EP4721896A1

EP4721896A1 - Mold powder for continuous casting and method for producing steel

Info

Publication number: EP4721896A1
Application number: EP24856152.4A
Authority: EP
Inventors: Hiroyuki Ohno; Tomoya Odagaki; Shosei Ichiseki; Wataru Todo; Ryusuke Kishimoto; Makoto Fujimura; Shohei ONO
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-08-24
Filing date: 2024-07-04
Publication date: 2026-04-08
Also published as: JP7640981B1; TW202508735A; WO2025041457A1; CN121586621A; JP2025036126A; EP4721896A4; KR20260006655A

Abstract

Provided is a mold powder for continuous casting that enables production of high-quality cast steel and prevention of operational problems when continuously casting high-Al steel. The mold powder for continuous casting has a chemical composition including at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂, aggregate carbon, Al₂O₃, fluorine compound, and Li₂O, with the balance being inevitable impurity. Al₂O₃ content is 25 mass% to 35 mass%, total content of Ca, Ba, Mg, La, and Zr as oxide equivalent is 40 mass% to 60 mass%, F content is 3 mass% to 10 mass%, Li content as Li₂O equivalent is 6 mass% to 12 mass%, total content of the inevitable impurity is 0 mass% to 2.0 mass%, and Y, expressed as -12.7 × (%C) + 51.8, is 5 to 35.

Description

TECHNICAL FIELD

The present disclosure relates to a mold powder for continuous casting and a method of producing steel.

BACKGROUND

When molten steel is continuously cast using a continuous casting machine, first, the molten steel is poured from a ladle through a tundish into a mold, and an initial solidification shell is formed in the mold. Then, in a subsequent secondary cooling zone, the molten steel is cooled and solidification progresses to the interior. During casting, mold powder for continuous casting (hereinafter sometimes referred to as powder) is constantly being introduced into the mold, and the powder in a molten state penetrates between the mold and the solidified shell. In continuous casting, the powder plays a role in improving lubrication between the solidified shell and the mold and in maintaining the temperature of the molten steel.
Typically, such powder is composed mainly of CaO and SiO₂, and mixed with components such as Al₂O₃, Na₂O, fluorine compounds, and C depending on the desired properties. Here, SiO₂ is added for the purpose of lowering the melting point and promoting vitrification. However, when Al concentration in the molten steel is high, a thermite reaction occurs during continuous casting, and oxides in the powder, particularly SiO₂ and the like, are reduced by the Al in the molten steel. Here, the thermite reaction between SiO₂ and Al is indicated by the following expression (1).

4[Al] + 3(SiO₂) → 2(Al₂O₃) + 3[Si] ... (1)
Here, [Al] and [Si] refer to components in the molten steel, and (SiO₂) and (Al₂O₃) refer to components in the powder.
The thermite reaction increases the Al₂O₃ concentration in the powder. It is known that an increase in the Al₂O₃ concentration in the powder causes various problems. For example, when continuous casting is carried out using powder containing CaO and SiO₂, an increase in the amount of Al₂O₃ causes the formation of 2CaO·Al₂O₃·SiO₂ (Gehlenite), which has a high melting point, in the powder. Gehlenite significantly decreases the lubrication of the powder, increasing the risk of an operational problem known as breakout, in which the initial solidification shell breaks during casting, causing molten steel to leak out. Further, as the Al₂O₃ content increases, the melting point increases, and a sintered mass of powder known as slag rim is produced on the mold. When the slag rim becomes coarse, it pushes into the initial solidification shell, which can cause depressions on the cast steel surface.
In view of this background, various powders have been proposed for continuous casting of high-Al steel, with the aim of producing high-quality cast steel and preventing operational problems.
For example, in Patent Literature (PTL) 1, a mold powder for continuous casting of steel is proposed that has an F content of 16 mass% to 25 mass%, a mass ratio of CaO to SiO₂ (CaO/SiO₂) of 1.0 to 1.8, an Al₂O₃ content of 5 mass% or less (including zero), and a MgO content of 1.5 mass% or less (including zero). This powder is said to be able to prevent restrictive breakouts and enlargement of slag rim caused by large composition variation. Here, the composition variation refers to a decrease in SiO₂ and an increase in Al₂O₃ in the molten slag.
Further, in PTL 2, a mold powder for continuous casting is proposed that contains CaO: 10 wt% to 35 wt%, Al₂O₃: 10 wt% to 35 wt%, TiO₂: 3 wt% to 15 wt%, Li₂O: 3 wt% to 20 wt%, BaO: 5 wt% to 40 wt%, F: 15 wt% or less, Na₂O: 20 wt% or less, and further contains one or more of BN: 0.5 wt% to 4.0 wt% or C: 0.5 wt% to 4.0 wt% as aggregate, with the balance being inevitable impurity. This powder is said to be able to prevent degradation of cast steel surface quality and breakouts caused by poor lubrication (alteration) of the powder when continuously casting steel containing slag-reducing metal elements.

CITATION LIST

Patent Literature

PTL 1: JP 2017-170494 A
PTL 2: JP H05-185195 A

SUMMARY

(Technical Problem)

The powders described in the above Patent Literature have the following problems.
For example, the powder described in PTL 1 contains SiO₂, and therefore a thermite reaction occurs when used. The thermite reaction not only causes an adverse effect due to an increase in the Al₂O₃ concentration in the powder, but also causes an adverse effect due to heat generation. First, the solidification of the molten steel is delayed at the meniscus, causing the solidified shell to break (bleed) along the oscillation mark. This impairs the surface quality of the cast steel. Further, the occurrence of flame can lead to equipment failure and stopping of operations.
Further, the powder described in PTL 2 does not contain SiO₂ but does contain Na₂O and TiO₂, and a thermite reaction proceeds between these components and Al in the molten steel. Therefore, with the increase in heat generation and Al₂O₃ concentration, operational trouble occurs and the quality of the cast steel degrades.

(Solution to Problem)

We thus provide:

1. A mold powder for continuous casting that has a chemical composition comprising (consisting of):
- aggregate carbon,
- Al₂O₃,
- at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂,
- fluorine compound, and
- Li₂O,
- with the balance being inevitable impurity, wherein
- Al₂O₃ content is 25 mass% to 35 mass%,
- total content of Ca, Ba, Mg, La, and Zr as oxide equivalent is 40 mass% to 60 mass%,
- F content is 3 mass% to 10 mass%,
- Li content as Li₂O equivalent is 6 mass% to 12 mass%,
- total content of the inevitable impurity is 0 mass% to 2.0 mass%, and
- when content of the aggregate carbon in mass% is represented as (%C), Y, as indicated in the following expression (2), is 5 or more and 35 or less, $Y = - 12.7 \times (% C) + 51.8$
2. The mold powder for continuous casting according to 1, above, wherein the chemical composition further comprises at least one carbonate of an element selected from the group consisting of
Li, Ca, Ba, Mg, La, and Zr.
3. The mold powder for continuous casting according to 1 or 2, above, wherein a maximum particle size of the mold powder is 250 µm or less.
4. The mold powder for continuous casting according to any one of 1 to 3, above, wherein 50 mass% or more of the mold powder is made of pre-melt raw material.
5. A method of producing steel, the method comprising continuous casting of steel containing 0.5 mass% or more of Al using the mold powder for continuous casting according to any one of 1 to 4, above.
6. The method of producing steel according to 5, above, wherein powder consumption per tonne of molten steel is 0.4 kg or more.

(Advantageous Effect)

According to the present disclosure, it is possible to provide a mold powder for continuous casting that enables production of high-quality cast steel and prevention of operational problems when continuously casting high-Al steel. Further, it is possible to provide a method of producing steel using the powder.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a graph illustrating a relationship between molten layer thickness and number of temperature reversals between an upper thermocouple and a lower thermocouple; and
FIG. 2 is a graph illustrating a relationship between the molten layer thickness and number density of powder defects.

DETAILED DESCRIPTION

The present disclosure is described in detail below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.
As described above, when Al₂O₃ concentration in powder increases, Gehlenite that has a high melting point is formed and slag rim becomes coarse, which impairs operational stability and reduces the surface quality of cast steel. Therefore, in order to obtain stable productivity and quality in the continuous casting of high-Al steel, it is desirable that an increase in the Al₂O₃ concentration of the powder during casting be small.
Factors that cause the Al₂O₃ concentration in the powder to increase during casting include oxidation of Al in the molten steel by oxides in the powder and oxidation of Al in the molten steel by air. Here, it is difficult to prevent oxidation by air, and therefore in order to make the increase in the Al₂O₃ concentration small, it is necessary to prevent as much as possible the oxidation of Al in the molten steel by oxides in the powder. Further, in order to produce steel without causing problems related to operation and quality, it is necessary to set conditions for producing powder and producing steel, taking into consideration that the amount of Al₂O₃ in the powder increases to some extent during casting.
As a result of investigations with the above perspective, the inventors have focused on a solidification start temperature after the powder undergoes a composition change during casting. The inventors reached the conclusion that it is preferable to lower the solidification start temperature after the composition change (hereinafter, the solidification start temperature after the component change may be referred to simply as the solidification start temperature). In order to lower the solidification start temperature, the powder chemical composition and steel production conditions are important.
The chemical composition of the powder according to an embodiment of the present disclosure is described below.
First, the content of each component is described. Hereinafter, the content of each component is a normalized value such that the sum of the content of the metal element as oxide equivalent and the content of elements excluding the metal element and O (oxygen) is 100 %. The above normalization is applied to total C content, the Al₂O₃ content, content of Ca, Ba, Mg, La, and Zr as oxide equivalent, F content, Li content as Li₂O equivalent, aggregate carbon content, and content of inevitable impurity.
For example, in the case of a mold powder whose component elements are C, Al, Ba, Ca, Li, Mg, F, and O, the content is determined as follows.
First, the content of each component other than O in 100 mass% of the mold powder (%T.C', %Al', %Ba', %Ca', %Li', %Mg', %F') is measured by the method described below.
Next, using %Al', %Ba', %Ca', %Li', and %Mg', the content of Al, Ba, Ca, Li, and Mg as oxide equivalent (%Al₂O₃', %BaO', %CaO', %Li₂O', %MgO') is determined.
Next, a coefficient (100 (mass%) ÷ A (mass%)) is calculated from a total A (mass%) of the content of Al, Ba, Ca, Li, and Mg as oxide equivalent, the C content, and the F content, and the content of each component is normalized by multiplying the content of each component by the coefficient. Specifically, the content of each component is normalized by the following expressions. $A = % T . C' + {%Al}_{2} O_{3}' + % BaO' + % CaO' + {%Li}_{2} O' + % MgO' + % F'$ $% T . C = % T . C' \div A \times 100$ ${%Al}_{2} O_{3} = {%Al}_{2} O_{3}' \div A \times 100$ $% BaO = % BaO' \div A \times 100$ $% CaO = % CaO' \div A \times 100$ ${%Li}_{2} O = {%Li}_{2} O' \div A \times 100$ $% MgO = % MgO' \div A \times 100$ $% F = % F' \div A \times 100$
Here, %T.C and %F are the total C content and the F content, respectively, after normalization. Further, %Al₂O₃, %BaO, %CaO, %Li₂O, and %MgO are the Al₂O₃ content, the Ba content as BaO equivalent, the Ca content as CaO equivalent, the Li content as Li₂O equivalent, and the Mg content as MgO equivalent, respectively, after normalization. When the aggregate carbon content after normalization is %C and the aggregate carbon content in 100 mass% of the mold powder is %C', the following expression is also established. $% C = % C' \div A \times 100$
The same applies when the molding powder further contains inevitable impurity. For example, when the inevitable impurity is composed of SiO₂ and the SiO₂ content in 100 mass% of the mold powder is %SiO₂', the following expression is used instead of the above expression (3-1). A = %T.C' + %Al₂O₃' + %BaO' + %CaO' + % Li₂O' + %MgO' + %F' + %SiO₂'
Further, the content %Im of inevitable impurity matches the SiO₂ content after normalization (%SiO₂) and is determined by the following expression. $% Im = {%SiO}_{2} = {%SiO}_{2}' \div A \times 100$
Reasons for the limitation of each element are described below.

[Aggregate carbon]

The aggregate carbon plays a role in adjusting the melting rate of the powder. Examples of aggregate carbon include, but are not limited to, carbon black and coke powder. The aggregate carbon content is described later.
When the total C content is low, the effect as aggregate cannot be obtained. From this viewpoint, the total C content is preferably 1.3 % or more. On the other hand, when the total C content exceeds 5.0 %, then when a thermite reaction occurs, the C burns and a flame is generated, which may cause equipment failure and operation shutdown. Therefore, from the viewpoint of avoiding operational trouble, the total C content is preferably 5.0 % or less. Here, the total C content can be determined as the sum of the aggregate carbon content and the C content derived from carbonates, described below. The carbon derived from carbonates is considered to be the sum of carbon derived from carbonates remaining in the pre-melt raw material, described later, and carbon derived from carbonates added other than the pre-melt raw material. The total C content can be measured by a combustion method.

[Al₂O₃]

Al₂O₃ is added to increase the activity of Al₂O₃ in the molten powder and to suppress the oxidation reaction of Al in the molten steel. When the Al₂O₃ content is less than 25 %, such an effect cannot be obtained. The Al₂O₃ content is therefore 25 % or more. The Al₂O₃ content is preferably 28 % or more. The Al₂O₃ content may be 25.0 % or more, and may be 28.0 % or more. On the other hand, when the Al₂O₃ content is higher than 35 %, the solidification start temperature of the powder becomes very high, taking into account that the Al₂O₃ content will further increase during casting. The Al₂O₃ content is therefore 35 % or less. The Al₂O₃ content is preferably 33 % or less. The Al₂O₃ content may be 35.0 % or less, and may be 33.0 % or less. The Al₂O₃ content is determined by converting the Al content determined by fluorescence X-ray analysis into Al₂O₃.

[At least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂]

As a result of investigations by the inventors, it has been found that the solidification start temperature of the powder can be lowered by including at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂. This is believed to be because of the following reasons. First, the oxides all have a standard free energy of formation at 1300 °C lower than that of Al₂O₃. An average temperature of the powder in the molten state poured into the mold is 1300 °C. Here, oxides that have a standard free energy of formation lower than that of Al₂O₃ at 1300 °C hardly react with Al in molten steel, and therefore an increase in the Al₂O₃ concentration in the powder during casting is suppressed. Further, the oxides forms complex oxides with Al₂O₃, lowering the solidification start temperature of the powder. Therefore, in order to suppress an increase in the Al₂O₃ concentration and lower the solidification start temperature of the powder, it is necessary to include an oxide whose standard free energy of formation at 1300 °C is lower than that of Al₂O₃. For the above reasons, in an embodiment of the present disclosure, the powder contains at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂.

[Total content of Ca, Ba, Mg, La, and Zr as oxide equivalent: 40 mass% to 60 mass%]

When the total content of Ca, Ba, Mg, La, and Zr as oxide equivalent is less than 40 %, the above-mentioned effects of suppressing the increase in the Al₂O₃ concentration and lowering the solidification start temperature of the powder cannot be obtained. The total content is therefore 40 % or more. The total content is preferably 45 % or more. The total content may be 40.0 % or more, and may be 45.0 % or more. On the other hand, when the total content exceeds 60 %, the content of the other components is decreased instead, and the effects of including the other components cannot be obtained. The total content is therefore 60 % or less. The total content is preferably 55 % or less. The total content may be 60.0 % or less, and may be 55.0 % or less. The total content is calculated by determining the content of Ca, Mg, La, and Zr elements by X-ray fluorescence analysis, determining the content of the Ba element by ICP emission spectrometry, converting into CaO, MgO, La₂O₃, ZrO₂, and BaO, respectively, and then summing together.
Among the above oxides, CaO, BaO, and MgO are advantageous in terms of cost, and can more efficiently form a liquid phase at a low temperature even when the content of Al₂O₃ in the powder increases. From this viewpoint, it is preferable that the oxides include at least one component selected from the group consisting of CaO, BaO, and MgO. In this case, the total content of Ca, Ba, and Mg as oxide equivalent is preferably 40 % or more. The total content is more preferably 45 % or more. The total content may be 40.0 % or more, and may be 45.0 % or more. On the other hand, the total content is preferably 60 % or less. The total content is more preferably 55 % or less. The total content may be 60.0 % or less, and may be 55.0 % or less.
An upper limit and a lower limit of the content of Ca and Ba, respectively, are not particularly limited. However, the Ca content as CaO equivalent may be 20.0 % or more. The Ca content as CaO equivalent may be 45.0 % or less. The Ba content as BaO equivalent may be 10.0 % or more. The Ba content as BaO equivalent may be 35.0 % or less.
An upper limit and a lower limit of the content of Mg, La, and Zr, respectively, are not particularly limited. However, these elements have an effect of lowering the melting point. Therefore, when Mg is contained, the Mg content as MgO equivalent is preferably 1.0 % or more. When La is contained, the La content as La₂O₃ equivalent is preferably 0.5 % or more. When Zr is contained, the Zr content as ZrO₂ equivalent is preferably 0.5 % or more. On the other hand, when contained in excess, the melting point increases. Accordingly, the Mg content as MgO equivalent is preferably 5.0 % or less. The La content as La₂O₃ equivalent is preferably 3.0 % or less. The Zr content as ZrO₂ equivalent is preferably 3.0 % or less.

[Fluorine compound]

The fluorine compound is added to lower the solidification start temperature of the powder and viscosity, described later. The fluorine compound is not particularly limited, and examples include fluorides such as CaF₂, LiF, and BaF₂. At least one selected from the group consisting of CaF₂, LiF, and BaF₂ can be used, but from the viewpoint of cost, it is preferable to use CaF₂.

[F content: 3 mass% to 10 mass%]

When the F content of the powder is less than 3 %, the above-mentioned effects of the fluorine compound cannot be obtained. The F content of the powder is therefore 3 % or more. The F content may be 3.0 % or more. On the other hand, when the F content of the powder exceeds 10 %, not only does the effect saturate, but the risk of oxidation and degradation of the continuous casting machine increases dramatically. The F content of the powder is therefore 10 % or less. The F content may be 10.0 % or less. The F content can be measured by absorption photometry.

[Li₂O]

Li₂O has a higher standard free energy of formation at 1300 °C than Al₂O₃, but is an oxide that is relatively difficult to reduce with Al, and is also effective in lowering the solidification start temperature, and is therefore added.

[Li content as Li₂O equivalent: 6 mass% to 12 mass%]

When the Li content as Li₂O equivalent is less than 6 %, the effect of lowering the solidification start temperature is not exhibited. The Li content as Li₂O equivalent is therefore 6 % or more. The Li content as Li₂O equivalent is preferably 8 % or more. The content may be 6.0 % or more, and may be 8.0 % or more. On the other hand, when the Li content as Li₂O equivalent exceeds 12 %, Li₂O is easily reduced by Al in the molten steel, resulting in an increase in Al₂O₃, and therefore the effect of increasing the solidification start temperature due to the increase in Al₂O₃ exceeds the effect of decreasing the solidification start temperature due to the addition of Li₂O. The Li content as Li₂O equivalent is therefore 12 % or less. The Li content as Li₂O equivalent is preferably 11 % or less. The content may be 12.0 % or less, and may be 11.0 % or less. The content is calculated by determining the content of Li element by ICP emission spectrometry and conversion into Li₂O.

[Carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr]

The powder may include a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr. This is for the following reasons. In the production of the powder, the chemical composition of intermediate raw material such as pre-melt raw material may deviate from a target. In such a case, the content of an element can be adjusted by mixing the carbonate with the intermediate raw material. Here, even when elements such as Li, Ca, Ba, Mg, La, and Zr are contained as carbonates, equivalent effects as when oxides of the elements are contained can be obtained.
When elements such as Ca, Ba, Mg, La, and Zr are contained as carbonates, the total content of the elements as oxide equivalent is calculated by adding together the content of the oxides of the elements and the content obtained by converting the carbonates of the elements into oxides. Further, when Li carbonate is contained, the Li content as Li₂O equivalent is calculated by adding up the Li₂O content and the content obtained by converting Li₂CO₃ into oxide.
Here, the carbonate may be a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, and Mg, and may be a carbonate of at least one element selected from the group consisting of Li and Ba.
The total content of the carbonate is not limited as long as the total content of Ca, Ba, Mg, La, and Zr as oxide equivalent and the content of Li as Li₂O equivalent are satisfied. However, by decreasing the total content of the carbonate, the contents of other components can be increased, and changes in melting property during casting due to carbon dioxide gas can be suppressed. Further, the generation of slag rim can be further suppressed. The total content of the carbonate in the powder is therefore preferably 10 % or less. The total content of the carbonate in the powder is more preferably 5 % or less. On the other hand, a lower limit of the total content of the carbonate is not particularly limited, and may be 0 %, meaning the carbonate need not be contained. From the same viewpoint, the content of Li carbonate in the powder, as Li₂O equivalent, is preferably 10 % or less. A lower limit of the content is not limited and may be 0 %. From the same viewpoint, it is preferable that the content of each of the elements Ca, Ba, Mg, La, and Zr in the powder is 10 % or less as oxide equivalent of the carbonate of the element. The lower limit of the content of each element is not particularly limited and may be 0 %.
Further, from the same viewpoint, the proportion of the content of Li carbonate as Li₂O equivalent to the content of Li as Li₂O equivalent is preferably 50 % or less. A lower limit of the proportion is not particularly limited, and may be 0 %, meaning the Li carbonate need not be contained. From the same viewpoint, for each of the elements Ca, Ba, Mg, La, and Zr, it is preferable that the proportion of the content of the carbonate of the element, as oxide equivalent, to the content of the element, as oxide equivalent, is 50 % or less. A lower limit of the proportion is not particularly limited, and may be 0 %, meaning the carbonate of each element need not be contained.
The chemical composition of the powder according to an embodiment of the present disclosure has been described. The powder according to an embodiment of the present disclosure contains the components described above, with the balance being inevitable impurity.

[Total content of inevitable impurity: 0 mass% to 2.0 mass%]

The inevitable impurity include oxides (excluding Li₂O) having a standard free energy of formation at 1300 °C higher than that of Al₂O₃. Examples of the oxides include SiO₂, Na₂O, TiO₂, and Fe₂O₃. The oxides may unavoidably be mixed into the powder raw material, and therefore it is difficult to completely remove them. However, in order to prevent the thermite reaction with Al in the molten steel, it is necessary to avoid intentionally adding the oxides and to keep the content as low as possible. The total content of the inevitable impurity is therefore 2.0 % or less.

[Y: 5 or more and 35 or less]

According to the present disclosure, it is necessary to determine the chemical composition of the powder so that Y, as indicated in the expression (2), is 5 or more and 35 or less.
First, a molten layer thickness of the powder during casting is described. When the molten layer thickness of the powder is less than 5 mm, flow between the mold and the solidified shell becomes unstable, resulting in a localized powder supply shortage and increasing the risk of the molten steel coming into direct contact with the mold, a phenomenon known as sticking.
On the other hand, when the molten layer thickness exceeds 35 mm, the following problems arise.
First, sensor failure makes it difficult to control the molten metal surface position. Normally, a continuous casting machine includes a sensor for detecting the molten metal level in an upper portion of the mold to control the molten metal level. When the molten layer thickness becomes too thick, the temperature of the sensor body increases, increasing the risk of sensor failure. In continuous casting machines, it is common to control the amount of molten steel poured into the mold by detecting the level of the molten steel, but when the sensor fails, automatic control of the molten steel level stops and operations come to a halt.
Further, quality problems can occur when the powder gets entrapped in the molten steel. Normally, the molten powder floats on top of the molten steel, with unmelted powder sitting on top of that. However, when the molten layer thickness becomes too thick, solid phase that precipitates from the molten powder sinks within the molten powder, increasing the risk of entrapment in the molten steel.
As a result of investigations by the inventors, these phenomena are observed only when the molten layer thickness exceeds 35 mm, and therefore the molten layer thickness needs to be controlled to 35 mm or less.
Here, the molten layer thickness of the powder can be controlled by the melting rate of the powder. That is, when the melting rate is slow, the molten layer thickness of the powder is thin, and conversely, when the melting rate is fast, the molten layer thickness of the powder is thick. Further, as mentioned above, the melting rate of the powder can be adjusted by the amount of aggregate added to the powder. The inventors investigated the relationship between the molten layer thickness Y (unit: mm) and the aggregate carbon content %C (unit: mass%) for powders in the composition range of the present disclosure, and found that the expression (2) holds true in an error range of 10 %.
Generally, the molten layer thickness and the melting rate of the powder can be adjusted by changing the melting points of the powder and the components other than the aggregate. However, when other chemical compositions are taken into consideration, it may be considered that the molten layer thickness and the melting rate of the powder of the present disclosure can be adjusted only by the content of aggregate carbon. That is, according to the present disclosure, by appropriately using a powder having a defined chemical composition, it is possible to control the solidification start temperature of the powder between the mold and the solidified shell. Here, the solidification start temperature is determined by the composition of the powder after composition change. On the other hand, the molten layer thickness of the powder on the molten steel is determined by the composition before the composition change. Therefore, from the viewpoint of controlling the composition both before and after the composition change, it may be considered that the molten layer thickness and the melting rate of the powder can be adjusted only by the content of aggregate carbon.
Therefore, when the content of the aggregate carbon in mass% is represented as (%C), Y, as indicated in the expression (2), is 5 or more and 35 or less. This makes it possible to produce high-quality cast steel and prevent operational problems. Y is preferably 10 or more. Further, Y is preferably 30 or less. Here, Y is a value rounded to the nearest whole number. Further, the aggregate carbon content is preferably 1.3 % or more. The aggregate carbon content is more preferably 1.5 % or more. The aggregate carbon content is preferably 3.7 % or less. The aggregate carbon content is more preferably 3.5 % or less.
As described above, the powder according to an embodiment of the present disclosure has a chemical composition comprising:

aggregate carbon,
Al₂O₃,
at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂,
fluorine compound, and
Li₂O,

Al₂O₃ content is 25 mass% to 35 mass%,
total content of Ca, Ba, Mg, La, and Zr as oxide equivalent is 40 mass% to 60 mass%,
F content is 3 mass% to 10 mass%,
Li content as Li₂O equivalent is 6 mass% to 12 mass%,
total content of the inevitable impurity is 0 mass% to 2.0 mass%, and
when content of the aggregate carbon in mass% is represented as (%C), Y, as indicated in the following expression (2), is 5 or more and 35 or less.

Further, the chemical composition may further include:
carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr.
The powder according to an embodiment of the present disclosure can be produced, for example, by mixing raw material so as to have the chemical composition described above. Here, the raw material may be composed of aggregate carbon, Al₂O₃, at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂, a fluorine compound, and Li₂O. The raw material may further contain a carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr. Some or all of the raw material may consist of pre-melt raw material. Further, the particle size of the powder may be adjusted by techniques such as grinding or classification into particle sizes.

[Pre-melt raw material]

Pre-melt raw material refers to material that has been melted and ground in advance. As mentioned above, carbonates such as Li₂CO₃ and BaCO₃ may be added as raw material of the powder. However, when carbonate is added, gas is generated when the powder melts during casting, which may have adverse effects on the quality of the cast steel, such as pinhole defects and inclusion entrapment defects, or may have adverse effects on operation, such as melting of the submerged entry nozzle. The pre-melt raw material is used to decrease such effects and form a stable molten layer. Therefore, in the powder according to an embodiment of the present disclosure, preferably 30 mass% or more of the powder is made of pre-melt raw material. More preferably 50 mass% or more of the powder is made of pre-melt raw material. On the other hand, an upper limit of the proportion of the pre-melt raw material is not particularly limited, and may be 100 mass%. However, in consideration of adding of aggregate carbon, it is preferable that 99.5 mass% or less is the pre-melt raw material. It is more preferable that 90 mass% or less is the pre-melt raw material.
The powder according to an embodiment of the present disclosure can be produced by adding additional material with the pre-melt raw material in order to adjust the chemical composition of the powder. For example, when Al₂O₃ is insufficient, Al₂O₃ may be added. When CaO is insufficient, CaF₂ or CaO may be added. When the F content is insufficient, CaF₂ may be added. When BaO is insufficient, BaCO₃ may be added. When Li₂O is insufficient, Li₂CO₃ may be added.
Here, aggregate carbon cannot be used as normal pre-melt raw material. That is, the proportion of the pre-melt raw material to the total amount of aggregate carbon may be 0 %.
When carbonate is added after pre-melt to adjust the composition, the total C content increases, which decreases the content of other components, and the molten state changes due to the influence of carbon dioxide gas. This may also result in the generation of dust and flames. In other words, by decreasing the mix proportion of carbonate, it is possible to further improve operability and quality. Therefore, with respect to the carbonate of at least one element selected from the group consisting of Li, Ca, Ba, Mg, La, and Zr, the proportion of the carbonate other than the pre-melt raw material relative to the total amount of the powder is preferably 20 % or less. The proportion is more preferably 10 % or less. A lower limit of the proportion is not particularly limited, and may be 0 %.

[Particle size]

The use of finely ground powder makes it easier for the powder to melt during casting, forming a stable molten layer. The powder according to an embodiment of the present disclosure therefore preferably has a maximum particle size of 250 µm or less. Here, the maximum particle size of 250 µm or less means that all particles pass through a No. 60 mesh (aperture 250 µm) standard sieve (The Iida Testing Sieve, produced by Sieve Factory Iida Co., Ltd.) as specified in JIS Z 8801. That is, it is preferable that the powder has a particle size that allows all of the powder to pass through a No. 60 mesh sieve.

[Method of producing steel]

The following describes a method of producing steel according to an embodiment of the present disclosure. The method of producing steel according to an embodiment of the present disclosure is a method of continuous casting of steel containing 0.5 mass% or more of Al using the powder described above.
When the powder is used in the continuous casting of high-Al steel, occurrence of the thermite reaction can be minimized, and high-Al steel of stable quality can be produced with high productivity. Therefore, the steel subjected to continuous casting by the method of producing steel according to an embodiment of the present disclosure contains Al in an amount of 0.5 mass% or more. The steel preferably contains Al in an amount of 1.0 mass% or more. In other words, the method of producing steel according to an embodiment of the present disclosure is a method of continuously casting steel containing 0.5 mass% or more of Al. The method is preferably a method of continuously casting steel containing 1.0 mass% or more of Al.

[Solidification start temperature]

In continuous casting, in order to produce high-quality cast steel while preventing operational problems, it is advantageous to lower the solidification start temperature of the powder during casting. Lowering the solidification start temperature results in higher quality cast steel and further decreases the risk of operational problems.
The average temperature of the molten powder during casting is 1300 °C. The powder therefore preferably has a solidification start temperature of 1300 °C or lower. In other words, it is preferable to control the powder so that it becomes a completely liquid phase at 1300 °C during casting.

[Viscosity]

Further, according to the present disclosure, in order to produce high-quality cast steel, it is preferable to decrease the viscosity of the powder at 1300 °C during casting (hereinafter, the viscosity of the powder at 1300 °C during casting may be referred to simply as viscosity). This is because lowering the viscosity at 1300 °C, which is the average temperature in the molten state, secures uniform flow when the powder is added, and decreases surface roughness of the cast steel. From this viewpoint, it is preferable to control the viscosity to 10 poise or less. Note that 1 poise = 0.1 Pa·s.
The solidification start temperature and the viscosity are measured as follows. First, the molten powder is collected during casting. In the early stages of casting, the powder composition is changing and the measured values fluctuate, so the sampling is carried out just before the end of the casting when the powder composition change has reached a steady state.
The solidification start temperature and the viscosity are then measured. The solidification start temperature can be measured using differential thermal analysis. Specifically, a sample is cooled from a molten state at a constant cooling rate, the differential heat peak is measured using differential thermal analysis, and the starting point on the high temperature side of the peak (peak start temperature) is taken as the solidification start temperature. Further, the viscosity can be measured by a rotational viscometer.

[Molten layer thickness]

As described above, in the continuous casting of high-Al steel, the molten layer thickness is preferably 5 mm or more. The molten layer thickness is preferably 35 mm or less. The molten layer thickness can be measured, for example, by inserting a copper wire into the powder, removing it, and then observing the state of deposits.

[Powder consumption]

In the method of producing steel according to an embodiment of the present disclosure, the powder is added to the meniscus when molten steel that has been melted to have the Al content described above is poured from a ladle via a tundish into a mold. After the powder is supplied to the meniscus, the powder becomes molten, penetrates into the gap between the mold and the solidified shell, and is withdrawn from the rear of the mold together with the cast steel. During the continuous casting process, the molten steel and the powder come into contact with each other at the meniscus, and only while this contact is occurring does the Al in the molten steel react with the powder. Therefore, when the powder consumption is small, the reaction time with molten steel per unit amount of powder becomes long, and the Al₂O₃ concentration in the powder increases, resulting in a higher solidification start temperature. Further, when powder consumption is low, the increase in Al₂O₃ leads to higher viscosity, which at 1300 °C leads to higher viscosity.
Here, the reaction between the powder and molten steel does not reach equilibrium instantaneously, but changes gradually, and the components in the powder saturate at a certain value lower than the equilibrium value. Here, the balance equation for component i in the powder is expressed as follows using powder consumption: $W \cdot ({dX}_{i} / dt) = Q_{P} \cdot (X_{i, 0} - X_{i}) + k_{i} \cdot A \cdot (X_{i, E} - X_{i})$

W: weight of the molten powder layer (kg)
X_i: concentration of component i in the powder
X_i,0 : initial concentration of component i in the powder
X_i,E: equilibrium concentration of component i between molten steel and powder
t: time (s)
Q_P: powder consumption (kg/s)
A: reaction area between molten steel and powder (m²)
k_i: reaction rate constant of component i (kg/m²/s)

When the concentration of component i no longer changes (dX_i/dt = 0), the concentration X_i of component i can be expressed as follows: $X_{i} = (Q_{P} X_{i, 0} + k_{i} \cdot A \cdot X_{i, E}) / (Q_{P} + k_{i} \cdot A)$
From the above expression, it can be seen that as the powder consumption Q_P increases, X_i decreases. In other words, when component i is Al₂O₃, increasing the powder consumption dilutes the amount of Al₂O₃ that increases during casting, decreasing the Al₂O₃ concentration at saturation, and ultimately achieving a low solidification start temperature and low viscosity.
Therefore, in order to further decrease the solidification start temperature and viscosity and achieve more stable operation, the powder consumption may be increased.
The inventors have found that when steel containing 0.5 mass% or more of Al is continuously cast, when powder is introduced at a powder consumption of less than 0.4 kg per tonne of molten steel, the solidification start temperature exceeds 1300 °C. On the other hand, by introducing the powder according to an embodiment of the present disclosure at a powder consumption of 0.4 kg or more per tonne of molten steel, the solidification start temperature can be made 1300 °C or less and the viscosity can be made 10 poise or less. This allows stable operation and makes it possible to produce cast steel with no problems in surface quality.
From this perspective, in the method of producing steel according to an embodiment of the present disclosure, it is preferable to set the powder consumption per tonne of molten steel to 0.4 kg or more.
The method of controlling the powder consumption is not particularly limited. However, the mold typically vibrates in the casting direction, and therefore it may suffice to control the amplitude or frequency of the vibration of the mold. Further, the powder consumption may be controlled by adjusting the physical properties of the powder. Specifically, the powder consumption can be controlled by adjusting the viscosity and crystallization temperature. Further, the vibration conditions of the mold, such as the amplitude or the frequency, may be changed depending on the physical properties of the powder.

EXAMPLES

More detailed description is given below based on examples. However, the present disclosure is not restricted to the following examples and appropriate modifications may be made within the range conforming to the spirit of the present disclosure, all such modifications being included within the technical scope of the present disclosure.
First, mold powders for continuous casting were prepared having the 12 types of chemical compositions listed in Table 1. In the preparation, first, alumina (Al₂O₃), quicklime (CaO), fluorite (CaF₂), barium carbonate (BaCO₃), magnesium carbonate (MgCO₃), and lithium carbonate (Li₂CO₃) were added as required, and then melted to prepare pre-melt raw material. Next, alumina, quicklime, and fluorite were mixed with the pre-melt raw material as required, and aggregate carbon was further mixed in to obtain a chemical composition listed in Table 1. Carbon black was used as the aggregate carbon. Powder C further contained SiO₂ and Na₂O. The contents of Al, Ca, Mg, and Si were measured by fluorescence X-ray analysis. The total C content was measured by a combustion method. The content of Ba, Li, and Ti were measured by ICP emission spectrometry. The Na content was measured by atomic absorption spectroscopy. The F content was measured by absorption photometry. The values of Al₂O₃, Li₂O, CaO, BaO, MgO, SiO₂, Na₂O, and TiO₂ in Table 1 indicate the content of each element as oxide equivalent. Further, the carbonates were converted to oxides by the pre-melt, and the raw material added after the pre-melt was all oxides or fluorides, so that none of the powders contained much carbon derived from carbonates. Therefore, the total C in Table 1 corresponds to the amount of carbon added as aggregate. The proportion of pre-melt raw material in each of the powders listed in Table 1 was 50 % or more, and the maximum particle size was 250 µm or less.

[Table 1]

Table 1

Powder	Components (mass%)								Y	Remarks
Powder	Total C	Al₂O₃	Li₂O	F	Total content of Ca, Ba, Mg, La, and Zr as oxide equivalent	SiO₂	Na₂O	TiO₂	Y	Remarks
A	3.0	30.6	9.6	6.8	49.3 (CaO=35.7+BaO=13.6)	0.3*	0.2*	0.2*	14	Example
B	2.0	25.5	8.0	9.0	54.9 (CaO=27.0+BaO=25.2+MgO=2.7)	0.3*	0.1*	0.2*	26	Example
C	3.1	5.3	3.5	7.3	37.6 (CaO=37.6)	35.9	7.1	0.2*	12	Comparative Example
D	3.4	18.5	11.6	8.1	57.7 (CaO=26.5+BaO=31.2)	0.3*	0.2*	0.2*	9	Comparative Example
E	2.1	38.2	10.5	7.5	41.3 (CaO=21.2+BaO=20.1)	0.1*	0.1*	0.2*	25	Comparative Example
F	3.2	30.6	14.2	1.2	50.1 (CaO=34.0+BaO=16.1)	0.3*	0.2*	0.2*	11	Comparative Example
G	2.9	25.3	5.5	17.3	48.6 (CaO=38.2+BaO=10.4)	0.2*	0.1*	0.1*	15	Comparative Example
H	3.8	30.4	9.4	6.6	49.3 (CaO=35.7+BaO=13.6)	0.1*	0.2*	0.2*	4	Comparative Example
I	1.0	31.6	9.8	7.0	49.7 (CaO=35.9+BaO=13.8)	0.3*	0.4*	0.2*	39	Comparative Example
J	3.7	27.9	8.4	7.3	52.4 (CaO=38.1+BaO=13.2+MgO=1.1)	0.1*	0.1*	0.1*	5	Example
K	1.3	34.0	8.1	6.5	49.7 (CaO=36.1+BaO=13.6)	0.2*	0.1*	0.1*	35	Example
L	1.2	34.3	7.9	6.7	49.5 (CaO=36.1+BaO=13.4)	0.2*	0.1*	0.1*	37	Comparative Example

* Inevitable impurity content

Next, molten steel having a C concentration of 0.0040 % and an Al concentration of 5 % was produced, and continuous casting of a slab having a mold cross-section size of 200 mm × 1000 mm was carried out under the condition of a cast steel withdrawal rate of 0.7 m/min. At this time, a powder listed in Table 2 was supplied while adjusting the vibration conditions of the mold so that a powder consumption per tonne of molten steel listed in Table 2 was achieved. Further, two thermocouples (upper thermocouple and lower thermocouple) were inserted into the mold at positions 90 mm and 200 mm below the surface of the molten metal. When casting is proceeding smoothly, the temperature of the upper thermocouple is higher than that of the lower thermocouple, but when the molten steel comes into direct contact with the mold, the temperatures of the upper and lower thermocouples is reversed. When the temperature reversed, there was a risk of breakout, so the slab withdrawal rate was decreased during casting, and the casting velocity was returned to normal after the thermocouple temperature stabilized.
Subsequently, the solidification start temperature and viscosity were measured by the above-mentioned methods. Further, as an operational evaluation, occurrence of smoke and flames during continuous casting was investigated. In addition, the resulting cast steel was observed to check the number of bleeds (per m²). Further, trimming yield rate was investigated as an evaluation of the quality of the cast steel. The trimming yield rate is a value calculated by the following expression (5). Trimming yield rate (%) = (weight of cast steel after trimming)/ (weight of cast steel before trimming) × 100
When defects such as depressions occur on the surface of cast steel, the surface of the cast steel needs to be scraped with a scarf or grinder to remove the defects, resulting in a decrease in yield rate. That is, the higher the trimming yield rate, the higher the quality of the cast steel.
Results are listed in Table 2. For Comparative Example No. 4, breakout occurred about 10 minutes after the start of casting. Accordingly, it was not possible to collect the molten powder during casting, and it was not possible to measure the solidification start temperature or the viscosity. Further, it was not possible to evaluate the number of bleed occurrences or the trimming yield rate. For Comparative Example No. 8, corrosion progressed throughout the continuous casting line, making it difficult to continue. For Comparative Example No. 9, due to an insufficient molten layer thickness of the powder, the molten steel stuck onto the mold immediately after the start of casting, and further, a breakout occurred from the stuck portion. Accordingly, it was not possible to collect the molten powder during casting, and it was not possible to measure the solidification start temperature or the viscosity. Further, it was not possible to evaluate the number of bleed occurrences or the trimming yield rate. For Comparative Example No. 10, the sensor for detecting the molten metal level failed about five minutes after the start of pouring, and pouring was stopped. Accordingly, it was not possible to collect the molten powder during casting, and it was not possible to measure the solidification start temperature or the viscosity. Further, it was not possible to evaluate the number of bleed occurrences or the trimming yield rate. For Comparative Example No. 13, no particular operational troubles were observed during casting, and the number of bleed occurrences in the cast steel and the trimming yield rates were equivalent to those for the Examples according to the present disclosure, but powder defects, a type of surface defect, occurred frequently in the steel material after rolling. This was because the thick molten layer thickness caused a lot of powder entrapment.

[Table 2]

Table 2

No.	Powder	Powder consumption (kg/t)	Powder during casting			Smoke and flame occurrence	Bleed occurrence (per m²)	Trimming yield rate (%)	Important notes	Remarks
No.	Powder	Powder consumption (kg/t)	Molten layer thickness (mm)	Solidification start temperature (°C)	Viscosity (at 1300 °C) (poise)	Smoke and flame occurrence	Bleed occurrence (per m²)	Trimming yield rate (%)	Important notes	Remarks
1	A	0.55	14	1260	4.0	almost none	0	100		Example
2	B	0.53	26	1280	6.5	almost none	0.2	99.5		Example
3	A	0.32	14	1330	30	little	0.8	97.3		Example
4	C	0.49	20	-	-	extremely frequent	-	-	casting stopped midway	Comparative Example
5	D	0.51	8	1310	20	little	1.8	96.3		Comparative Example
6	E	0.53	23	1430	150	almost none	3.6	91.8		Comparative Example
7	F	0.50	9	1420	100	a lot	3.8	91.3		Comparative Example
8	G	0.51	14	1280	8.0	almost none	0.6	97.9	after casting, apparatus was severely corroded and in need of repair	Comparative Example
9	H	0.42	4	-	-	almost none	-	-	casting stopped midway	Comparative Example
10	I	0.63	40	-	-	almost none	-	-	casting stopped midway	Comparative Example
11	J	0.41	5	1280	5.7	almost none	0.9	97.1		Example
12	K	0.65	35	1250	3.0	almost none	0.2	99.7		Example
13	L	0.67	36	1240	2.5	almost none	0.2	99.6	powder defects frequently occurred in rolled steel material	Comparative Example

As indicated in Table 2, for all of the Examples, high quality cast steel and stable operation was achieved. On the other hand, for all of the Comparative Examples, quality was inferior or more operational problems occurred.
FIG. 1 illustrates a relationship between the molten layer thickness in the continuous casting and the number of times the temperatures of the upper thermocouple and the lower thermocouple reversed. When the molten layer thickness was 5 mm or more, the above phenomenon never occurred, but when 4 mm, reversals occurred frequently, and casting was stopped when the number of occurrences reached 8.
Further, powder defects were examined after rolling the cast steel obtained by the above continuous casting. FIG. 2 illustrates a relationship between the molten layer thickness in the continuous casting and the number density of powder defects. When the molten layer thickness was 35 mm or less, almost no powder defects were observed in the steel material after rolling, but when the molten layer thickness was 36 mm, the number of powder defects due to powder entrapment increased sharply. When the molten layer thickness was 40 mm, the sensor for detecting the molten metal level failed midway through casting, and casting was halted. However, when the partially cast steel was rolled to produce steel, many powder defects were found.

Claims

A mold powder for continuous casting that has a chemical composition comprising:
aggregate carbon,

Al₂O₃,

at least one oxide selected from the group consisting of CaO, BaO, MgO, La₂O₃, and ZrO₂,

fluorine compound, and

Li₂O,

with the balance being inevitable impurity, wherein

Al₂O₃ content is 25 mass% to 35 mass%,

total content of Ca, Ba, Mg, La, and Zr as oxide equivalent is 40 mass% to 60 mass%,

F content is 3 mass% to 10 mass%,

Li content as Li₂O equivalent is 6 mass% to 12 mass%,

total content of the inevitable impurity is 0 mass% to 2.0 mass%, and

when content of the aggregate carbon in mass% is represented as (%C), Y, as indicated in the following expression, is 5 or more and 35 or less, $Y = - 12.7 \times (% C) + 51.8 .$
The mold powder for continuous casting according to claim 1, wherein the chemical composition further comprises at least one carbonate of an element selected from the group consisting of
Li, Ca, Ba, Mg, La, and Zr.
The mold powder for continuous casting according to claim 1 or 2, wherein a maximum particle size of the mold powder is 250 µm or less.
The mold powder for continuous casting according to any one of claims 1 to 3, wherein 30 mass% or more of the mold powder is made of pre-melt raw material.
A method of producing steel, the method comprising continuous casting of steel containing 0.5 mass% or more of Al using the mold powder for continuous casting according to any one of claims 1 to 4.
The method of producing steel according to claim 5, wherein powder consumption per tonne of molten steel is 0.4 kg or more.