WO2011071152A1 - Agent for maintaining surface temperature of molten steel and method for maintaining surface temperature of molten steel - Google Patents

Abstract

Disclosed is an agent for maintaining the surface temperature of molten steel, which is applied over the surface of molten steel that has a predetermined surface temperature. The agent for maintaining the surface temperature of molten steel contains two or more kinds of high melting point materials that have a melting point higher than the surface temperature of the molten steel, while containing 10-70% by mass of CaO, 10-60% by mass of Al₂O₃, 5-30% by mass of MgO and 0-10% by mass of SiO₂ so that the total is not less than 70% by mass and the ratio of CaO to Al₂O₃, namely CaO/Al₂O₃ is 0.5-2.0. The agent for maintaining the surface temperature of molten steel has a melting point lower than the surface temperature of the molten steel, and is composed of a powder that contains particles having a particle diameter of 30-100 μm in an amount of not less than 70% by mass.

Description

Molten steel surface thermal insulation and molten steel surface thermal insulation method

TECHNICAL FIELD The present invention relates to a molten steel surface heat insulating agent that coats a molten steel surface for the purpose of heat insulation / heat retention or air oxidation prevention when the molten steel is transferred or refined by a ladle or a tundish for continuous casting.
This application claims priority on December 10, 2009 based on Japanese Patent Application No. 2009-280206 for which it applied to Japan, and uses the content here.

Conventionally, when transporting or refining molten steel using a tundish or ladle for continuous casting, the surface of the molten steel is covered with a molten steel surface heat insulating agent to prevent heat dissipation from the molten steel and intrusion of outside air. Yes. As a molten steel surface heat insulating agent, shochu containing SiO ₂ and C as main components is widely used. When shochu is used as a molten steel surface heat insulating agent, SiO ₂ reacts with Al in the molten steel to generate Al ₂ O ₃ -based inclusions, which increases the surface defects of the product.

Therefore, as shown in Patent Document 1, an MgO-based molten steel surface heat insulating agent has been developed as a heat insulating agent with a small amount of SiO ₂ .

Japanese Examined Patent Publication No. 3-48152

However, since the molten steel surface heat insulating agent mainly composed of MgO has a high melting point and is mainly a solid phase at the operating temperature, the molten steel surface cannot be uniformly coated, and the reaction between the outside air and the molten steel surface causes Al ₂ O ₃ inclusions are produced.

The present invention solves the above-mentioned problems, and does not produce alumina inclusions in the molten steel due to the components derived from the molten steel surface heat insulating agent, and has a high melting rate on the molten steel surface, making the molten steel surface uniform. An object of the present invention is to provide a molten steel surface heat insulating agent that can be coated on the surface.

In order to solve the above problems, the present invention employs the following configurations and methods.

(1) A first aspect of the present invention is a molten steel surface heat insulating agent disposed on a molten steel surface having a predetermined molten steel surface temperature, wherein two or more types of high melting point raw materials having a melting point higher than the molten steel surface temperature are used. 10 to 70% by mass of CaO, 10 to 60% by mass of Al ₂ O ₃ , 5 to 30% by mass of MgO, and 0 to 10% by mass of SiO ₂ in total of 70% by mass or more. the CaO and is said _Al 2 _{O 3} ratio CaO / _Al 2 _{O 3} is 0.5 to a 2.0, a melting point lower than the molten steel surface temperature, the particle size not less than 70 wt% 30 - 100 [mu] m powder It is a molten steel surface heat insulating agent that is a body.

(2) A second aspect of the present invention is a molten steel surface in which the molten steel surface heat-retaining agent according to (1) is disposed on the molten steel surface so that the average molten layer thickness is in the range of 5 to 30 mm. It is a heat retention method.

According to the configuration described in (1) above, alumina inclusions are not generated in the molten steel due to the components of the molten steel surface heat insulating agent, and the molten steel surface heat insulating agent quickly melts to make the molten steel surface uniform. It is possible to suppress the formation of alumina inclusions due to contact between the molten steel and the atmosphere.

According to the method described in the above (2), the molten steel surface heat insulating agent can be melted quickly and the molten steel surface can be coated uniformly and surely, and the occurrence of shelves can be prevented. Generation of alumina inclusions due to contact can be suppressed.

This is the amount of change in the total oxygen amount in the tundish outlet side molten steel relative to the total oxygen amount in the tundish inlet side molten steel in one or two pans in continuous continuous casting. It is the average number of surface defects due to oxide inclusions present in one cold-rolled steel sheet coil obtained from steel slabs produced in 1-2 pans of continuous continuous casting.

The inventors examined a method for increasing the melting rate of the molten steel surface heat insulating agent in order to uniformly coat the molten steel surface with the molten steel surface heat insulating agent. As a result, it is a molten steel surface heat insulating agent having a melting point (melting point after mixing) produced by mixing two or more high melting point raw materials having a melting point higher than the molten steel surface temperature. When a molten steel surface heat insulating material whose particle size is 30-100 μm or more is used, solid diffusion between different high melting point raw materials is promoted, so that the molten steel surface heat insulating material in contact with the molten steel surface is rapidly melted. Discovered that it would be possible. Therefore, the molten steel surface insulation agent dissolved can be uniformly coat the surface of molten steel, the molten steel surface to prevent the formation of Al ₂ O ₃ based inclusions due to contact with the outside air. Hereinafter, the molten steel surface heat insulating agent which concerns on embodiment of this invention based on the said discovery is demonstrated in detail. The melting point of the molten steel surface heat insulating material is a temperature at which melting starts when the temperature of the substance is raised, and in the case of a multi-component substance, it is a melting point with an average composition corresponding to the solidus temperature. The molten steel surface temperature in a continuous casting tundish or ladle is 1550 ° C. to 1650 ° C.

The molten steel surface heat insulating agent according to the present embodiment is characterized in that 70 mass% or more of the molten steel surface is a powder having a particle size of 30 to 100 μm. The particle size here is a dimension of the sieve opening, and is a dimension that can pass through the sieve having the predetermined opening dimension. When 70 mass% or more of the molten steel surface heat insulating agent is larger than the particle size of 100 μm, the molten steel surface heat insulating agent does not melt quickly, so that the molten steel surface comes into contact with the outside air and Al ₂ O ₃ -based inclusions are generated. . On the other hand, if 70 mass% or more of the molten steel surface heat insulating agent is a powder having a particle size of 30 μm or less, the cost of making the raw material fine becomes enormous. More preferably, 70% by mass or more of the molten steel surface heat insulating agent is a powder having a particle size of 40 to 90 μm, and more preferably 70% by mass or more of the molten steel surface heat insulating agent is a powder having a particle size of 50 to 80 μm. Further, even when the powder having a particle size of 30 to 100 μm accounts for less than 70% by mass of the molten steel surface heat insulating agent, the molten steel surface is quickly covered with the solution because there are few molten steel surface heat insulating agents having a high dissolution rate. The surface of the molten steel comes into contact with the outside air.
The molten steel surface heat insulating agent should just be put in the bag in the state in which different high melting-point raw materials were mixed uniformly. The molten steel surface heat insulating agent in the state of being put in the bag can be put into the molten steel surface together with the bag. When the raw material particle size is greatly different, for example, when a high melting point material of 30 to 100 μm of the molten steel surface heat insulating material according to the present embodiment and a different type of high melting point material of less than 30 μm or more than 100 μm are mixed, Since the raw materials are unevenly distributed in the inside, it becomes difficult for different raw materials to come into contact with each other and the reaction is slowed down, so that the melt is not rapidly generated on the molten steel surface, and the molten steel surface comes into contact with the outside air.

When the melting point (melting point in average composition) of the molten steel surface heat insulating agent is higher than the molten steel surface temperature, the molten steel surface heat insulating agent does not reach a completely molten state, and the spreadability on the molten steel surface deteriorates, and the molten steel surface Will come into contact with outside air. For this reason, melting | fusing point of the molten steel surface heat insulating agent which concerns on this embodiment is set lower than molten steel surface temperature.

When using a molten steel surface heat insulating agent that is completely melted, melting of refractories such as ladle and tundish becomes a problem. Therefore, in the molten steel surface heat insulating agent according to the present embodiment, melting of the tundish is prevented by using a raw material containing magnesia (MgO) used in a ladle or a tundish coating material. When the amount of magnesia contained is less than 5% by mass, the melting rate of the ladle or tundish coating material is increased, which hinders operation. On the other hand, when the amount of magnesia contained is higher than 30% by mass, the melting point increases, so that the molten steel cannot be uniformly coated. Therefore, in the molten steel surface heat insulating agent according to the present embodiment, the content of magnesia is regulated to 5 to 30% by mass. The content of magnesia is more preferably 7 to 25% by mass.

The molten steel surface heat insulating agent according to this embodiment has a composition after mixing (average composition) of 10 to 70 mass%, preferably 15 to 65 mass%, more preferably 20 to 60 mass% CaO, 10 to 60 mass%. The main component is preferably 15 to 55% by mass, more preferably 20 to 50% by mass of Al ₂ O _3, 5 to 30% by mass of MgO, and 0 to 10% by mass of SiO ₂ . However, CaO / Al ₂ O ₃ = 0.5 to 2.0. This is because the melting point (melting point of the average composition) of the molten steel surface heat insulating agent is minimized in the range of CaO / Al ₂ O ₃ = 0.5 to 2.0. Moreover, if the mass of SiO ₂ occupying the molten steel surface heat insulating agent exceeds 10 mass%, the reaction with Al in the molten steel generates Al ₂ O ₃ -based inclusions, which increases the surface defects of the product. End up. The content of MgO is as described above. “Containing as a main component” means that the corresponding component occupies 70% by mass or more of the whole. In the molten steel surface heat insulating agent according to the present embodiment, the total of the above components may be 80% by mass or 90% by mass or more of the whole.

Examples of the high melting point raw material include MgO produced by firing magnesite, electromelted MgO, CaO, Al ₂ O ₃ , SiO ₂ , SrO, ZrO ₂ , Al ₂ O ₃ —MgO, and CaO—. MgO can be used.

As the molten steel surface heat retaining method, it is desirable to dispose the above-described molten steel surface heat insulating agent on the surface of the molten steel so that the average melt thickness is in the range of 5 to 30 mm. This is because when the average melt thickness is less than 5 mm, the molten steel surface is not sufficiently blocked from the outside air. In addition, when the average melt thickness exceeds 30 mm, the upper part of the molten steel surface heat insulating agent located at a position away from the molten steel as the heat source is cooled, so that the temperature of the ladle or tundish is lower than that of the molten steel. This is because the molten steel surface heat insulating agent solidifies and adheres to the surface of the object, and a gap is formed between the molten steel and the molten steel surface heat insulating agent. This is called shelving. When shelving occurs, a gap is formed between the molten steel and the molten steel surface heat insulating agent, so that the molten steel surface comes into contact with the outside air. The molten steel surface heat insulating agent may be arranged on the molten steel surface so that the average melt thickness is in the range of 7 to 25 mm, or 9 to 20 mm.

Next, the present invention will be described based on examples, but the conditions in the examples are one condition example adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to these condition examples. It is not limited to.
The present invention can adopt various conditions or combinations of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

The molten steel with 1 charge of 280 t was subjected to hot metal preliminary treatment, converter decarburization, and vacuum degassing treatment with RH to produce ultra-low carbon steel. A slab was produced by continuous casting using a tundish having a capacity of 60 t. Casting was performed continuously for 15 charges of molten steel. The molten steel surface temperature was 1560 to 1580 ° C. The molten steel surface heat-retaining agent of the present invention or the comparative example was used to keep the molten steel in the tundish from the beginning of casting. As for the molten steel surface heat insulating agent, 500 kg of the bag was added to the tundish in both the examples and comparative examples. One slab has a thickness of 250 mm, a length of 7000 mm, and a width of 1500 mm. The slab was made into a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm through a commonly used hot rolling and cold rolling process. Tables 1 to 4 show data of examples and comparative examples. Although divided into four tables for the sake of layout, Table 2 is a continuation of Table 1, Table 3 is a continuation of Table 2, and Table 4 is a continuation of Table 3.

In Table 1, * 1 CaO—Al ₂ O ₃ is 50% by mass of CaO and 50% by mass of Al ₂ O ₃ .
In Table 1, * 2 CaO—SiO ₂ is 55% by mass of CaO and 45% by mass of CaO—SiO ₂ .
* 3 SrO—SiO _{2 in} Table 1 is 50% by mass of SrO and 50% by mass of SiO ₂ .
* 4 Al ₂ O ₃ —CaO—MgO in Table 1 is 50% by mass of Al ₂ O ₃ , 45% by mass of CaO and 5% by mass of MgO.
* 5 Al ₂ O ₃ —CaO—SiO _{2 in} Table 1 is 50% by mass of Al ₂ O ₃ , 45% by mass of CaO and 5% by mass of SiO ₂ .
* 6 Al ₂ O ₃ —CaO—ZrO _{2 in} Table 1 is 50% by mass of Al ₂ O ₃ , 45% by mass of CaO and 5% by mass of ZrO ₂ .
* 7 Al ₂ O ₃ —MgO—SiO _{2 in} Table 1 is 25% by mass of Al ₂ O ₃ , 25% by mass of MgO, and 50% by mass of SiO ₂ .
In Table 2, * 8 Al ₂ O ₃ —MgO is 75% by mass of Al ₂ O _{3 and} 25% by mass of MgO.
In Table 2, * 9 CaO—MgO is 70% by mass of CaO and 30% by mass of MgOs.
The thickness of the molten layer * 10 in Table 4 was obtained by immersing an iron bar in molten steel and setting the thickness of the molten steel surface heat insulating agent attached thereto as the molten layer thickness.
* 11 ΔT. In Table 4 O is a change amount of the total oxygen amount in the tundish molten steel with respect to the total oxygen amount in the molten steel after RH treatment (after vacuum degassing treatment) in 1 to 2 pans of continuous continuous casting.
* 12 In Table 4, the number of defects is the average number of surface defects due to oxide inclusions present in each cold-rolled steel sheet coil obtained from steel slabs produced in 1-2 pans of continuous continuous casting. It is.

As shown in FIG. 1, in the example, the total oxygen amount in the tundish molten steel is 1 to 2 pans of continuous continuous casting, compared to the total oxygen amount in the molten steel after RH treatment (after vacuum degassing treatment). is decreasing. This is because the molten steel surface heat insulating agent melts quickly and uniformly coats the tundish surface, so that the formation of alumina inclusions due to contact between the molten steel and the atmosphere is suppressed, and the alumina inclusions in the molten steel surface. This is because it was removed from the molten steel.
In addition, as shown in FIG. 2, in the example, surface defects caused by oxide inclusions present in one cold-rolled steel sheet coil obtained from a steel piece produced by continuous casting in one or two pans are used. The average number is greatly reduced as compared with the conventional comparative example. This is also because the molten steel surface heat insulating agent quickly melts and uniformly coats the tundish surface, thereby suppressing generation of alumina inclusions due to contact between the molten steel and the atmosphere.

According to the present invention, due to the component derived from the molten steel surface heat insulating agent, there is no generation of alumina inclusions in the molten steel, and the melting rate on the molten steel surface is high, and the molten steel surface is uniformly coated. It is possible to provide a molten steel surface heat insulating agent capable of

Claims (2)

1. A molten steel surface heat insulating agent disposed on a molten steel surface having a predetermined molten steel surface temperature,
   Containing two or more high melting point raw materials having a melting point higher than the surface temperature of the molten steel,
   10 to 70% by mass of CaO, 10 to 60% by mass of Al ₂ O ₃ , 5 to 30% by mass of MgO, and 0 to 10% by mass of SiO ₂ are contained in total of 70% by mass or more,
   The ratio CaO / _Al 2 _{O 3} of the _Al 2 _{O 3} and the CaO is 0.5 to 2.0
   The melting point is lower than the surface temperature of the molten steel,
   A molten steel surface heat-retaining agent characterized in that 70 mass% or more is a powder having a particle size of 30 to 100 μm.
2. The molten steel surface heat insulating agent according to claim 1, wherein the molten steel surface heat insulating agent is disposed on the molten steel surface so that an average molten layer thickness is in a range of 5 to 30 mm.

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