JPH05293612A - Method for adjusting temperature of molten steel in tundish - Google Patents

Abstract

PURPOSE:To prevent the clogging of a tundish nozzle and to reduce large size inclusion defect in continuous casting of a steel. CONSTITUTION:At the time of adding ferro-alloy into molten steel in the tundish in order to lower the molten steel temp. in the continuous casting of the molten steel complex-deoxidized by adding Ca after Al deoxidizing, Al content in the ferro-alloy is adjusted to <0.1% and the ferro-alloy is added to prevent the nozzle clogging and the development of the inclusion defect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the temperature of molten steel in a tundish to prevent clogging of a tundish nozzle and produce a slab with a large number of inclusion defects in continuous casting of steel.

[0002]

2. Description of the Related Art In continuous casting of steel, the temperature of molten steel in the tundish is closely related to the center segregation of the continuously cast slab and inclusion defects. ing. For example, in order to reduce center segregation, low-temperature casting is applied because refinement of the solidification structure of the slab is effective, while from the viewpoint of reducing inclusions, floating separation in the tundish or in the mold. High temperature casting is applied for the purpose of promoting As a result, casting is generally performed within an appropriate temperature range that satisfies both conditions.

As a method for actively lowering the molten steel temperature, a method of adding a coolant to the molten steel in the tundish is used. As the type of the coolant in the coolant addition method, high carbon steel having a melting point lower than that of the molten steel to be cast is often used in order to accelerate the melting rate and facilitate melting.
For the same reason, as the shape, a granular, linear or strip-shaped material having a large specific surface area is used.

As a conventional known technique for controlling the molten steel temperature in the tundish, as disclosed in Japanese Patent Publication No. 54-21816, a method of continuously supplying metal filaments into the tundish, or Japanese Patent Laid-Open No. As disclosed in Japanese Patent Publication No. 14539, a method of adding a metal, a metal oxide or the like to molten steel in a tundish has been proposed.

[0005]

Generally, the temperature of molten steel in the tundish in continuous casting is high in the first half of the casting and decreases with the progress of casting, and sharply decreases after the end of pouring the ladle where the supply of molten steel is stopped. The average drop rate of the molten steel temperature in the tundish with the progress of casting is approximately 0.1 to 0.3 ° C./minute, although it depends on the heat size. For example, if the casting time is 60 minutes / heat, the temperature drop is about 6-18.
° C / heat.

Therefore, if low temperature casting is attempted from the initial stage of casting to reduce center segregation and the like, the temperature tends to drop further at the end of casting, and problems such as nozzle clogging are likely to occur. On the other hand, the basic idea of the molten steel temperature control method in the tundish by adding a coolant is to set the temperature at the beginning of casting high so that nozzle clogging does not occur at the end of casting, and Coolant is added and cooled. From this idea, a relatively large amount of coolant is added to the molten steel in the tundish in the early stage of casting. However, when a large amount of coolant is added, nozzle clogging may occur even if the molten steel temperature is not too low, which is a problem.

[0007]

The present invention is to solve the above-mentioned problems. That is, in the continuous casting of molten steel that has been deoxidized by Al and then compounded by adding Ca, when adding an iron alloy to the molten steel in the tundish for the purpose of lowering the molten steel temperature, the Al content in the iron alloy is The amount is adjusted to 0.1% or less and added.

Similarly, the present invention is an iron alloy for the purpose of lowering the molten steel temperature in the tundish in continuous casting of molten steel in which Al ≦ 0.005% or less and Si in the range of 0.01 to 0.30%. When adding A, A in the iron alloy
The content of 1 is adjusted to less than 0.1% and added.

[0009]

The present invention will be described in detail below. According to the measurement by the inventors, when an iron alloy containing 0.1% or more of Al is added as a coolant to the molten steel in the tundish as shown in FIG. 1, nozzle clogging (inclusions in the stopper nozzle portion) occurs. It was found that (adhesion) proceeded in proportion. Al
-In the casting of Ca complex deoxidized steel, a sample was taken from the molten steel in the tundish and the composition of inclusions was investigated. As a result, the composition of inclusions when no coolant was added was CaO-Al ₂
In contrast to the O ₃ -based low-melting-point oxide, when the coolant was added, the proportion of Al ₂ O ₃ -based inclusions in the total inclusions increased as the amount of coolant added increased. And
It has been known that this tendency remarkably occurs when Al in the iron alloy is contained in an amount of 0.1% or more and is not observed when Al is less than 0.1%.

From the above measurement results, the cause of nozzle clogging and the generation of large inclusion defects are caused by the reaction between Al in the iron alloy and the oxides unavoidably contained in the slag in the tundish ((1) below). considered to be due to formation of Al ₂ O ₃ by reaction with oxygen in the formula), or an atmosphere (the following equation (2)). 2Al + 3 (FeO) = Al ₂ O ₃ (s) + 3Fe (l) ... (1) 2Al + 3 / 2O ₂ (g) = Al ₂ O ₃ (s) ... (2)

Now, 0.1% Al is contained in the iron alloy,
When the amount added to the molten steel is 10 kg / T, the Al concentration in the molten steel increases by 0.001%, and then Al ₂ O ₃ is produced according to the equations (1) and (2). Newly formed Al ₂ O ₃
Is the composition of inclusions in the molten steel of low-melting CaO-Al ₂ O ₃
CaO-Al rich in Al ₂ O _{3 with} high melting point from oxides
_The change to ₂ O ₃ oxide causes nozzle clogging and defects of large inclusions.

Similarly, in the continuous casting of molten steel in which Al ≦ 0.005% or less and Si in the range of 0.01 to 0.30%, 0.1% or more of Al is contained in the iron alloy for cooling. In that case, the occurrence of nozzle clogging was observed with the addition of the iron alloy. This is because the addition of the iron alloy causes Al ₂ O ₃ to be newly generated by the reactions of the above formulas (1) and (2), which causes nozzle clogging. For the above reasons, in the present invention, the Al content in the iron alloy added to the molten steel in the tundish is specified to be less than 0.1%.

According to the present invention, it is possible to perform low temperature casting by adding a coolant to the molten steel in the tundish while preventing the occurrence of nozzle clogging and inclusion defects, and not only to reduce center segregation but also to stabilize nozzles without clogging. It is possible to secure the above casting and to manufacture a slab with few inclusion defects.

[0014]

EXAMPLES Examples of the present invention are shown below. When melting 260 tons / heat of mechanical structural steel (S45C) for steel bars using a converter and degassing equipment, Al deoxidation is performed to adjust the components and dehydrogenate, and then iron is coated on the molten steel in the ladle. Ca
Si alloy wire was added to perform Al-Ca composite deoxidation.
The chemical composition is 0.45% C-0.20% Si-0.80.
% Mn-0.015% P-0.020% S-0.030
% Al-10 ppm Ca.

The molten steel thus melted was cast by a 6-strand curved billet continuous casting machine having a slab cross section size of 162 mm × 162 mm. The casting speed was constant at 2.5 m / min, and the molten steel temperature in the tundish was 1,515 ° C (TD-Δ
(T = 25 ° C.), a chemical composition of 0.9% C-1.0% Si-0.
Particle size 1 consisting of 9% Mn-0.007% Al-balance Fe
Granulated iron of ~ 5 mmφ was cast while adding 1 to 10 Kg / T-Steel basic unit. This coolant has a melting point of 1,460 ° C., which is lower than the casting steel grade of 1,490 ° C. As shown in FIG. 1, the thickness of inclusions adhered to the nozzle portion due to the addition of this coolant was 2 mm or less, and stable low temperature casting could be carried out. As a result of measuring the obtained slab, both the center segregation and the inclusion level were good.

On the other hand, for comparison, the chemical composition is 0.9% C-1.0% Si-0.9% Mn-0.12%.
In the heat of casting granular iron consisting of Al-the balance Fe and having a grain size of 1 to 5 mm while adding the basic unit of 1 to 10 kg / T-Steel,
Although the molten steel temperature was lowered as in the case of the present invention, FIG.
Nozzle clogging progressed as shown in FIG. As a result of investigating the results of inclusions, it was confirmed that the inclusions deteriorate in proportion to the amount of coolant added.

Similarly, Al ≦ 0.005%, Si;
Si killed steel adjusted to 20% was melted and cast. For this steel type as well, in the case of adding a coolant having an Al content of less than 0.1%, the thickness of the inclusions adhered to the nozzle portion was within a negligible range, and stable low temperature casting could be carried out, and at the same time, The obtained slab also had good center segregation and inclusion levels. On the other hand, for comparison, Al
In the heat to which the iron alloy containing 0.1% or more is added,
In some cases, the Al value in molten steel exceeds 0.005%,
Progression of nozzle clogging and deterioration of inclusion level were observed.

[0018]

The method for controlling the temperature of molten steel in a tundish according to the present invention described above is extremely effective in stabilizing casting and improving quality.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a nozzle unit clogging and a basic unit for adding a coolant to molten steel in a tundish.

Claims (2)

[Claims] 1. In continuous casting of molten steel deoxidized by Al and then complex deoxidized by adding Ca, when an iron alloy for lowering the molten steel temperature is added to the molten steel in a tundish, the Al in the iron alloy is added. A method for controlling the temperature of molten steel in a tundish, which is characterized in that the content is adjusted to less than 0.1% and added. 2. As the deoxidizing element, Al ≦ 0.005%,
In continuous casting of molten steel with Si adjusted to the range of 0.01 to 0.30%, when adding an iron alloy that lowers the molten steel temperature into the molten steel in the tundish, the Al content in the iron alloy is set to 0. A method for controlling the temperature of molten steel in a tundish, which is characterized by adding less than 1%.