JP4486526B2 - Molten steel surface heat insulating material and continuous casting method of steel using the same - Google Patents

Description

  The present invention provides a molten steel surface heat insulating material for covering a molten steel surface for the purpose of heat insulation and heat insulation or prevention of air oxidation when the molten steel is transferred or refined by a tundish or ladle for continuous casting. The present invention relates to the continuous casting method used.

When molten steel is transferred or refined by a tundish for continuous casting or a ladle, the surface of the molten steel is covered with a heat insulating material to prevent heat dissipation from the molten steel and intrusion of outside air into the molten steel. Conventionally, shochu made from steamed rice husk has been mainly used as a heat insulating material, and the main components are SiO ₂ and C. SiO ₂ has a low thermal conductivity and has a heat retention effect, and C has an excellent oxygen blocking effect because it converts oxygen to CO gas. For this reason, shochu is a heat insulating material characterized by having a heat insulating effect and an air blocking effect and being inexpensive. However, for the purpose of improving workability, the C concentration in the steel sheet is reduced as much as possible. For example, when using shochu as a heat insulating material in an extremely low carbon steel having a C concentration of 50 ppm or less, the C component in the heat insulating material is contained in the molten steel. There are known disadvantages of being picked up and degrading the properties of the steel. Moreover, since the SiO ₂ component in the heat insulating material reacts with Al in the molten steel to generate Al ₂ O ₃ -based inclusions, there is a risk of increasing surface defects. Conventionally, in order to solve these drawbacks of shochu, an MgO-based heat insulating material is used as a heat insulating material having a small amount of C and SiO ₂ components, as described in Patent Document 1, for example. Further, since MgO itself has a high thermal conductivity, various methods for producing foamed MgO imparted with thermal insulation have been studied and described in Patent Document 2 and the like.
Japanese Patent Publication No. 3-48152 Japanese Patent Publication No. 48-7485

However, the heat insulating material mainly composed of MgO such as Patent Document 1 and Patent Document 2 has a high melting point, and is mainly present as a powder or sintered solid phase at the use temperature of continuous casting of steel. A uniform coating state on the surface of the molten steel cannot be obtained, and Al ₂ O ₃ inclusions are generated by the reaction between the outside air and the molten steel. On the other hand, a method in which a part of MgO is replaced with a low melting point material such as SiO ₂ , Na ₂ O, or CaF ₂ to melt the heat insulating material and uniformly coat the molten steel surface is considered. ₂ and Na ₂ O react with Al in the molten steel to produce Al ₂ O ₃ inclusions, and CaF ₂ increases the inclusion due to the refractory by melting the tundish refractory.

  In view of these problems, an object of the present invention is to provide a heat insulating material that can prevent molten steel contamination caused by the reaction between air oxidation and the heat insulating material, and a continuous casting method for obtaining a slab excellent in cleanliness. Is.

In order to solve the above problems, the present invention is summarized as follows. That is,
(1) In the molten steel surface thermal insulation material, SiO ₂ content of not more than 10 wt%, a CaO / _Al 2 _{O 3} weight ratio of 0.5 to 2.0, and containing TiO ₂ 5 to 30 wt% It is the molten steel surface heat insulating material characterized by having carried out.

( 2 ) A continuous casting method for steel, wherein the molten steel surface heat insulating material described in ( 1 ) is added onto the molten steel surface in the tundish.

  According to the continuous casting method using the molten steel surface heat insulating material of the present invention, contamination of the molten steel is reduced and the casting quality is greatly improved. In addition, since the refractory is not melted and the heat insulating material is improved, the continuous casting method that is effective in terms of operation can be provided.

Conditions that should be satisfied as a heat insulating material covering the surface of the molten steel are to reliably prevent contamination of the molten steel due to the reaction between the air oxidation and the heat insulating material while ensuring the heat retaining property of the molten steel. As a result of studying the heat insulating material to satisfy these basic conditions, the present inventors are effective to reduce SiO _{2 in} order to prevent the reaction between the heat insulating material and molten steel, and to suppress air oxidation. Found that it is effective to contain a small amount of TiO ₂ after optimizing the mass ratio of CaO / Al ₂ O ₃ .

Since the SiO ₂ in the heat insulating material reacts with Al and Ti in the molten steel as shown in the following formulas 1 and 2, the heat insulating material can be reduced in SiO ₂ due to the presence of Al ₂ O ₃ type inclusions and titania type inclusions. Has the effect of preventing generation.

When the SiO ₂ content in the heat insulating material is 10% by mass or less, the reaction rate of the above formulas 1 and 2 rapidly decreases, and the Al ₂ O ₃ inclusions and titania inclusions that are substantially generated are reduced. Therefore, the SiO ₂ content needs to be 10% by mass or less. Since the reaction of Formula 1 and Formula 2 does not occur if no SiO ₂ is contained, the lower limit of SiO ₂ includes 0% by mass.

In addition, in order to prevent air oxidation of molten steel in the tundish, the temperature of the heat insulating material is lowered and the viscosity is also reduced, and the surface of the molten steel is uniformly coated with the heat insulating material to block air from entering the molten steel. It is important to. In order to lower the melting point of the heat insulating material, the CaO / Al ₂ O ₃ mass ratio needs to be in the range of 0.5 to 2.0, and the CaO / Al ₂ O ₃ mass ratio is less than 0.5 or 2 If it exceeds 0.0, the melting temperature of the heat insulating material is not lower than 1536 ° C., which is the melting point of steel. The melting temperature of the heat insulating material is the temperature at which the temperature of the heat insulating material tablet having a diameter of 10 mm × height of 10 mm is raised in a heating furnace and the height of the heat insulating material tablet is halved. Furthermore, when TiO ₂ is added to a heat insulating material mainly composed of CaO and Al ₂ O ₃ , the viscosity of the heat insulating material is lowered, and it is easier to spread on the surface of the molten steel, and a reliable air blocking by a uniform heat insulating material coating is possible. It becomes. In the present invention, the “main component” means that 50% by mass or more is contained.

A preferred range of addition of the TiO ₂ to the thermal insulation material composed mainly of the CaO and _Al 2 _{O 3} is 2.0 to 30 mass%. When the content of TiO ₂ is less than 2% by mass, the viscosity of the CaO—Al ₂ O ₃ heat insulating material is difficult to decrease, whereas when it exceeds 30% by mass, Al in the molten steel reacts with TiO ₂ to react with the Al ₂ O ₃ system. There is a risk that inclusions may be produced in a small amount. However, when the heat insulating material of the present invention is used in Ti deoxidized molten steel, the reaction between TiO _{2 in} the heat insulating material and Ti in the molten steel does not occur, so the upper limit of the TiO ₂ content need not be considered.

Although the basic composition of the heat insulating material is as described above, other oxides such as MgO and ZrO ₂ and non-oxides such as CaCl ₂ should be added as long as the function of the product of the present invention is not deteriorated. Is also possible.

When the heat insulating material of the present invention is added onto the surface of the molten steel in the tundish and the continuous casting of the steel is performed, it is possible to prevent air oxidation and molten steel contamination due to the reaction between the heat insulating material and the molten steel. It is also difficult to cause melting damage. Further, it is not necessary to lower the melting point by adding CaF ₂ as in the conventional case, and it is possible to prevent the tundish refractory from being rapidly melted by CaF ₂ . Furthermore, since the heat insulating material of the present invention has a low viscosity in the liquid phase, there is an advantage that the heat-dissipating property of the heat insulating material is greatly improved even when the tundish is repeatedly used in the hot state.

  Hereinafter, the present invention will be described with reference to examples and comparative examples, but the present invention is not limited thereto.

Example 1
Low carbon aluminum killed steel having a carbon concentration of 0.04 mass% by adding 400 kg of a heat insulating material of 3 mass% SiO ₂ , 32 mass% CaO, 60 mass% Al ₂ O ₃ , 5 mass% TiO _{2 to} a tundish with a capacity of 60 t. Was cast for 400 minutes. The molten steel temperature in the tundish was 1558 ° C. The casting dimensions were 245 mm thickness x 1500 mm width and cut to a length of 8500 mm to make one coil unit. This 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 air oxidation prevention effect and the contamination prevention effect of the heat insulating material were evaluated by the amount of increase in the total oxygen concentration in the molten steel on the entry side and the exit side of the tundish and the number of surface defects generated in the cold rolled steel sheet. As a result of the thermal insulation of the present invention preventing air oxidation and molten steel contamination by the thermal insulation, and suppressing the refractory melting, the total oxygen concentration on the tundish entry and exit sides is 0.0034% by mass and 0. At 0032% by mass, there was no increase in total oxygen concentration and no occurrence of surface defects.

(Example 2)
400 kg of heat retention material of ₂ % by mass SiO ₂ , 48% by mass CaO, 25% by mass Al ₂ O ₃ , 25% by mass TiO ₂ was added to a tundish with a capacity of 60 t, and an extremely low carbon titanium killed carbon having a carbon concentration of 0.003 mass%. The steel was cast for 400 minutes. In addition, the molten steel temperature in a tundish is 1560 degreeC. The casting dimensions were 245 mm thickness x 1500 mm width and cut to a length of 8500 mm to make one coil unit. This 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. As a result of preventing air oxidation and molten steel contamination due to the heat insulating material by the heat insulating material of the present invention and suppressing the refractory melt damage, the total oxygen concentration in the molten steel on the tundish inlet and outlet sides is 0.0034% by mass. At 0.0033 mass%, there was no increase in total oxygen concentration and no occurrence of surface defects.

(Comparative Example 1)
400 kg of a heat insulating material of 100% by mass MgO was added to a tundish having a capacity of 60 t, and a low carbon aluminum killed steel having a carbon concentration of 0.04% by mass was cast for 400 minutes. The casting dimensions were 245 mm thickness x 1500 mm width and cut to a length of 8500 mm to make one coil unit. This 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. Since MgO-based conventional heat insulating material was used, oxygen entered through the heat insulating material layer and reoxidation of the molten steel occurred. As a result, the total oxygen concentration in the molten steel from 0.0036% by mass to 0% from the tundish inlet side to the outlet side. The average number of surface defects was 6 / coil.

(Comparative Example 2)
400 kg of a heat insulating material of 20 mass% SiO ₂ and 80 mass% MgO was added to a tundish having a capacity of 60 t, and an ultra-low carbon aluminum killed steel having a carbon concentration of 0.0028 mass% was cast for 400 minutes. The casting dimensions were 245 mm thickness x 1500 mm width and cut to a length of 8500 mm to make one coil unit. This 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. Since MgO-based conventional heat insulating material containing SiO ₂ was used, molten steel was contaminated by reoxidation of molten steel by air and reaction of SiO ₂ and molten steel in the heat insulating material. The total oxygen concentration increased from 0.003 mass% to 0.006 mass%, and an average of 8 surface defects / coil was generated.

(Comparative Example 3)
400 kg of heat insulating material of 16% by mass SiO ₂ , 58% by mass CaO and 26% by mass Al ₂ O ₃ was added to a tundish having a capacity of 60 t, and ultra low carbon aluminum killed steel having a carbon concentration of 0.0028% by mass was cast for 400 minutes. The casting dimensions were 245 mm thickness x 1500 mm width and cut to a length of 8500 mm to make one coil unit. This 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. Since a high CaO-low Al ₂ O ₃ type heat insulating material containing a large amount of SiO ₂ was used, as a result of the reoxidation of the molten steel by air and the reaction between the SiO ₂ and the molten steel in the heat insulating material, molten steel contamination occurred. From the side to the outlet side, the total oxygen concentration in the molten steel increased from 0.0032 mass% to 0.0063 mass%, and an average of 8.3 surface defects / coil was generated.

Claims (2)

In the molten steel surface thermal insulation material, SiO ₂ content of not more than 10 wt%, a CaO / _Al 2 _{O 3} weight ratio of 0.5 to 2.0, and and containing TiO ₂ 5 to 30 wt% A molten steel surface heat insulating material characterized by that. A continuous casting method for steel, wherein the molten steel surface heat insulating material according to claim 1 is added onto the molten steel surface in the tundish.