JP6874521B2 - Inclusion morphology control steel and its manufacturing method - Google Patents

Description

The present invention relates to high-clean steel used for shipbuilding materials, line pipes, construction steel materials, automobile steel, etc. and a refining method thereof. The present invention relates to an inclusion morphology control steel having excellent productivity and function for suppressing nozzle blockage and a manufacturing method thereof.

For the purpose of improving corrosion resistance, toughness, workability, lamellar tear resistance, etc., many techniques for reducing and detoxifying non-metal inclusions (hereinafter referred to as inclusions) in steel have been conventionally developed. In particular, MnS generated by the reaction of S and Mn in steel in the casting solidification process tends to be the starting point of various defects, and therefore many techniques for reducing them have been developed.

The main method of suppressing MnS formation is a technique of adding REM such as Ca, La, Ce, and Nd to steel and fixing S in steel as a sulfide such as CaS and LaS to suppress the reaction between Mn and S. Is widely used. For example, Patent Documents 1, 2 and 3 show sour steel pipe steel having excellent weld toughness by setting the REM concentration and the Ca concentration in the steel within an appropriate range.

On the other hand, since inclusions generated by Ca and REM may block the immersion nozzle during continuous casting, many nozzle blockage avoidance techniques have been proposed. For example, Patent Document 4 discloses a method of suppressing nozzle blockage by adding an additive composed of Ca and REM after controlling the molten steel to an appropriate composition by secondary refining.

Japanese Unexamined Patent Publication No. 2016-125137 Japanese Unexamined Patent Publication No. 2005-240051 Japanese Unexamined Patent Publication No. 2014-218707 Japanese Unexamined Patent Publication No. 2012-233220

As described above, many techniques have been proposed in which Ca and REM are used to suppress MnS formation and prevent nozzle blockage during continuous casting.

However, it has been difficult to sufficiently achieve both of them with the existing technology. In general, it is advantageous to increase the concentration of REM, which has a stronger affinity for S, to increase the concentration of REM oxide in inclusions in order to suppress the formation of MnS, and increase the Ca concentration to prevent clogging of one nozzle. It is considered advantageous to increase the Ca oxide concentration in the inclusions. That is, the appropriate inclusions are different between the suppression of MnS generation and the prevention of nozzle blockage.

However, when REM and Ca are added to molten steel and refining is performed focusing only on the molten steel component as in the conventional technology, the inclusions consist of REM-Ca-OS-based REM and Ca acid sulfide. In many cases, only one type of complex acid sulfide is produced. If the addition interval between REM and Ca is shortened or added at the same time with particular emphasis on productivity, the number of inclusions becomes almost one type. As described above, when there is only one type of inclusions, CaO in the composite acid sulfide contributes to the prevention of nozzle blockage, and the REM oxide in the same composite acid sulfide inclusion contributes to the suppression of MnS formation. .. As a result, since CaO and REM oxide are present in the same inclusions, the melting point of the reaction product between the inclusions and the nozzle refractory increases and the nozzle clogging prevention effect decreases as compared with the case of CaO alone, and further, REM Since the activity is reduced as compared with the case of the oxide alone, the reaction with S is suppressed, so that the effect of suppressing MnS production is also reduced.

Therefore, it is necessary to independently generate a plurality of types of inclusions in order to suppress MnS generation and prevent nozzle blockage at the same time. there were.

In view of the above problems, the present invention is to provide a steel in which nozzle blockage and MnS, which is a harmful inclusion, are thoroughly reduced, and a method for producing the same.

As a result of intensive research to achieve the above object, the present inventor has generated two types of specific inclusions and made the abundance ratio appropriate to significantly improve MnS suppression and nozzle blockage prevention at the same time. I found.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In terms of mass%, C: 0.002% or more and 0.4% or less, Mn: 0.1% or more and 2% or less, Si: 0.001% or more and 1% or less, S: 0.001% or more and 0 .005% or less, Al: 0.005% or more and 0.50% or less, O: 0.0013% or more and 0.005% or less, Ca: 0.0005% or more and 0.0045% or less, and La , Ce, Nd, one or more of which has a total concentration of 0.0005% or more and 0.0035% or less, and the balance is Fe and unavoidable impurities.
Regarding the composition of non-metal inclusions in steel, the Ca compound is defined as CaO in terms of oxide, and the compound consisting of one or more of La, Ce, and Nd is designated as RO in terms of oxide, and each non-metal is defined as RO. When the total content (% by mass) of the three components _{of Al 2} O ₃ , Ca O and RO in the inclusions is _{100%, the converted contents are (% Al 2} O ₃ ), (% CaO) and (% CaO), respectively. % R-O) and represent, inclusions B, and the number per unit area of the inclusions a group of such translation content is defined by (1) N _a, the conversion content is defined by the equation (2) per unit area number N _B of the group is, (3) satisfy the relationship of expression, the total inclusions number in steel (excluding MnS), the ratio of the inclusions group B a total number of the inclusions group a inclusions form control steel, characterized in der Rukoto 60%.
Inclusion group A: 10% ≤ (% Al ₂ O ₃ ) ≤ 30% and 50% ≤ (% RO) ... (1)
Inclusion B group: 30% ≤ (% CaO) ≤ 50% and (% RO) ≤ 30% ... (2)
_{0.6 ≦ N B / (N A} + N B) ≦ 0.75 ... (3)
[2] Further, in terms of mass%, Cu ≦ 1%, Ni ≦ 10%, Ti ≦ 0.7%, Nb ≦ 0.5%, V ≦ 0.5%, Cr ≦ 1.5%, Mo ≦ 1% The inclusion morphology control steel according to [1], which contains one or more of the above.
[3] Using a vacuum degassing refining facility, a flux containing CaO as a main component is sprayed or blown onto the molten steel prepared by adjusting components other than Ca and La, Ce, Nd, and then from La, Ce, Nd. The production of the inclusion morphological control steel according to [1] or [2], wherein one or more kinds of metals or alloys selected from the above group are added, and then Ca or a Ca alloy is added. Method.

According to the present invention, high-performance steel containing no MnS can be efficiently and stably produced without causing nozzle blockage.

Of the inclusions A and B in the ternary phase diagram of oxide RO consisting of one or more _{of Al 2} O ₃ , CaO and La, Ce, Nd in the non-metal inclusions. It is a figure which shows the composition range. It is a diagram showing the relationship between inclusions number ratio _{N B / (N A + N} B) and MnS number index and nozzle clogging index.

Hereinafter, the present invention will be described in detail.
First, the steel components other than Fe to be treated in the present invention were specified for the following reasons. In the present specification, "%" in the steel composition means "mass%" unless otherwise specified.

C: C acts as a deoxidizing element under reduced pressure and also affects the activity of S and N. Therefore, if C is less than 0.002%, the hypoxic effect becomes unstable, and if it exceeds 0.4%, the activity of S changes significantly and the reaction mechanism changes. Therefore, C was set to 0.002% or more and 0.4% or less.

Mn: Mn is also a deoxidizing element and is an essential element because it improves the properties of various steel materials. If it is less than 0.1%, deoxidation becomes unstable, and if it exceeds 2%, the activity of S decreases, making desulfurization difficult. Therefore, the Mn concentration was set to 0.1% or more and 2% or less.

Si: Si is also an element that is indispensable for deoxidation stability like Mn, but if it is less than 0.001%, deoxidation becomes unstable, and if it exceeds 1%, the SiO ₂ concentration in the inclusions increases. It becomes difficult to control the composition of inclusions intended by the present invention. Therefore, Si is set to 0.001% or more and 1% or less.

Al: Al is an element having the strongest deoxidizing power, so 0.005% or more is required to obtain this deoxidizing effect. On the other hand, if the concentration exceeds 0.50%, it may be difficult to control the CaO concentration in the inclusions, so 0.50% or less is required.

S: S is an element to be removed, but when it becomes higher than 0.005%, the interfacial tension changes, and the nozzle clogging prevention effect by the inclusion control intended by the present invention decreases. On the other hand, if it is less than 0.001%, the amount of desulfurizing agent used increases significantly, which increases the cost. Therefore, in the present invention, molten steel of 0.001% or more and 0.005% or less is treated.

When O: Si, Al and Mn are in the above concentration range, if the O concentration is higher than 0.005%, a large amount of inclusions will be present in the molten steel. Therefore, the O concentration was set to 0.005% or less. Further, when the O concentration is less than 0.0013%, the amount of inclusions is small and nozzle clogging is unlikely to occur, and the amount of oxide inclusions that capture S and suppress MnS formation becomes too small and the suppression of MnS formation becomes unstable. Since the treatment cost is significantly increased to reduce the O concentration to less than 0.0013%, the present invention is excluded. In the present invention, O is T.I. Means O.

Ca: Ca is an element effective for deoxidation and desulfurization, and at the same time, it is also effective for controlling the morphology of inclusions. If the Ca concentration is less than 0.0005%, the deoxidizing power is insufficient for REM. When the Ca concentration exceeds 0.0045%, the formation of CaS inclusions becomes active, and nozzle clogging due to CaS may occur. Therefore, in the present invention, Ca is set to 0.0005% or more and 0.0045% or less.

In addition, Cu ≦ 1%, Ni ≦ 10%, Ti ≦ 0.7%, Nb ≦ 0.5%, V ≦ 0.5%, Cr ≦ 1.5%, Mo ≦ for the purpose of ensuring strength and corrosion resistance. One or more components of 1% may be added as needed.

Next, La, Ce, and Nd will be described. In the present invention, one or more of La, Ce, and Nd are hereinafter referred to as "REM".
La, Ce, Nd: These REMs are constituent elements of inclusions for obtaining the clean steel which is the object of the present invention, and one kind or two or more kinds are added. If these concentrations are less than 0.0005% in total, REM compounds cannot be formed in the inclusions. On the other hand, if it exceeds 0.0035%, the activity of S is reduced, and it is predicted that the reaction rate between S and inclusions will decrease. Therefore, in the present invention, the total concentration of La, Ce, and Nd is set to 0.0005% or more and 0.0035% or less.

Next, the composition of non-metal inclusions in steel will be described. Among the non-metal inclusions, the Ca compound (Ca acid sulfide) is converted into CaO in terms of oxide, and the compound (acid sulfide) consisting of one or more of La, Ce, and Nd is converted into oxide. Let it be RO. When the total content (mass%) of the three components _{of Al 2} O ₃ , Ca O and RO in the non-metal inclusions is _{100%, the converted contents are (% Al 2} O ₃ ) and (% CaO), respectively. , (% RO). Then, the inclusions whose conversion content is defined by the equation (1) are defined as inclusions A group, and the inclusions whose conversion content is defined by the equation (2) are defined as inclusions B group. Furthermore, the number per unit area of the inclusions group A N _A, the number per unit area of the inclusions group B and N _B. After having thus defined the inclusion group, in the present invention, it is filled inclusions group A per unit area number N _A and inclusions group B per unit area number N _B is the (3) Relationship It is characterized by. In addition, since it is _{converted into the ternary mass concentration of Al 2} O ₃ , Ca O and RO, the balance of the formula (1) is CaO and the balance of the formula (2) is Al ₂ O ₃ . Further, (% Al ₂ O ₃ ) + (% CaO) + (% RO) = 100 (mass%).
Inclusion group A: 10% ≤ (% Al ₂ O ₃ ) ≤ 30% and 50% ≤ (% RO) ... (1)
Inclusion B group: 30% ≤ (% CaO) ≤ 50% and (% RO) ≤ 30% ... (2)
_{0.6 ≦ N B / (N A} + N B) ≦ 0.75 ... (3)

Hereinafter, the reason why the inclusion composition in the steel is defined in this way will be described.
The reason why it is difficult to achieve both prevention of nozzle blockage and suppression of MnS production simply by controlling the Ca concentration and REM concentration in steel is as described above. We thought that it was necessary for inclusions to coexist, and that it was necessary to optimize the abundance ratio of these multiple inclusions. Therefore, it was necessary to clarify the types of inclusions required and their abundance ratios.

Therefore, by changing the addition amount, addition order, and addition interval of REM and Ca, steel containing various inclusions was manufactured using RH and a continuous casting machine, and the effects on nozzle blockage and the number of MnS were investigated. did. The nozzle blockage was the thickness of deposits in the nozzle during continuous casting, and the number of MnS was evaluated by observing the slabs obtained by the continuous casting machine. As for inclusions other than MnS, the composition and number of inclusions in the obtained slab were analyzed using SEM and EPMA. The thickness of deposits in the nozzle is determined by cutting the immersion nozzle after continuous casting of 300 tons of molten steel for one charge in a vertical cross section and investigating the thickness of deposits in the nozzle, and determining the maximum thickness of deposits in the nozzle. Was defined as. For the number and composition of MnS and inclusions other than MnS, cut out four 20 mm × 20 mm × 10 mm samples from the slab, observe the 20 mm × 20 mm surface with SEM-EPMA, and determine the number and composition of inclusions of 1 μm or more. It was quantified by analysis. As for the sampling position from the slab, assuming that the slab width is W and the casting side thickness is t, a sample is taken from each position of 1 / 2W, 1 / 4W, and 3/4W in the width direction from the 1 / 2t position, and 3 The sum of the measured values of the samples was defined as the number of inclusions in the slab.

The inclusions other than MnS are REM-Ca-Al-OS systems, and their composition distribution range varies from very narrow to wide, or in which two or more kinds of inclusions exist independently. However, the types and composition ranges of inclusions involved in nozzle blockage prevention and MnS suppression were specified, and the relationship between the abundance ratio of the specified inclusion composition, the number of MnS, and the nozzle blockage was investigated. As a result, the inclusions that strongly affect the nozzle blockage and MnS formation are the inclusions A group represented by the equation (1) and the inclusions B group represented by (2), and the inclusion group A and the inclusions B. It was found that the nozzle clogging prevention and the MnS generation suppression can be achieved at the same time by satisfying the relationship of the _{number (3) in the relationship of the number N A} and N _B per unit area of each group.

When inclusions other than inclusions A and B were generated, they acted on either nozzle blockage prevention or MnS formation suppression, or did not act on either. When the inclusion group A and the inclusion group B are projected onto the ternary diagram of (% RO)-(% CaO)-(% Al ₂ O ₃ ), the inclusion group A and the inclusions are projected as shown in FIG. Since the composition range of the product B group is a distant region, it is considered that they showed different effects on MnS and nozzle blockage. The effect of the present invention can be exhibited when the ratio of the total number of inclusions A group and B group to the total number of inclusions in steel (excluding MnS) is 60% or more.

Next, the abundance ratio of inclusion group A and inclusion group B will be described. The experimental results are shown in FIG. The horizontal axis shows the total number accounted number of group B (N _B) ratio of (1) and (2) being defined group A and the number per unit area of the group B respectively by formula N _A, N _B, vertical The axis shows the MnS number index or the nozzle blockage index. MnS number is _{N B / (N B + N} A) = 1 or 1 a MnS number when all of the inclusions in comparison with the inclusion of groups A and B was group B, nozzle clogging index N _{B / (N B + N a)} = 0 that is, all of the inclusions were shown as nozzle clogging index within deposit thickness nozzles indexed as 1 when was group a.

From FIG. 2, it can be seen that the nozzle blockage index decreases as the ratio of the inclusions B group increases, and the nozzle blockage index becomes 0 when the value given by Eq. (3) increases beyond 0.6. This is because the inclusions B group are inclusions having a relatively high CaO concentration, and therefore, when they come into contact with the nozzle refractory, they form a low melting point oxide phase and do not adhere to the nozzle. Next, focusing on the MnS number index, it can be seen that the number of MnS increases sharply when the value given by Eq. (3) exceeds 0.75. It is considered that this is because the inclusions of group A having a high REM oxide concentration and a high ability to capture S in steel are reduced.

From the above, by forming inclusions of groups A and B in the steel and further satisfying the equation (3) by the number of inclusions of groups A and B, the suppression of MnS generation and the prevention of nozzle blockage are remarkably improved at the same time. I understand that I will do it.

Next, a method for producing the inclusion morphology control steel of the present invention for controlling inclusions satisfying the equations (1) to (3) will be described. By applying the manufacturing method of Process A or Process B shown below, the inclusion morphologically controlled steel of the present invention can be obtained. In both process A and process B, the ratio of the total number of inclusions A group and inclusion group B group to the total number of inclusions in the steel can be 60% or more.

First, process A will be described. For example, in the secondary refining, after adjusting the components other than Ca and La, Ce, and Nd, REM is added to the molten steel in two or more divided portions, the molten steel is held for 10 minutes or more, and then Ca is added to 0. There is a method of adding at a low speed of 01 to 0.15 kg / (molten steel tons / minute). The reason for adding REM separately is to control the inclusions in group A. If a large amount of REM is added _{all at once, the Al 2} O ₃ concentration in Group A may decrease too much. Further, the reason why the molten steel is retained and Ca is added at a low speed is to generate the inclusions of the B group while retaining the inclusions of the A group. If the holding time is too short, REM remains in the molten steel, so that deoxidation by Ca does not work, and it becomes difficult to generate group B having a high CaO concentration in inclusions. In addition, when Ca is added at high speed, the Ca concentration in the molten steel temporarily increases, and a part of the previously generated group A is reduced to Ca, which deviates from the composition range of the group A and changes to the high CaO concentration side. It ends up.
By the above method, it is possible to control inclusions satisfying the equations (1) to (3).

Next, process B will be described below as a method that is more efficient than process A. After spraying or blowing a flux mainly composed of quicklime to molten steel using a vacuum degassing refining facility, one or more metals or alloys selected from the group consisting of La, Ce, and Nd are added, and further. After this, a new method of adding Ca or Ca alloy was devised. The main mechanism of action is as follows.

As described above, in process A, the inclusions of group A are slowly generated and then retained to avoid interference between groups A and B, and the inclusions of group B are stably added by slowly adding Ca. It is necessary to generate it. In this method, the inclusions in group A are controlled by REM in molten steel, and the inclusions in group B are controlled by Ca in molten steel. Therefore, low-speed addition and retention are required to reduce mutual interference due to the interaction between REM and Ca. You will need it. However, it was considered that the interaction between REM in molten steel and Ca in molten steel was alleviated by further coexisting oxides affecting both inclusions in groups A and B, and slow addition and retention became unnecessary. When REM is added to Al-containing molten steel in the presence of CaO, inclusions are defined by the REM-Al-Ca-O equilibrium. On the other hand, when Ca is added to molten steel containing Al and REM, inclusions are defined by the REM-Al-Ca-O equilibrium. Moreover, since CaO is thermodynamically stable, the change due to REM and Al is slight. That is, if CaO is added to the molten steel, the equilibrium reaction that defines the inclusions becomes the same, so that it is not necessary to add or retain CaO at a low speed.

An embodiment of the present invention using a converter, RH, and a continuous casting machine will be described. After the converter processing is completed, the molten steel is taken out to the ladle. An alloy such as Si or Mn may be added at the time of steel ejection, or a slag-forming agent such as quicklime may be added. Further, a slag modifier or Al may be used for the purpose of reducing slag intermediate and lower oxides at the time of steel ejection. At this time, it is desirable that the amount of slag in the ladle is 10 kg / ton or more. This is because when the amount of slag is small, the coating effect on the surface of the molten steel is small, and it is easily reoxidized and absorbed by the atmosphere. Further, the slag composition preferably has a total of FeO and MnO in the slag of 10% by mass or less, more preferably 7% by mass or less. If the concentrations of FeO and MnO in the slag are high, the oxygen potential is unstable and easily fluctuates, so that it may be difficult to accurately control the group A and the group B. By adjusting the amount of the slag modifier and Al added at the time of steel ejection, the amount of slag and the slag composition of the slag in the ladle can be set within the above preferable ranges.

Immediately after transferring the ladle to RH, the process is started. In the treatment with RH, the present invention is carried out after performing necessary treatments such as vacuum degassing such as dehydrogenation, molten steel temperature adjustment, and component adjustment.

First, process A will be described.
First, REM is added to the molten steel. The amount of REM added may be determined according to the target concentration, but in order to obtain the inclusions specified in the present invention more accurately, it is desirable to divide the addition into two or more times. However, the effect is saturated even if it is divided into four or more times. The REM to be added may be in any form such as a metal such as metal Ce or an alloy such as mischmetal.

Subsequently, Ca is added to the molten steel, but in order to obtain the inclusions specified in the present invention more accurately, it is desirable to add Ca after circulating the molten steel for 10 minutes or more after the addition of REM. If necessary, the alloy components other than REM, S, and Ca may be adjusted during reflux, but it is necessary to avoid adjusting the temperature of the molten steel using an oxidizing gas. This is because the oxygen activity fluctuates greatly during the temperature adjustment of the molten steel, and the inclusions of group A generated by the addition of REM change significantly.

Ca may be added in any form such as metallic Ca and CaSi alloy. Further, the addition method may be any method such as blowing into molten steel or adding a wire. Further, Ca addition may be carried out by RH, or may be carried out by an atmospheric pressure ladle smelting device at another treatment position after the RH treatment is completed.

However, in order to obtain the inclusions specified in the present invention more accurately, the Ca addition rate may be 0.01 kg / (molten steel ton / min) or more and 0.15 kg / (molten steel ton / min) or less in terms of pure Ca content. It is desirable, and more preferably 0.05 kg / (ton / min of molten steel) or less. If it is less than 0.01 kg / (ton / min of molten steel), the Ca evaporation rate is faster than the addition rate, so that the Ca activity in the molten steel cannot be sufficiently increased, and the control accuracy for the B group is lowered. If it exceeds 0.15 kg / (ton / minute of molten steel), the Ca activity in the molten steel becomes too high, and the inclusions of group A that have been generated in advance may change to the side of group B.

After the addition of Ca is completed, the molten steel is recirculated or stirred for about 3 minutes in order to achieve uniform mixing, and the ladle is transferred to a continuous casting machine for prompt casting.

Secondly, the refining method (process B) according to claim 3 will be described. Immediately after transferring the ladle to RH, the process is started. In the treatment with RH, the present invention is carried out after performing necessary treatments such as vacuum degassing such as dehydrogenation, molten steel temperature adjustment, and component adjustment. First, a flux containing 60% or more of CaO is added by RH. In addition to CaO, the flux may be CaO mixed with or premelted with oxides such as _{MgO, Al 2} O ₃ and _{Ca F 2.}

The method of addition may be any method such as a method of spraying the surface of the molten steel in the vacuum chamber through a top blowing lance or a method of immersing the injection lance in the molten steel in the ladle and blowing it into the molten steel. After that, Ca is added after the REM addition, but it is not necessary to divide the REM addition in particular, and it is not necessary to hold the molten steel for 10 minutes after the REM addition. This is because the CaO controls the equilibrium reaction that defines the inclusions, so that the molten steel-inclusion reaction approaches equilibrium in a short time.

300 tons of hot metal is charged into an upper bottom blown converter, rough decarburization is performed until the C content in molten iron reaches 0.03 to 0.2%, and the end point temperature is 1630 to 1650 ° C. Steel was dispensed into the ladle, and various deoxidizers and alloys were added at the time of steel ejection to adjust the molten steel composition in the ladle. Further, quicklime was added at the time of steel _{removal to adjust the mass ratio of CaO / Al 2} O _{3 in} the slag to 1.2 to 1.5 and the total concentration of FeO and MnO in the slag to 8% or less.

After that, the ladle was transferred to RH, and the RH treatment was started promptly. In RH, the temperature was adjusted first, and then the molten steel components such as Ni, Nb, and Cu were adjusted (alloy addition).

After that, refining was carried out by three kinds of methods.
In process A, after adding REM in two portions, recirculation was performed for 10 minutes, the RH treatment was completed, and a CaSi wire containing a CaSi alloy (35% Ca-65% Si) was added to molten steel with a pure Ca content of 0 in a ladle refining apparatus. It was added at 0.05 kg / (molten steel ton / min).
In process B, CaO powder was blown over 1.2 kg / ton of molten steel at a rate of 0.7 kg / (ton of molten steel) from a top-blown lance installed in the RH vacuum chamber, then REM was added all at once and refluxed for 3 minutes. After that, the CaSi alloy was added to the molten steel in the pan at a pure Ca content of 0.15 kg / (tons of molten steel).
In Process C, REM was added all at once and then refluxed for 3 minutes, and then the CaSi alloy was added to the molten steel in the ladle at a pure Ca content of 0.15 kg / (tons of molten steel).
The amount of REM added is 0.02 to 0.1 kg / ton of molten steel, and the amount of CaSi added is 0.1 to 0.4 kg / ton of molten steel.

After each process was completed, the ladle was transferred to a continuous casting machine and cast. The thickness of deposits in the nozzle was investigated from the immersion nozzle after 1-charge casting by the method described above, and inclusions were investigated from the obtained slab by the method described above to evaluate the nozzle blockage and inclusions. The molten steel composition after treatment, showing inclusions number ratio of _{(N B / (N A +} N B)) and producing results in Table 1. Numerical values and items outside the scope of the present invention are underlined. In both Process A and Process B, the ratio of the total number of inclusions A group and inclusion group B group to the total number of inclusions (excluding MnS) in the steel was 62.5% or more.

The MnS index was standardized with the result of test number 12 as 1, and the occlusion index with the result of test number 13 as 1. Test numbers 1 to 11 satisfying claim 1 of the present invention suppress MnS formation and blockage. On the other hand, in Comparative Examples 12 to 24 that did not satisfy claim 1 of the present invention, it can be seen that either one or both of MnS formation and occlusion cannot be suppressed. Further, Test Nos. 4, 9, 10 and 11 use the process B specified in claim 3 of the present invention as a manufacturing method, but MnS and nozzle blockage are further suppressed, and by satisfying claim 3. It can be seen that a higher effect can be obtained.

Claims (3)

By mass%, C: 0.002% or more and 0.4% or less, Mn: 0.1% or more and 2% or less, Si: 0.001% or more and 1% or less, S: 0.001% or more and 0.005% Hereinafter, Al: 0.005% or more and 0.50% or less, O: 0.0013% or more and 0.005% or less, Ca: 0.0005% or more and 0.0045% or less are contained, and La, Ce, A steel containing one or more of Nd having a total concentration of 0.0005% or more and 0.0035% or less, and the balance being Fe and unavoidable impurities.
Regarding the composition of non-metal inclusions in steel, the Ca compound is defined as CaO in terms of oxide, and the compound consisting of one or more of La, Ce, and Nd is designated as RO in terms of oxide, and each non-metal is defined as RO. When the total content (% by mass) of the three components _{of Al 2} O ₃ , Ca O and RO in the inclusions is _{100%, the converted contents are (% Al 2} O ₃ ), (% CaO) and (% CaO), respectively. % R-O) and represent, inclusions B, and the number per unit area of the inclusions a group of such translation content is defined by (1) N _a, the conversion content is defined by the equation (2) per unit area number N _B of the group is, (3) satisfy the relationship of expression, the total inclusions number in steel (excluding MnS), the ratio of the inclusions group B a total number of the inclusions group a inclusions form control steel, characterized in der Rukoto 60%.
Inclusion group A: 10% ≤ (% Al ₂ O ₃ ) ≤ 30% and 50% ≤ (% RO) ... (1)
Inclusion B group: 30% ≤ (% CaO) ≤ 50% and (% RO) ≤ 30% ... (2)
_{0.6 ≦ N B / (N A} + N B) ≦ 0.75 ... (3) Further, in terms of mass%, one type of Cu ≦ 1%, Ni ≦ 10%, Ti ≦ 0.7%, Nb ≦ 0.5%, V ≦ 0.5%, Cr ≦ 1.5%, Mo ≦ 1% The inclusion morphological control steel according to claim 1, further comprising the above. From the group consisting of La, Ce, and Nd after spraying or blowing a flux mainly containing CaO onto the molten steel prepared by adjusting components other than Ca and La, Ce, and Nd using a vacuum degassing and refining facility. The method for producing an inclusion morphologically controlled steel according to claim 1 or 2, wherein one or more selected metals or alloys are added, and then Ca or a Ca alloy is added.

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