Method for determining secondary refining conditions and continuous casting conditions, and method for producing steel

Abstract

To suppress poor inflow of mold powder.SOLUTION: When steel having a dissolved Al content of 0.3 mass% or more and a Ca content of less than 10 ppm is subjected to continuous casting, a range of Al2O3 content in mold powder inside a mold, in which poor inflow of the mold powder does not occur, is determined by using a consumption amount of the mold powder, and values of A, B, and a to d satisfying the following equation are determined when the Al2O3 content X (mass%) is within the determined range of the Al2O3 content: 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B=X, wherein A: dissolved Al content (mass%) in molten steel, B: casting speed (m / min), a: Ca content (mass%) in the alloy added to molten steel until completion of secondary refining, b: amount of oxygen gas (Nm3) supplied to molten steel during secondary refining, c: circulation gas flow rate for molten steel (NL / min) during secondary refining, and d: circulation time (min) for molten steel during secondary refining.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for determining secondary refining conditions and continuous casting conditions, and to a method for producing steel. [Background technology]

[0002] In continuous casting of steel, mold powder is added to the molten steel in the mold. The mold powder melts on the molten steel and flows into the gap between the mold and the solidified shell. The mold powder that flows into the gap between the mold and the solidified shell provides lubrication between the mold and the solidified shell.

[0003] When steel containing Al is continuously cast, Al2O3 is produced by a reaction between Al in the molten steel and SiO2 in the molten mold powder. The produced Al2O3 is absorbed into the molten mold powder, causing the mold powder to become highly viscous, which results in poor flow of the mold powder between the mold and the solidified shell.

[0004] If the lubrication between the mold and the solidified shell is impaired due to poor mold powder flow, the solidified shell will stick to the mold, causing surface defects on the cast slab. In the worst case, breakouts may occur.

[0005] Therefore, mold powders to be used when continuously casting steel containing Al have been proposed. For example, Patent Document 1 states that "Al2O3 was increased by 20% in mass ratio from the composition before melting, and SiO2 was reduced to correspond to the increased Al2O3, and then the powder was melted at 1400°C, cooled to 850°C at 10°C / min, and then air-cooled to room temperature. The crushed sample was subjected to X-ray diffraction analysis. As a result, in a comparison of the first peak heights of the diffracted X-ray intensity, the first peak height of Ca2Al2SiO7 was found to be higher than that of Ca2Al2SiO7. 12 Al 14 O 32The mold powder has been described as having a first peak height lower than that of F2 (see, for example, claim 1 of Patent Document 1). By using such a mold powder, it is possible to suppress poor flow of the mold powder when continuously casting steel containing Al. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-122326 Summary of the Invention [Problem to be solved by the invention]

[0007] Although the above-mentioned method using special molding powder is known, it is desirable to propose a different method, for example, a method that does not use special molding powder.

[0008] An object of the present invention is to provide a method that can suppress poor inflow of mold powder when continuously casting steel containing Al, by a method different from conventional methods. [Means for solving the problem]

[0009] In continuous casting of steel containing Al, the deterioration of mold powder that causes poor flow of mold powder is thought to be caused by the formation of Al2O3 due to the reaction between Al in the molten steel and SiO2 in the molten mold powder, as well as the floating of inclusions in the molten steel. These inclusions are Al2O3 inclusions that contain Ca (such as CaO-Al2O3 inclusions).

[0010] The formation of Al2O3 by the reaction between Al in molten steel and SiO2 in the molten mold powder depends on the amount of dissolved Al in the molten steel, and is unavoidable in high-Al steels with a high amount of dissolved Al.

[0011] On the other hand, the above-mentioned inclusions in molten steel are formed during secondary refining. The inclusions contain CaO, which lowers their melting point and results in a solid-liquid coexistence or liquid phase composition, making them difficult to float and separate during secondary refining. The inclusions remain in the molten steel when the continuous casting process begins. During continuous casting, the inclusions are absorbed into the molten mold powder in the mold, causing the mold powder to deteriorate.

[0012] In order to reduce the impact of the inclusions, the inventors of the present invention considered reducing the amount of the inclusions by converting them to a solid phase composition that facilitates floating and separation during secondary refining. This would prevent the deterioration of mold powder caused by the inclusions in the mold during continuous casting. To achieve this, they focused on the "oxygen flow rate" as a factor that converts the inclusions to a solid phase composition during secondary refining. They also focused on the reflux conditions as a factor that promotes the floating and separation of the inclusions that have been converted to a solid phase composition during secondary refining.

[0013] The present invention was made based on the above findings. The method of determining the secondary refining conditions and continuous casting conditions disclosed in the present specification involves, when continuously casting steel with a dissolved Al content of 0.3 mass% or more and a Ca content of less than 10 ppm using mold powder containing SiO2, using the mold powder consumption amount to determine the range of the Al2O3 content of the mold powder in the mold that does not cause poor flow of the mold powder between the mold and the solidified shell, and then determining secondary refining conditions a to d and continuous casting conditions A and B that satisfy the following formula when the Al2O3 content X (mass%) is within the determined range of the Al2O3 content. 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X Where, A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Gas flow rate for circulating molten steel in secondary refining (NL / min) d: Circulation time of molten steel in secondary refining (min) is.

[0014] The deterioration of mold powder, which causes poor mold powder flow, occurs when Al2O3, produced by the reaction between Al in molten steel and SiO2 in the molten mold powder, and Al2O3 inclusions containing Ca, are absorbed into the mold powder in the mold. The present inventors focused on the Al2O3 content absorbed into the mold powder in the mold. They also focused on secondary refining conditions a through d and continuous casting conditions A and B in the above formula (I) as factors that affect the Al2O3 content of the mold powder in the mold. Secondary refining conditions a through d are related to the inclusions and include conditions that change the solid phase composition of the inclusions to one that facilitates floating and separation during secondary refining, thereby promoting the floating and separation of the inclusions. Continuous casting conditions A and B are related to the Al2O3 absorbed into the mold powder. They discovered the above formula (I), which shows the relationship between these secondary refining conditions a through d and continuous casting conditions A and B and the Al2O3 content X of the mold powder in the mold.

[0015] According to the above method, the "range of Al2O3 content in the mold powder in the mold" in which poor mold powder flow does not occur is determined using the "mold powder consumption amount," which is an index of the lubrication between the mold and solidified shell, and secondary refining conditions a to d and continuous casting conditions A and B that satisfy the above formula (I) when the Al2O3 content X is within the determined Al2O3 content range are determined. The determined secondary refining conditions a to d and continuous casting conditions A and B are conditions in which poor mold powder flow does not occur. By carrying out secondary refining under the above secondary refining conditions a to d and then carrying out continuous casting under the above continuous casting conditions A and B, poor inflow of mold powder can be suppressed.

[0016] Furthermore, the above conditions are conditions that can suppress poor inflow of mold powder regardless of the composition of the mold powder. The above conditions are conditions that can suppress poor inflow of mold powder even when a general mold powder containing SiO2 is used. By performing secondary refining under the above secondary refining conditions a to d and then performing continuous casting under the above continuous casting conditions A and B, poor inflow of mold powder can be suppressed regardless of the composition of the mold powder, even when a general mold powder containing SiO2 is used.

[0017] The method for producing steel disclosed in the present specification involves continuously casting steel having a dissolved Al content of 0.3 mass % or more and a Ca content of less than 10 ppm using mold powder containing SiO, using the mold powder consumption amount to determine the range of the Al2O3 content of the mold powder in the mold that does not cause poor flow of the mold powder between the mold and the solidified shell, and performing secondary refining and continuous casting under secondary refining conditions a to d and continuous casting conditions A and B that satisfy the following formula when the Al2O3 content X (mass %) is within the determined range of the Al2O3 content. 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X Where, A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Gas flow rate for circulating molten steel in secondary refining (NL / min) d: Circulation time of molten steel in secondary refining (min) is.

[0018] According to the above method, similar to the above-mentioned method for determining the secondary refining conditions and continuous casting conditions, secondary refining conditions a to d and continuous casting conditions A and B that do not cause poor inflow of mold powder can be obtained. After secondary refining is performed under these secondary refining conditions a to d, continuous casting is performed under continuous casting conditions A and B, thereby making it possible to suppress poor inflow of mold powder.

[0019] According to the above method, poor inflow of mold powder can be suppressed by controlling the secondary refining conditions and continuous casting conditions. According to the above method, poor inflow of mold powder can be suppressed regardless of the composition of the mold powder. According to the above method, poor inflow of mold powder can be suppressed even when a general mold powder containing SiO2 is used. [Effects of the Invention]

[0020] When continuously casting steel containing Al, poor inflow of mold powder can be suppressed by a method different from conventional methods. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 10 is a diagram showing an example of determining the mold powder consumption amount and the allowable Al2O3 pickup amount (the range of Al2O3 content of the mold powder in the mold in which poor flow of the mold powder does not occur). [Figure 2] FIG. 10 is a diagram showing another example of determining the allowable Al2O3 pickup amount. [Figure 3] FIG. 10 is a diagram showing another example of determining the allowable Al2O3 pickup amount. DETAILED DESCRIPTION OF THE INVENTION

[0022] Preferred embodiments of the present invention will be described below. Note that the embodiments described below are examples of specific embodiments of the present invention and are not intended to limit the present invention.

[0023] This embodiment is directed to the case where high-Al steel having a dissolved Al content of 0.3 mass% or more (s.Al≧0.3 mass%) is continuously cast. This embodiment is also directed to the case where steel having a Ca content of less than 10 ppm (less than 0.001 mass%) in the molten steel is continuously cast. This embodiment is directed to the case where steel having a trace amount of Ca is continuously cast. This embodiment is directed to the case where an alloy containing Ca as a main component is not intentionally added for the purpose of adjusting the composition in secondary refining.

[0024] This embodiment deals with the case where the above steel is continuously cast using mold powder containing SiO2. Common mold powders contain SiO2. When high-Al steel is continuously cast, Al2O3 is generated in the mold by a reaction between Al in the molten steel and SiO2 in the molten mold powder, and Al2O3 is absorbed (picked up) by the mold powder in the mold. Furthermore, inclusions in the molten steel (Al2O3 inclusions containing Ca) rise to the surface and are absorbed (picked up) by the mold powder in the mold. These factors can cause the mold powder to deteriorate, leading to poor flow of the mold powder between the mold and the solidified shell. This embodiment deals with a case where poor mold powder flow is likely to occur.

[0025] The method for determining the secondary refining conditions and the continuous casting conditions according to this embodiment includes the following steps. (1) Using the amount of mold powder consumed in the mold, the range of Al2O3 content of the mold powder in the mold is determined so that poor flow of the mold powder does not occur. Hereinafter, the "Al2O3 content of the mold powder in the mold" may be referred to as the "Al2O3 pick-up amount" or the "Al2O3 pick-up amount of the mold powder." Furthermore, the "range of Al2O3 content in the mold powder in the mold in which poor flow of the mold powder does not occur" is sometimes referred to as the "allowable Al2O3 pickup amount." (2) When the Al2O3 content X is within the determined range of the Al2O3 content, secondary refining conditions a to d and continuous casting conditions A and B that satisfy the following formula (I) are determined. 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X...(I) Where, A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Gas flow rate for circulating molten steel in secondary refining (NL / min) d: Circulation time of molten steel in secondary refining (min)

[0026] In addition, in the method for producing steel according to this embodiment, the following step (3) is carried out after the above step (1). (3) When the Al2O3 content X is within the determined range of the Al2O3 content, secondary refining and continuous casting are carried out under the secondary refining conditions a to d and continuous casting conditions A and B that satisfy the above formula (I).

[0027] The above method will be explained below.

[0028] (1) Determining the allowable amount of Al2O3 pickup The Al2O3 absorbed (picked up) into the mold powder changes the mold powder to a high viscosity, causing poor mold powder flow. It is believed that the greater the Al2O3 pickup amount, the greater the mold powder changes, making it more likely to have poor mold powder flow. The inventors of the present application focused on the "Al2O3 pickup amount" and decided to determine the range of Al2O3 pickup amount that can suppress poor mold powder flow ("allowable Al2O3 pickup amount").

[0029] "Mold powder consumption" is used to determine the "allowable Al2O3 pickup amount." "Mold powder consumption" is the amount of mold powder that flows between the mold and the solidified shell. Generally, an amount of mold powder corresponding to the mold powder consumption is supplied to the mold, so mold powder consumption is often the mold powder supply amount. "Mold powder consumption" is used as an indicator of the lubricity between the mold and the solidified shell. When "mold powder consumption" is low, insufficient mold powder inflow causes the solidified shell to stick to the mold. This leads to surface defects and breakouts on the slab.

[0030] In high-Al steel, it is thought that the lower the "mold powder consumption," the greater the "Al2O3 pickup amount." This is because the lower the mold powder consumption, the longer the contact time between the molten steel and mold powder, which accelerates the reaction between the Al in the molten steel and the SiO2 in the molten mold powder, increasing the viscosity. It is thought that the higher the "mold powder consumption," the lower the "Al2O3 pickup amount."

[0031] The "mold powder consumption" used to determine the "allowable Al2O3 pickup amount" may be, for example, a value obtained through experiments, a known value described in literature, or a value obtained by other methods. The "mold powder consumption" tends to vary depending on the mold vibration conditions.

[0032] An example of a method for determining the "allowable amount of Al2O3 pickup" from the "amount of mold powder consumed" will be described below.

[0033] Figure 1 shows the results of casting high-Al steel using mold powder A under the same casting speed and mold vibration conditions. Figure 1 also shows the mold powder consumption (kg / t) and Al2O3 pickup amount (mass%). "Mold powder consumption (kg / t)" refers to the amount of mold powder used when continuously casting one ton of steel (mold powder consumption). After casting, the slab was checked for surface defects caused by poor mold powder flow, and samples with no surface defects were marked with "Good", and samples with surface defects were marked with "Poor".

[0034] From Figure 1, of the two "X" where surface defects occurred, no surface defects occurred above the "X" where a large amount of mold powder was consumed. At the "X" where a large amount of mold powder was consumed, the mold powder consumption was 0.32 kg / t. From this, it is thought that poor mold powder flow will not occur above the "high mold powder consumption 'x'", that is, when the mold powder consumption exceeds 0.32 kg / t.

[0035] From the above, when mold powder A is used, the "Al2O3 pickup amount range" when mold powder consumption exceeds 0.32 kg / t is considered to be the "allowable Al2O3 pickup amount" (the range of Al2O3 pickup amount in which poor mold powder flow does not occur).

[0036] In Figure 1, there are two "◯" mold powder consumptions on the mold powder consumption line of 0.32 kg / t where no surface defects occurred. The Al2O3 pickup amounts for these two "◯" were 12.7 mass% and 12.8 mass%. The Al2O3 pickup amounts for the other "◯"s were less than 12.7 mass%. From this, it can be determined that the "allowable Al2O3 pickup amount" when using mold powder A is less than 12.7 mass%.

[0037] Alternatively, an approximation line can be obtained from the "◯" and "X" symbols shown in Figure 1 using the least squares method. This approximation line shows the relationship between mold powder consumption and Al2O3 pickup amount. The range of Al2O3 pickup amount in this approximation line where mold powder consumption exceeds 0.32 kg / t may be determined as the "allowable Al2O3 pickup amount." As another method, the approximate line is shifted downward while maintaining the same slope to obtain a line passing through the "○" or "X" that is the furthest downward from the approximate line. The range of Al2O3 pickup amounts on this line where the mold powder consumption exceeds 0.32 kg / t may be determined as the "allowable Al2O3 pickup amount."

[0038] Next, an example of a method for determining the "allowable Al2O3 pickup amount" when using another mold powder will be described using the "mold powder consumption amount" obtained when using mold powder A.

[0039] Figure 2 shows the results when mold powder B, which has a different composition from mold powder A, was used. Continuous casting was performed under the same mold vibration conditions (sine waveform with an amplitude of 3.0 mm) as in the experiment shown in Figure 1. Figure 2 shows the relationship between mold powder consumption (kg / t) and Al2O3 pickup amount (mass%). Poor mold powder flow was not investigated here.

[0040] From the relationship between mold powder consumption (kg / t) and Al2O3 pickup amount (mass%) shown in Figure 2, an approximate straight line (L1) was obtained by the least squares method. The approximate line (L1) is expressed by the following equation (a1). y=-0.0282x+1.0899 (a1) Here, x is the Al2O3 pickup amount (mass %). y is the mold powder consumption (kg / t).

[0041] When mold powder consumption exceeds 0.32 kg / t, poor mold powder flow does not occur. From the approximate line (L1) and the above formula (a1), the range of Al2O3 pickup amounts (mass%) where mold powder consumption exceeds 0.32 kg / t can be determined as the "allowable Al2O3 pickup amount" (the range of Al2O3 pickup amounts where poor mold powder flow does not occur).

[0042] In addition, the allowable Al2O3 pickup amount can be determined with higher accuracy so that poor mold powder flow does not occur, by the following method. The further down the approximate line (L1) in Figure 2 is moved, the smaller the Al2O3 pickup amount for the same mold powder consumption. Moving the approximate line (L1) downward yields line (L2). Line (L2) has the same slope as the approximate line (L1) and passes through the plot furthest downward from the approximate line (L1). From line (L2), the range of Al2O3 pickup amounts where mold powder consumption exceeds 0.32 kg / t is less than 21.9 mass%. Therefore, the "allowable Al2O3 pickup amount" when using mold powder B can be determined to be less than 21.9 mass%.

[0043] When mold powder C, which has a composition different from mold powder A and mold powder B, is used, the "allowable Al2O3 pickup amount" can be determined in the same way as when mold powder B is used.

[0044] The results using mold powder C are shown in Figure 3. Here, continuous casting was performed under the same mold vibration conditions (sine waveform with an amplitude of 3.0 mm) as in the experiment shown in Figure 1. From the relationship between mold powder consumption (kg / t) and Al2O3 pickup amount (mass%) shown in Figure 3, an approximate straight line (L11) was obtained by the least squares method. The approximate line (L11) is expressed by the following formula (a2). y=-0.017x+0.5994 (a2) Here, x is the Al2O3 pickup amount (mass %). y is the mold powder consumption (kg / t).

[0045] From the approximate line (L11) and the above formula (a1), the range of Al2O3 pickup amounts where the mold powder consumption exceeds 0.32 kg / t can be determined as the "allowable Al2O3 pickup amount" (the range of Al2O3 pickup amounts where poor mold powder flow does not occur).

[0046] In addition, the allowable Al2O3 pickup amount can be determined with higher accuracy so that poor mold powder flow does not occur, by the following method. By shifting the approximate line (L11) downward, we obtain the line (L12). The line (L12) has the same slope as the approximate line (L11) and passes through the plot that is the furthest downward from the approximate line (L11). From the line (L12), it was found that the range of Al2O3 pickup amounts where mold powder consumption exceeded 0.32 kg / t was less than 15.9 mass%. From this, we can determine that the "allowable Al2O3 pickup amount" when using mold powder C is less than 15.9 mass%.

[0047] In the above, when determining the "allowable Al2O3 pickup amount" for mold powders A to C, the "mold powder consumption amount" obtained from the experimental results shown in Figure 1 was used. However, a different "mold powder consumption amount" may also be used. For example, a "mold powder consumption amount" determined from previous experiments, empirical values, or literature values ​​may also be used. For example, based on the disclosures of JP 2003-88942 A (claim 5,

[0033] , etc.) and JP 2011-218411 A (

[0078] ), the "mold powder consumption amount" at which mold powder flow failure does not occur may be set to 0.3 kg / t or more. In this case, the "allowable Al2O3 pickup amount" may be determined to be the range of Al2O3 pickup amount at which the "mold powder consumption amount" is 0.3 kg / t or more when casting under the same mold vibration conditions as those described in these documents.

[0048] Furthermore, there are no particular limitations on how to determine the "mold powder consumption" and "Al2O3 pickup amount" in the experiment. For example, the "mold powder consumption" can be the powder supply amount per surface area of ​​the slab, and the "Al2O3 pickup amount" indicates the state of the molten powder, but can also be determined from the solidified powder film.

[0049] (2) Determining secondary refining conditions and continuous casting conditions Using the "allowable Al2O3 pickup amount" determined in (1) above, secondary refining conditions and continuous casting conditions that can suppress poor mold powder inflow are determined.

[0050] In the steelmaking process, after refining in a converter (primary refining), the molten steel tapped from the converter undergoes secondary refining, and then steel is produced through processes such as continuous casting. During secondary refining, the molten steel undergoes adjustment of its composition, degassing, deoxidation, and impurity removal.

[0051] In continuous casting, the factors that affect the "Al2O3 pickup amount" of mold powder in the mold are thought to be the "amount of dissolved Al in the molten steel," "casting speed," and "amount of Ca in the molten steel." The reasons for this are explained below.

[0052] The greater the "amount of dissolved Al in molten steel," the more Al2O3 is generated due to the reaction between Al in the molten steel and SiO2 in the mold powder, and the greater the "amount of Al2O3 picked up."

[0053] From past experience, it has been found that the faster the "casting speed," the greater the "Al2O3 pickup amount." The reason for this is not clear, but it is thought that the faster the "casting speed," the greater the amount of molten steel immediately after being discharged into the mold that comes into contact with the mold powder.

[0054] The "amount of Ca in molten steel" is an indicator of the amount of Al2O3 inclusions containing Ca (such as CaO-Al2O3 inclusions) in the molten steel. The higher the "amount of Ca in molten steel," the more of the above inclusions there are in the molten steel, and therefore the "amount of Al2O3 picked up" increases.

[0055] From the above, it is thought that the "amount of dissolved Al in molten steel," "casting speed," and "amount of Ca in molten steel" in continuous casting affect the "amount of Al2O3 pick-up." Therefore, the following experiment was conducted to investigate the relationship between the "amount of dissolved Al in molten steel," "casting speed," "amount of Ca in molten steel," and the "amount of Al2O3 pick-up."

[0056] After secondary refining, continuous casting was carried out and the amount of Al2O3 picked up by the mold powder in the mold was investigated.

[0057] In the secondary refining, degassing treatment (RH treatment) was carried out using a reflux-type RH vacuum degasser. During the degassing treatment, oxygen was supplied to the molten steel in the vacuum vessel. Tables 1 and 2 show the conditions for secondary refining.

[0058] [Table 1]

[0059] [Table 2]

[0060] The "amount of Ca in the alloy added to the molten steel" shown in Table 2 was calculated by the following method. When the Ca contents of the various alloys added to the ladle by the end of secondary refining are x1 (mass%), x2 (mass%), x3 (mass%)..., the amounts of the various alloys added are y1 (kg), y2 (kg), y3 (kg)..., and the weight of the molten steel in the ladle is M (kg), the "amount of Ca in the alloys added to the molten steel" was calculated using the following formula.

number

[0061] Tables 3 and 4 show the continuous casting conditions. The mold powder types (A to C) shown in Table 3 are mold powders A to C used in the casting shown in Figures 1 to 3. Table 3 also shows the Al2O3 pickup amount of mold powder in the mold. Table 4 shows the components of the molten steel other than Al.

[0062] [Table 3]

[0063] [Table 4]

[0064] Conditions and methods other than those mentioned above were carried out in accordance with the usual methods used by those skilled in the art.

[0065] It is believed that the continuous casting conditions "amount of dissolved Al in molten steel," "casting speed," and "amount of Ca in molten steel" shown in Table 3 affect the "amount of Al2O3 picked up." Therefore, using the results in Table 3, the three continuous casting conditions "amount of dissolved Al in molten steel," "casting speed," and "amount of Ca in molten steel" were used as explanatory variables, and the "amount of Al2O3 picked up" was used as the objective variable, and the following approximate formula (i) was obtained by the least squares method. 20.33A+13.75B+0.58C=X...(i) Where, A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) C: Amount of Ca in molten steel (mass%) X: Al2O3 pickup amount (Al2O3 content of mold powder in the mold) (mass%) is.

[0066] From the above, the relationship between the three continuous casting conditions A to C and the "Al2O3 pickup amount X" was obtained, as shown in equation (i). From equation (i) above, it is possible to determine the continuous casting conditions A to C that satisfy equation (i) above when the "Al2O3 pickup amount X" on the left side is the "allowable Al2O3 pickup amount". The determined continuous casting conditions A to C can be said to be conditions that can suppress poor inflow of mold powder. By performing continuous casting under the determined continuous casting conditions A to C, poor inflow of mold powder can be suppressed.

[0067] Incidentally, the "amount of Ca in the molten steel" in C above is an index of the amount of Al2O3 inclusions containing Ca (such as CaO-Al2O3 inclusions) in the molten steel. These inclusions are formed in secondary refining, and it is believed that the conditions of secondary refining affect the formation of these inclusions. Factors that affect the formation of these inclusions in secondary refining are thought to be the "amount of Ca in the added alloy," "amount of oxygen fed," "flow rate of molten steel reflux gas," and "molten steel reflux time." The reasons for this are explained below.

[0068] The Ca contained in the inclusions is mainly Ca contained in the alloy added for composition adjustment in secondary refining. The more Ca added to the alloy by the end of secondary refining, the greater the amount of inclusions.

[0069] In secondary refining, oxygen gas is sent to the molten steel (oxygen supply). The purpose of oxygen supply is, for example, to generate Al2O3 by reacting the oxygen gas with Al in the molten steel, or to modify the composition of the inclusions mentioned above. Once the composition of the inclusions has been modified to Al2O3 by oxygen gas, they float and separate in the reflux degassing process described below. The more oxygen is supplied to the molten steel, the more the inclusions in the molten steel are modified to Al2O3 and float and separate, reducing the amount of inclusions in the molten steel.

[0070] In secondary refining, degassing (RH treatment) is performed using a reflux-type RH vacuum degasser. In the degassing process, reflux gas is blown into the molten steel, causing it to reflux between the vacuum tank and the ladle. Inclusions in the molten steel float up and separate as the molten steel refluxes. The higher the reflux gas flow rate, the more the reflux of the molten steel is promoted, and the more inclusions float up, reducing the amount of inclusions in the molten steel. The longer the reflux time of the molten steel, the more inclusions float up, reducing the amount of inclusions in the molten steel.

[0071] From the above, it is thought that the "Ca content of added alloys," "oxygen supply rate," "molten steel reflux gas flow rate," and "molten steel reflux time" in secondary refining affect the "Ca content in molten steel" in continuous casting. Therefore, the relationship between these four secondary refining conditions and the "Ca content in molten steel" in continuous casting was investigated.

[0072] The above four secondary refining conditions are shown in Table 2. The "Ca content in molten steel" in continuous casting is shown in Table 3. Using the results in Tables 2 and 3, the following approximate equation was obtained by the least squares method, with the four secondary refining conditions as explanatory variables and the "Ca content in molten steel" in continuous casting as the target variable. C=7000a-0.00079b-0.00067c-0.038d+9.63 (ii) Where, C: Ca content (mass%) in molten steel (in continuous casting) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Gas flow rate for circulating molten steel in secondary refining (NL / min) d: Circulation time of molten steel in secondary refining (min)

[0073] Using the above formula (ii), the above formula (i) can be expressed as the following formula (I). 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X...(I)

[0074] From the above, the relational expression (I) between the secondary refining conditions a to d, the continuous casting conditions A and B, and the "Al2O3 pick-up amount X" was obtained. From the above formula (I), when the "Al2O3 pickup amount X" on the left side is the "allowable Al2O3 pickup amount," it is possible to determine the secondary refining conditions a to d and continuous casting conditions A and B that satisfy the above formula (I). The determined secondary refining conditions a to d and continuous casting conditions A and B are conditions that can suppress poor inflow of mold powder. The secondary refining conditions a to d and the continuous casting conditions A and B are determined by the above method.

[0075] (3) Secondary refining and continuous casting From the above formula (I), secondary refining conditions a to d and continuous casting conditions A and B are obtained that satisfy the above formula (I) when the "Al2O3 pickup amount X" on the left side is the "allowable Al2O3 pickup amount." The obtained secondary refining conditions a to d and continuous casting conditions A and B are conditions that can suppress poor inflow of mold powder. After secondary refining is performed under the obtained secondary refining conditions a to d, continuous casting is performed under continuous casting conditions A and B.

[0076] As described above, the method for determining the secondary refining conditions and the continuous casting conditions according to this embodiment includes the following steps. (1) Using the amount of mold powder consumed in the mold, the allowable amount of Al2O3 pickup that does not cause poor mold powder flow (the range of Al2O3 content in the mold powder in the mold that does not cause poor mold powder flow) is determined. (2) Secondary refining conditions a to d and continuous casting conditions A and B are determined that satisfy the following formula (I) when the Al2O3 content X is within the determined allowable Al2O3 pickup amount (within the range of the Al2O3 content). 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X...(I) Where, A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Gas flow rate for circulating molten steel in secondary refining (NL / min) d: Circulation time of molten steel in secondary refining (min)

[0077] The determined conditions are secondary refining conditions and continuous casting conditions that do not cause poor mold powder flow. After secondary refining is performed under the determined secondary refining conditions a to d, continuous casting is performed under continuous casting conditions A and B, thereby preventing poor mold powder flow.

[0078] In addition, in the method for producing steel according to this embodiment, the following step (3) is carried out after the above step (1). (3) When the Al2O3 content X is the determined allowable Al2O3 pickup amount (within the range of the Al2O3 content), secondary refining conditions a to d and continuous casting conditions A and B that satisfy the above formula (I) are obtained. Secondary refining and continuous casting are performed under the obtained secondary refining conditions a to d and continuous casting conditions A and B.

[0079] The obtained secondary refining conditions a to d and continuous casting conditions A and B are conditions under which poor inflow of mold powder does not occur. By performing secondary refining under secondary refining conditions a to d and then performing continuous casting under continuous casting conditions A and B, poor inflow of mold powder can be suppressed.

[0080] Furthermore, according to the method of this embodiment, by controlling the secondary refining conditions and continuous casting conditions, it is possible to suppress poor inflow of the mold powder, regardless of the composition of the mold powder. According to the method of this embodiment, it is possible to suppress poor inflow of the mold powder, even when using a mold powder containing general SiO2.

[0081] In the above formula (I), "a: amount of Ca in the alloys added to the molten steel by the end of the secondary refining" is the total amount of Ca in all alloys added to the ladle by the end of the secondary refining. For example, when refining using a ladle refining apparatus such as an LF (LF treatment, etc.) and degassing using an RH vacuum degassing apparatus (RH treatment) are performed, the total amount of Ca in all alloys added by the end of all these treatments is "a: amount of Ca in the alloys added to the molten steel by the end of the secondary refining."

[0082] "a: amount of Ca in the alloy added to the molten steel by the end of secondary refining" is calculated by the following formula. Let the Ca content of each of the alloys added to the ladle by the end of secondary refining be x1 (mass%), x2 (mass%), x3 (mass%)..., the amount of each alloy added be y1 (kg), y2 (kg), y3 (kg)..., and the weight of molten steel in the ladle be M (kg),

number

[0083] In addition, although oxygen was fed during the degassing treatment in the above experiments, the timing of oxygen feeding is not limited to during the degassing treatment. The "amount of oxygen fed to molten steel in secondary refining" in b of the above formula (I) is not limited to the amount of oxygen fed during the degassing treatment.

[0084] Although the embodiments of the present invention have been described above with reference to the drawings, it should be understood that the specific configurations are not limited to these embodiments. The scope of the present invention is defined by the claims rather than the above description, and includes all modifications within the meaning and scope of the claims.

[0085] For example, conditions other than the secondary refining conditions and continuous casting conditions of the above formula (I) are not limited to the conditions of the above experiment, and may be carried out as would normally be done by a person skilled in the art. For example, specific steps, means, conditions, etc. of secondary refining other than the secondary refining conditions of the above formula (I) are not particularly limited, and may be carried out as would normally be done by a person skilled in the art.

[0086] The secondary refining may be a degassing treatment (RH treatment) using a reflux-type degassing device alone, or may be a combination of refining using a ladle refining device such as an LF (LF treatment, etc.) and a degassing treatment (RH treatment) using a reflux-type degassing device, or may involve two or more LF treatments, etc.

[0087] The present invention is directed to continuous casting of steel having a dissolved Al content of 0.3 mass % or more and a Ca content of less than 10 ppm in the molten steel. The components and composition other than Al and Ca can be changed within the scope of the common technical knowledge of a person skilled in the art.

Claims

1. Steel having a dissolved Al content of 0.3 mass % or more and a Ca content of less than 10 ppm is subjected to SiO 2 In the case of continuous casting using a mold powder containing Using the mold powder consumption, the Al content of the mold powder in the mold is determined so that there is no inflow of the mold powder between the mold and the solidified shell. 2 O 3 Determine the range of content, The determined Al 2 O 3 Al content within the range 2 O 3 A method for determining secondary refining conditions and continuous casting conditions, which determines secondary refining conditions a to d and continuous casting conditions A and B that satisfy the following formula when the content is X (mass%): 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X Where A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Flow rate of gas for circulating molten steel in secondary refining (NL / min) d: molten steel reflux time in secondary refining (min) is.

2. Steel having a dissolved Al content of 0.3 mass % or more and a Ca content of less than 10 ppm is subjected to SiO 2 In the case of continuous casting using a mold powder containing Using the mold powder consumption, the Al content of the mold powder in the mold is determined so that there is no inflow of the mold powder between the mold and the solidified shell. 2 O 3 Determine the range of content, The determined Al 2 O 3 Al content within the range 2 O 3 A method for producing steel, characterized in that secondary refining and continuous casting are carried out under secondary refining conditions a to d and continuous casting conditions A and B that satisfy the following formula when the content X (mass%) is 20.33A+0.58×(7000a-0.00079b-0.00067c-0.038d+9.63)+13.75B = X Where A: Amount of dissolved Al in molten steel (mass%) B: Casting speed (m / min) a: Amount of Ca in the alloy added to the molten steel by the end of secondary refining (mass%) b: Amount of oxygen fed to molten steel in secondary refining (Nm 3 ) c: Flow rate of gas for circulating molten steel in secondary refining (NL / min) d: molten steel reflux time in secondary refining (min) is.

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