected to secondary refining, that is, vacuum degassing treatment by a VOD device, prior to continuous casting. Then, the obtained 180 t / ch molten steel was continuously cast by a slab continuous casting machine. The VOD treatment time is 20 to 90 minutes, continuous casting is performed with a casting time of 80 to 120 minutes, and the molten steel superheat degree (° C) in the dandysh is measured at regular intervals to reduce the molten steel temperature. I asked.

The results are shown in FIG. 3 showing the temperature change during casting when the secondary refining time was 60 minutes as the example of the present invention and the secondary refining time was 20 minutes as the conventional example.

As can be seen from the figure, when the casting time was 120 minutes, the molten steel temperature certainly dropped from 40 ° C to 40 ° C in the invention method, but it was still within the proper range. Therefore, casting was possible. But,
In the conventional method, the casting temperature was 65 ° C. when 40 minutes passed, but the molten steel superheat degree decreased to 25 ° C. when 120 minutes passed, making it difficult to continue casting.

[0020]

As described above, according to the present invention,
Even when performing continuous casting for a long time, no special heat application equipment such as a molten steel heating device in the tundish is required, and in order to secure the molten steel temperature at the end of casting, the temperature at the beginning of casting (molten steel Since it is not necessary to raise the superheat degree excessively, it is possible to solve variations in the quality of the slab and problems in operation. In addition, a stable casting operation can be secured in the entire casting section.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between secondary refining time and ladle inner wall temperature.

FIG. 2 is a graph showing the effect of secondary refining time on the temperature drop of molten steel during continuous casting.

FIG. 3 is a graph showing the relationship between molten steel temperature change during casting and molten steel superheat degree in Example.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroyasu Morioka             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4E004 MB20                 4K013 FA00 FA03

Claims (3)

[Claims] 1. When performing continuous casting for a long time, the treatment at the secondary refining stage in the ladle prior to this continuous casting is performed for a time longer than the temperature at which the temperature of the refractory material inside the ladle is substantially stabilized. A characteristic pretreatment method for long-term continuous casting. 2. The time required for continuous casting of one charge is
The pretreatment method according to claim 1, which is about 70 minutes / ch or more. 3. The pretreatment method according to claim 1, wherein the time for stabilizing the temperature of the refractory material on the inner wall of the ladle is about 40 minutes or more.