JPH05293616A - Insulating material for molten steel surface - Google Patents

Abstract

(57) [Abstract] [Purpose] The present invention provides a heat retaining agent which surely prevents the contamination of molten steel due to the reaction between air oxidation and the heat retaining agent, and on which the refractory is not damaged or melted. [Constitution] In the present invention, the contents of CaO and Al ₂ O ₃ are changed to Ca
The present invention relates to a molten steel surface heat retaining agent, wherein O / Al ₂ O ₃ is 0.5 to 2.0, SiO ₂ content is 5% or less, and ZrO ₂ content is 10 to 50%. ..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a molten steel surface heat insulation for coating the 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 tundish for continuous casting or a ladle. It is related to agents.

[0002]

2. Description of the Related Art When a molten steel is transferred or refined by a continuous casting tundish or ladle, a heat insulating agent is used to cover the surface of the molten steel to prevent heat dissipation from the molten steel and invasion of outside air. .. Conventionally, as the heat retaining agent, burned rice obtained by steaming rice husks has been mainly used, and its main components are SiO ₂ and C. SiO ₂ has a low thermal conductivity and has a heat retaining effect, and C has an excellent effect of blocking external air because it converts oxygen into CO gas. Therefore, burned rice has a heat retaining effect and a deaeration effect, and is a cheap heat retaining agent. However, for the purpose of improving workability, the C concentration in the steel sheet has been reduced as much as possible, for example, in an extremely low carbon steel with a C concentration of 50 ppm or less, the C component in the heat retaining agent is picked up in the molten steel and the characteristics of the steel material are deteriorated. There are known drawbacks. Also, the SiO ₂ component in the heat retaining agent reacts with Al in the molten steel to produce A
Since the l ₂ O ₃ -based inclusions are generated, there arises a problem that surface defects are increased.

Conventionally, in order to solve these drawbacks of rice hulls,
As a heat retaining agent having a small amount of C and SiO ₂ components, for example, as disclosed in Japanese Patent Publication No. 3-48152, Mg
An O-based heat retaining agent is used. Further, since MgO itself has a high thermal conductivity, foamed MgO provided with heat insulation properties
Various manufacturing methods have been investigated, and Japanese Patent Publication No.
No. 85, etc.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Since the heat-insulating agent containing as a main component has a high melting point and is mainly a solid phase at the operating temperature, a uniform coating state of the molten steel surface cannot be obtained, and Al ₂ O ₃ -based inclusions are generated by the reaction between the outside air and the molten steel. To generate. In the tundish, a stopper is used to control the supply of molten steel into the mold.
Since the O-based heat retaining agent is sintered between particles to form a strong slag layer, it becomes difficult to control the stopper, and when it is severe, the stopper is broken. On the other hand, a method of lowering the MgO content and lowering the melting point can be considered, but in this case, the SiO ₂ content is relatively high and the reduction of SiO ₂ occurs in the molten steel. Further, since MgO lowers the viscosity of the slag, there is a problem that it penetrates into the pores of the tundish refractory and the stopper refractory and accelerates melting loss. In view of these problems, it is an object of the present invention to reliably prevent molten steel contamination due to the reaction of air oxidation and a heat retaining agent, and to provide a heat retaining agent that does not damage or melt the refractory. It is a thing.

[0005]

The present invention is directed to CaO and Al.
The content of ₂ O ₃ and 0.5 to 2.0 at CaO / Al ₂ O _3, and the SiO ₂ content of 5% or less, the ZrO ₂ content 1
The present invention relates to a molten steel surface heat retaining agent characterized in that it is set to 0 to 50%.

[0006]

[Function] A condition to be satisfied as a heat retaining agent for coating the surface of molten steel is that the contamination of molten steel caused by the reaction of air oxidation and the heat retaining agent is surely prevented, and further, there is no damage or melting loss of the refractory. . As a result of the inventors' investigations into heat insulation agents to satisfy these basic conditions, the liquid phase of the heat insulation agent is suppressed to suppress the air oxidation, and the low temperature is required to prevent the reaction between the heat insulation agent and molten steel. It has been found that conversion to SiO ₂ is effective. That is, lowering the melting point of the heat retaining agent to make it in the liquid phase makes the coating state on the surface of the molten steel uniform and enhances the deaeration ability of the heat retaining agent itself.

Further, since SiO ₂ in the heat retaining agent reacts with Al in the molten steel according to the equation (1), lowering the SiO ₂ content of the heat retaining agent is effective in preventing the formation of Al ₂ O ₃ type inclusions. 3SiO ₂ +4 Al = 2Al ₂ O ₃ +3 Si (1)

In view of the above points, a low melting point and a low SiO
Satisfies ₂ of further inclusions for heat retaining agent comprising an absorbent capacity result of extensive investigations, the content of CaO and Al ₂ O ₃ in the range of 0.5 to 2.0 at CaO / Al ₂ O ₃ , SiO
₂ It was clarified that it is optimal to set the content rate to 5% or less. Note that the CaO / Al ₂ O ₃ is set in the range of 0.5 to 2.0 because the heat insulating agent has a melting point below the molten steel temperature (1550 ° C.) in the tundish as shown in FIG. This is because. In addition, the SiO ₂ content is 5
The reason why the content is made less than or equal to% is that the reaction between the heat retaining agent and the molten steel can be prevented. However, it was found that when the heat retaining agent of this component was applied to the tundish, the stopper refractory melted rapidly and could not be used for a long time.

Therefore, the present inventors have added various components to the above-mentioned heat retaining agent and evaluated the dissolution rate of the stopper refractory. The result is shown in FIG. As is clear from the figure, the dissolution rate of the stopper refractory is 10% for ZrO _2.
Although it can be suppressed by the above addition, if the ZrO ₂ content exceeds 50%, the heat retaining agent becomes a solid phase and the deaeration effect decreases. Therefore, the optimum ZrO ₂ content in the heat retaining agent is 10 to 50%. We also conducted a detailed investigation on the melting mechanism of refractory materials, and the penetration of the heat retaining agent into the pores of the refractory material regulates the rate of melt loss, and by adding ZrO _{2 in an amount} of 10% or more, the viscosity of the heat retaining agent is increased. It was found that the permeation into the pores of the refractory material is suppressed and the penetration of the refractory material is suppressed.

The basic components of the heat retaining agent are as described above, but within the range of not impairing the function of the product of the present invention, Mg
It is also possible to add other components such as O, CaCl ₂ and CaF ₂ . As shown above, by using the heat retaining agent of the present invention, it is possible to surely prevent molten steel contamination caused by the reaction of air oxidation and the heat retaining agent, and to provide a molten steel heat retaining agent that does not damage or melt the refractory. Can be provided.

[0011]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples. 400 kg of heat retaining agent of the ingredients shown in Table 1
Was added to a tundish having a capacity of 60 tons, and low carbon aluminum killed steel was cast for 400 minutes. In both the example and the comparative example of the present invention, the casting dimensions are 245 mm in thickness and 1500 m in width.
In m, it was cut into a length of 8500 mm to make one coil unit. The slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1500 mm. The deaeration effect and reaction prevention effect of the heat retaining agent were evaluated by the amount of increase in total oxygen content on the tundish inlet side and outlet side and the number of surface defects generated on the cold rolled steel sheet. Regarding the melting loss of the refractory material, the amount of the melting loss of the stopper was measured after use, and the melting rate was calculated from the casting time.

[0012]

[Table 1]

[0013]

[Table 2]

As shown in Table 2, in the examples, CaO and Al were used.
The content of ₂ O ₃ and 0.5 to 2.0 at CaO / Al ₂ O _3, and the SiO ₂ content of 5% or less, the ZrO ₂ content 1
By setting it to 0 to 50%, it was possible to prevent the molten steel contamination due to the reaction of air oxidation and the heat retaining agent, so that the total oxygen content in the tundish was not increased and no surface defects were generated.
Further, it was confirmed that the stopper refractory hardly melts, so that it can sufficiently withstand use even when the number of castings increases continuously.

On the other hand, in Comparative Example 1, since the ZrO ₂ content was low, the melting loss of the stopper refractory could not be suppressed, and the casting was stopped 350 minutes after the heat insulating agent was added. Since Comparative Example 2 has a high ZrO ₂ content, Comparative Example 3 and Comparative Example 4
Since CaO / Al ₂ O ₃ is not in the range of 0.5 to 2.0, the heat retaining agent becomes a solid phase and a sufficient deaeration effect cannot be obtained, and the total oxygen content in the tundish increases and surface defects occur. Occurred. Further, in Comparative Example 5, the SiO ₂ content in the heat retaining agent is 5%.
As a result, the amount of Si and total oxygen increased due to the reaction with Al in the molten steel. As a result, surface defects caused by Al ₂ O ₃ -based inclusions occurred in the cold-rolled steel sheet.

[0016]

As described above, according to the molten steel heat retaining agent of the present invention, the molten steel is not contaminated at all, and the slab quality is extremely improved. In addition, since there is no damage or melting of refractories,
It is possible to provide an effective heat retaining agent in terms of operation.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between CaO / Al ₂ O _{3 of a} heat retaining agent and a melting point.

FIG. 2 is a diagram showing the relationship between the amount of ZrO ₂ added to a heat retaining agent and the dissolution rate.

Claims (1)

[Claims] 1. The content of CaO and Al ₂ O ₃ is CaO / A.
0.5 to 2.0 with 1 ₂ O ₃ and a SiO ₂ content of 5
A molten steel surface heat-retaining agent characterized by having a ZrO ₂ content of 10 to 50% by weight or less.