JP2010236030A - Method for refining molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for refining molten steel with which a nozzle clogging in the time of continuous-casting caused by low melting point inclusion and the surface defect on a rolled steel sheet, can be prevented. <P>SOLUTION: A simple ladle-refining method, with which the molten steel in the ladle is refined while performing Ar gas stirring in the atmospheric pressure, is used. Rare-earth metal elements (REM) are added into the molten steel deoxidized with Al or Al-Si, and the additional quantity of the REM supplied into the molten steel is in the range of 5-20 ppm to the molten steel mass, and the REM-additional timing is after adjusting the finish-components, and the time from the starting of this addition to the finishing of this treatment, is performed at the timing of uniform mixing time or shorter. Then, the formed inclusions are made to be high melting point and harmless by forming the inclusion composition with mass ratio into 1-25% CaO, 8-95% Al<SB>2</SB>O<SB>3</SB>and 3-90% REM oxides. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for refining molten steel by Al deoxidation or Al-Si deoxidation used for hot-rolled steel sheets and cold-rolled steel sheets.

  In general, a rolled steel sheet such as a hot-rolled steel sheet or a cold-rolled steel sheet is manufactured by deoxidizing an undeoxidized molten steel melted in a converter with Al or Al and Si. Inclusions mainly composed of alumina generated during deoxidation containing Al cause operational troubles such as nozzle clogging during continuous casting and surface defects in the rolling stage when removal from molten steel is insufficient. It is known.

  As a method of removing alumina from molten steel, a method of introducing Al as a deoxidizer at the time of steel output in a converter so that the floatation and separation time of alumina after deoxidation is as long as possible, and simple measures such as CAS are taken. In ladle refining such as the pot refining method or the RH vacuum degassing refining method, a method of intensively stirring molten steel to promote the floating and separation of alumina has been performed.

  However, there is a limit to the measures for flotation separation of alumina by these methods, and there is a problem that sliver flaws cannot be prevented because inclusions of several hundred μm or more cannot be completely removed.

Further, a method of adding Ca to modify the entire inclusions to CaO—Al ₂ O ₃ type low melting point inclusions is generally well known. However, since Ca is usually added as an inexpensive CaSi alloy. It is difficult to apply to steel plates with severe Si upper limit.

In order to solve the problem of nozzle clogging, Patent Documents 1 and 2 disclose T.A. A method is disclosed in which an appropriate amount of REM is added in accordance with the amount of O to reduce FeO and FeO.Al ₂ O _{3 serving} as binders. In Patent Document 1, the oxide inclusions are mainly composed of Al ₂ O ₃ and REM oxide, and the content of REM oxide is 0.5 to 15% by weight. In this composition range, aggregation and coalescence of Al ₂ O ₃ particles can be suppressed, and generation of coarse Al ₂ O ₃ clusters can be prevented. Patent Document 3 discloses a REM addition method that takes into account the amount of oxygen increase up to continuous casting.

  The simple ladle refining method refers to a ladle refining method in which molten steel in a ladle is refined while stirring Ar gas in an atmospheric pressure atmosphere. Specifically, as described in Non-Patent Document 1, a method of adding an alloy component while stirring molten steel in a ladle with argon gas, a SAB (Sealed Argon Bubbling) method, a CAB (Capped Argon Bubbling) method, a CAS (Composition Adjustment by Sealed Argon bubbling) method, immersion lance Ar blowing method and the like. It is characterized by being performed in an atmospheric pressure atmosphere, and does not include the RH vacuum degassing method in which refining is performed under reduced pressure.

  A cold crucible method is known as a method for evaluating the amount and composition of non-metallic inclusion particles in a sample collected from molten steel. Cold crucible is an apparatus in which an induction heating coil is wound around a water-cooled copper crucible divided in the circumferential direction as described in Patent Document 4. By applying a high-frequency AC current to the coil, a high-frequency AC magnetic field can be formed in the crucible without inductive loss due to the crucible. Can dissolve by exotherm. On the other hand, as a reaction of the pinch force acting on the molten steel, the outward volume force acts on the internal inclusions (insulators), and the inclusions can be collected on the metal surface. If the SEM-EDS analysis at a low magnification is performed on the portion where the inclusions thus obtained are accumulated, the average composition of the inclusions can be obtained with good representativeness.

JP 2004-52076 A JP 2004-52077 A JP 2007-254819 A International Publication WO96 / 28729 Pamphlet

Edited by the Japan Iron and Steel Association, “Third Edition Steel Handbook II Steel Making and Steel Making”, published on Mar. 15, 1989, pages 690 to 691 S. Ueda, K. Morita and N. Sano: ISIJ-International, vol. 38 (1998), No. 12, pp. 1292-1296

  None of the methods disclosed in Patent Documents 1 to 3 is intended to prevent clustering of alumina and reduce binder inclusions, and is not sufficient as a means for solving this problem.

In the simple ladle refining method, argon gas is blown from the bottom of the ladle for the purpose of stirring the molten steel. When the blown argon gas leaves the surface of the molten steel, the ladle slag on the surface of the molten steel is suspended in the molten steel by stirring the argon gas, and the ladle slag is caught in the molten steel. The ladle slag contains CaO. Moreover, since the alloy iron added for component adjustment contains impurities, such as CaO, these alloy iron impurities are also mixed in molten steel. Therefore, in the steel manufacturing method using the conventional simple ladle refining method, alumina can be reduced, but some of the inclusion composition changes to CaO—Al ₂ O ₃ inclusions. Since the ladle slag entrainment continues while the argon is blown, the formation reaction of the CaO-containing inclusions continues until the end of the simple ladle refining, so that the floating separation is not sufficiently performed.

FIG. 1 shows a photograph of inclusions collected by a cold crucible method for steel refined by a conventional simple ladle refining method. When the component analysis of inclusions was performed at two locations indicated by arrows in FIG. 1, the location where the CaO concentration was 10% and the Al ₂ O ₃ concentration was 90%, the CaO concentration was 50%, and the Al ₂ O ₃ concentration was 50%. The part of was seen. In other words, CaO—Al ₂ O ₃ -based inclusions with multiple compositions coexist in the molten steel. In addition, there are cases where MgO, SiO _{2 and the} like are contained, although the concentration is very small. Since the composition with a CaO concentration of 50% and an Al ₂ O ₃ concentration of 50% has a low melting point, it indicates that not only solid inclusions (high melting point) but also low melting point inclusions exist as liquids in the molten steel. In addition, it can be seen from the appearance of the inclusions shown in FIG. 1 that the solid inclusions (high melting point) are mixed with low melting point inclusions. Due to the inclusion of the low melting point, it adheres and accumulates on the ladle nozzle and the inner wall of the immersion nozzle, causing nozzle clogging. Moreover, a part is caught by the slab and causes surface defects in the steel sheet.

The present invention has been made to solve the above-described problems, and detoxifies a CaO—Al ₂ O ₃ inclusion having a plurality of compositions in which liquid inclusions and solid inclusions produced by a simple ladle refining method are mixed. The purpose of this is to prevent nozzle clogging during continuous casting in steel production and surface defects in rolled steel sheets.

That is, the gist of the present invention is as follows.
(1) In a molten steel refining method using a simple ladle refining method in which molten steel in a ladle is refined while stirring with Ar gas in an atmospheric pressure atmosphere, a rare earth element ( REM) is added and the amount of REM added to the molten steel is in the range of 5 to 20 ppm relative to the molten steel mass, and the REM addition time is after the final component adjustment in the simple ladle refining method, and The time from the addition to the end of the treatment is performed at a time equal to or less than the uniform mixing time, and the average inclusion composition in the steel is CaO% = 1 to 25%, Al ₂ O ₃ % = 8 to 95%, REM oxidation. A method for refining molten steel, characterized by comprising an oxide of a trace element as a residue.
(2) The molten steel is C: 0.01 to 1.5%, Si: 0.001 to 3.0%, Mn: 0.01 to 3.0%, P: 0.001 to 0% by mass. The refining of molten steel according to claim 1, characterized in that: 1%, S: 0.0001 to 0.05%, Al: 0.001 to 2.0%, the balance being Fe and inevitable impurities. Method.
(3) The molten steel is further in mass%, Nb: 0.001 to 0.1%, Ti: 0.001 to 0.2%, B: 0.0005 to 0.005%, one or more. The method for refining molten steel according to claim 2, comprising:

  By adding REM to the molten steel in the simple ladle refining method, the REM addition amount is in the range of 5 to 20 ppm with respect to the molten steel mass, and the REM addition time is immediately before the end of the simple ladle refining. Nozzle clogging due to inclusions and product defects can be greatly improved.

It is a SEM micrograph of the inclusion evaluated about the conventional example by the cold crucible method. It is a ternary phase diagram of CaO-Al ₂ O ₃ -REM. It is a figure which shows the relationship between the stirring time after completion of component adjustment by CAS method, and the number of inclusions in steel. It is a figure which shows the relationship between the manufacturing conditions in a CAS method, and a continuous casting nozzle clogging level. It is a case where it adds. It is a figure which shows the relationship between the amount of REM addition in the ladle of CAS, and a nozzle clogging level. It is a SEM micrograph of the inclusion evaluated about the example of this invention by the cold crucible method. It is a figure which shows the relationship between the amount of REM addition in the ladle of CAS, and a nozzle clogging level. In ternary phase diagram of CaO-Al ₂ O ₃ -REM, a diagram showing the distribution of the composition of inclusions of the present invention and comparative example embodiment.

Unlike the ultra-low carbon steel produced by the vacuum degassing method, the molten steel produced by the simple ladle refining method, in which the molten steel in the ladle is refined while stirring Ar gas in an atmospheric pressure atmosphere, is a simple ladle. Since the C concentration before being processed by the refining method is 0.01% or more, T.C. The amount of inclusions is low and the amount of inclusions produced is small. However, as a feature of the inclusion form in the molten steel melted by the simple ladle refining method, CaO—Al ₂ O ₃ inclusions are often generated. The same applies to both aluminum killed steel that is deoxidized only by Al and not subjected to Si deoxidation, and aluminum silicon killed steel that is deoxidized by Al and Si. As described above, the reason is that the ladle slag is relatively large by argon stirring in the simple ladle refining, and the impurities (especially Ca content) in the alloy iron added for component adjustment are separated under atmospheric pressure. It is hard to be done.

Therefore, by continuing Ar stirring in the ladle after adding the deoxidized component, the floating of inclusions and the separation time can be as long as possible. The method of promoting the floatation and separation of alumina has a problem that it rather promotes the generation of CaO—Al ₂ O ₃ in the secondary reaction from slag or the like.

FIG. 3 is a result of investigating the relationship between the stirring duration after the component adjustment of deoxidizing element input and the inclusion level in molten steel in refining using the simple ladle refining method by the CAS method. . The inclusion level immediately after the deoxidation element is added is 1. As shown in FIG. 3, the relationship between the stirring time and the amount of inclusions in the simple ladle refining method indicates that the amount of inclusions cannot be reduced even if the stirring time is extended beyond a certain level. FIG. 1 shows a result obtained by evaluating a sample collected from molten steel after 10 minutes of stirring time by a cold crucible method. As shown in FIG. 1, CaO—Al ₂ O ₃ inclusions in which liquid inclusions and solid inclusions are mixed remain.

  Accordingly, the inventors focused on the action of REM oxide to increase the melting point of inclusions, and conducted experiments and examinations on the melting point of inclusions by adding REM to the inclusions generated by the simple ladle refining method. Repeated. In the present invention, the rare earth element (REM) means one or more elements of Ce, La, Pr, or Nd.

  FIG. 4 shows the result of examining the REM addition time necessary for reducing the clogging of the immersion nozzle with the steel having the composition shown in Table 1. The submerged nozzle clogging level was obtained from the amount of change in the stopper opening of the continuous casting equipment. After the alloy component such as Al was finally added by CAS, argon stirring was continued for 6 minutes to complete CAS.

  The left end of FIG. 4 is a conventional example in which REM is not added. Immersion nozzle clogging occurred, and the nozzle clogging level at that time was normalized as “1”. The center and the right end are examples in which REM is added from the top of the pan corresponding to 6 ppm relative to the molten steel. About the center, it is a case where REM addition timing is set as the same period as component adjustment. The nozzle clogging level was not improved despite the addition of REM. On the other hand, when REM was added immediately before the end of CAS treatment (one minute before), it effectively functioned to reform the inclusions produced in the treatment process, and an effect was obtained. From the above results, it was found that the effect of adding REM cannot be obtained if the stirring time after adding REM with CAS is too long.

  Next, FIG. 5 shows the result of examining the REM addition amount necessary for reducing the clogging of the immersion nozzle with Al deoxidized steel having the composition shown in Table 1. Here, the REM addition amount represents the amount of REM supplied to the molten steel from the top of the pan in a mass ratio with the molten steel mass, and is a concept different from the REM content in the molten steel. REM addition in CAS was made 1 minute before the end of the treatment. When the amount of REM added is insufficient, the effect of improving clogging of the immersion nozzle is low. On the other hand, when the amount of REM added is excessive, clustering of the REM oxide alone occurs, so that the immersion nozzle clogging is worsened. Therefore, in order to stably reduce the clogging of the immersion nozzle in actual operation, it has been found that it is preferable that the amount of REM added by CAS from the pan is 5 to 20 ppm in terms of mass ratio. When the REM addition amount is 5 to 20 ppm, since the REM yield remaining in the steel is low, the REM concentration in the steel is 5 ppm or less. Moreover, if it is this REM addition amount range, clogging of a ladle nozzle will also be improved. In the present invention, the reason why the amount of REM added to the steel is not the REM content in the steel but the purpose is to control the composition of inclusions contained in the molten steel.

As described above, in order to achieve the inclusion composition by the simple ladle refining method, the treatment is performed after CaO-Al ₂ O ₃ is sufficiently generated by the treatment and after the final component adjustment and after the addition of REM. It is preferable that the time until completion is short and that no slag is involved. That is, it may be within the range of the uniform mixing time τ (generally, the uniform mixing time of CAS is about 100 to 200 s). Here, the uniform mixing time τ is estimated by the following equation using the stirring power value ε in the case of gas stirring.
ε = ((6.18 · V _g · T _l ) / M _l ) ln (1+ (h ₀ /(1.46×10 ^-5 · P ₀ )))
τ = 800 ・ ε ^-0.4
Here, V _g : gas flow rate (Nm ³ / min), M _l : molten steel mass (ton) in the ladle, T _l : molten steel temperature (K), h ₀ : gas blowing depth (m), P ₀ : Molten steel surface pressure (Pa), ε: stirring power value (W / ton), τ: uniform mixing time (s)

  In simple ladle refining, it is preferable to secure at least a uniform mixing time for the time from the final component adjustment of the alloy components other than REM to the end of the treatment.

For aluminum killed steel, the amount of REM added from the top of the pan in CAS is 5 to 20 ppm by mass, and the time from the end of REM addition to the end of treatment is within the range of uniform mixing time τ, the inclusions in the steel Evaluation was performed. FIG. 6 shows an inclusion form in which cold crucible evaluation was performed on a sample collected from molten steel in a continuous cast tundish. It should be noted that the time from the final adjustment of the alloy to the end of the treatment is 5 minutes, the amount of REM added = 5 ppm, and the time from the end of the REM addition to the end of the treatment is 1 minute (τ = 3.3 minutes). From the form of the inclusion appearing in the photograph of FIG. 6, it can be seen that the inclusion was solid at the molten steel temperature. When it was component analysis of inclusions on two positions indicated by arrows in FIG. 6, CaO concentration: 35% Al ₂ O ₃ concentration of 25% and part of the REM concentration 40% composition, CaO concentration 18% Al Values of ₂ O ₃ concentration 55% and REM concentration 27% were obtained. That is, this inclusion composition was solid at the molten steel temperature as shown in FIG. 2, and it was confirmed that the inclusion form in FIG. 6 was modified to a solid inclusion (high melting point).

In the present invention, the inclusion composition in the steel is, by mass ratio, CaO% = 1 to 25%, Al ₂ O ₃ % = 8 to 95%, REM oxide% = 3 to 90%, and trace elements as the residue It has been found that nozzle clogging can be prevented by modifying the inclusions to high melting point inclusions by using a composition containing this oxide. For the inclusion composition evaluation, a sample is taken from the molten steel in the continuous casting tundish, the inclusion is extracted by the cold crucible method, the inclusion composition analysis is performed by SEM-EDS, and the average value of the four locations in the sample is used. Can be calculated.

In the inclusion composition in steel, it is important not to intervene liquid phase inclusions. Therefore, it is a composition that avoids the low melting point region (hatched portion) in the phase diagram of CaO—Al ₂ O ₃ —REM oxide (FIG. 2). I just need it. Therefore, the CaO-Al ₂ O ₃ -REM oxide phase diagram basically has a CaO concentration of 0% at the bottom, a CaO concentration of 25% at the top, an Al ₂ O ₃ concentration of 0%, and a REM oxide. A trapezoidal portion surrounded by a concentration of 0% is a basic condition of the inclusion composition of the present invention.

Therefore, first, the upper limit value of the CaO concentration is 25%. In addition, it is known that the contact angle decreases from Al ₂ O ₃ to CaO—Al ₂ O ₃ inclusions and approaches 90 °. Clustering proceeds when a liquid phase is present, but clustering is difficult to occur with solid inclusions. The effect becomes remarkable when the CaO concentration is 1% or more. Therefore, the lower limit value of the CaO concentration is 1%. The lower limit value of the CaO concentration is more preferably 5%.

Since the effect becomes significant when the REM oxide concentration is 3% or more, the lower limit is set to 3%. In addition, by containing CaO, the effect of reducing clustering is seen even on the higher concentration side of the REM oxide concentration in the Al ₂ O ₃ -REM oxide. However, in the present invention, the amount of REM added is in the range of 5 to 20 ppm. Under these conditions, REM oxide alone is not seen, and therefore REM oxide alone is not included. Therefore, the upper limit of the REM oxide concentration is 90%.

Since the present invention uses a simple ladle refining method, the reaction between molten steel and slag is inevitable. Therefore, the inclusion of Al ₂ O ₃ alone is difficult to observe, so the upper limit of the Al ₂ O ₃ concentration is 95%.

In the present invention, components other than CaO, Al ₂ O ₃ and REM oxide in inclusions are 10% or less.

  One or more rare earth elements (REM) such as Ce and La are added to Al deoxidized or Al-Si deoxidized molten steel, and the amount of REM added from the pan on CAS is 5 to 20 ppm by mass, The REM addition time is after the final component adjustment in the simple ladle refining method, and the time from the addition to the end of the treatment is set to a time equal to or less than the uniform mixing time so that the inclusion composition is within the above preferred range. Can do.

  Al deoxidized steel and Al-Si deoxidized steel used in the present invention are C: 0.01 to 1.5%, Si: 0.001 to 3.0%, Mn: 0.01 by mass%. -3.0%, P: 0.001-0.1%, S: 0.0001-0.05%, Al: 0.001-2.0%, the balance being made of Fe and inevitable impurities Is a carbon steel characterized by If necessary, it is possible to use carbon steel in which trace elements such as Nb, Ti, and B are added to improve the characteristics of the steel.

  C is an element that stably improves the strength of steel, and the content is adjusted in the range of 0.01 to 1.5% according to the strength of the desired material. In simple ladle refining such as Ar bubbling and CAS, C is generally 0.01% or more. The reason why the content is 1.5% or less is that the workability of the steel material is deteriorated when the content exceeds this value.

  The reason why Si is set to 0.001 to 3.0% is that, if it is less than 0.001%, the refining cost burden is large and the economic efficiency is poor, and if it exceeds 3.0%, the surface properties of the steel material deteriorate. In Al deoxidized steel, the Si content is at the impurity level. In the Al—Si deoxidized steel, the Si concentration becomes a finite value.

  The reason why Mn is set to 0.01 to 3.0% is that if it is less than 0.01%, the refining cost burden is large and the economic efficiency is poor, and if it exceeds 3.0%, the workability of the steel material deteriorates.

  The reason why P is set to 0.001 to 0.1% is that if it is less than 0.001%, the refining cost burden is large and the economic efficiency is poor, and if it exceeds 0.1%, the workability of the steel material is deteriorated.

  The reason why S is set to 0.0001 to 0.05% is that, if it is less than 0.001%, the refining cost burden is large and the economy is poor, and if it exceeds 0.05%, the workability and corrosion resistance of the steel material deteriorate. .

  When Al is made 0.001 to 2.0%, if it is less than 0.001%, deoxidation becomes insufficient, bubbles are generated during casting, and castability is remarkably deteriorated. Further, if over 2.0%, the workability of the steel material deteriorates.

  If necessary, the steel used in the present invention is mass%, Nb: 0.001 to 0.1%, Ti: 0.001 to 0.2%, B: 0.0005 to 0.005%. Contains one or more.

  Nb is set to 0. By containing 001% or more, the strength improvement effect is shown. On the other hand, if added over 0.1%, the toughness may be impaired. Therefore, when Nb is added, the content range is set to 0.001 to 0.1%.

  Ti is set to 0. By containing 001% or more, the strength improvement effect is shown. On the other hand, if added over 0.2%, the toughness may be impaired. Therefore, when Ti is added, the content range is set to 0.001 to 0.2%.

  B is an element that improves the hardenability of the steel and increases the strength. By adding 0.0005% or more, it shows an effect of improving the strength. However, if added over 0.005%, the precipitate of B increases. Therefore, the toughness may be impaired, so the content is limited to 0.0005 to 0.005%.

The molten steel blown out in a 100 t converter was adjusted to the target component with CAS, which is a simple ladle refining facility. After completing the component adjustment and deoxidation other than REM, REM was added. Note that the REM-containing alloy used is an Fe-Si-REM alloy containing, in mass%, Si = 35%, La = 10%, and Ce = 20%. Casting of the molten steel that has undergone CAS treatment in a continuous casting facility under the conditions of a mold size of 250 mm thickness x 980 to 1800 mm width, a casting speed of 0.6 to 1.2 m / min, and a molten steel temperature in the tundish of 1520 to 1590 ° C The slab was manufactured. The specifications used for calculating the uniform mixing time τ are V _g = 0.23 to 0.32 Nm ³ / min, M _l = 100 ton, T _l = 1873 K, h ₀ = 2.6 m, and P ₀ = 101325 Pa. The time from the end of final component adjustment in CAS to the end of processing was uniformly 5 minutes.

Example 1

Using Al-Si deoxidized steel having the components shown in Table 2, steel was manufactured with various REM addition amounts. The argon gas flow rate V _{g in} CAS was 0.27 Nm ³ / min. The time from the addition of REM to the end of the treatment is 1 minute (τ = 3.3 minutes). FIG. 7 shows the clogging state of the immersion nozzle obtained from the change in the stopper opening of the continuous casting equipment. The evaluation of the immersion nozzle, that is, the situation was performed in the same manner as in the case of FIG. Similarly to the above-mentioned Al deoxidized steel, nozzle clogging is improved in Al-Si deoxidized steel by setting the REM addition amount to 5 to 20 ppm. Considering the addition cost of the REM alloy and the versatility of various steel components, it is preferable to manufacture at a REM addition amount of about 5 ppm.

(Example 2)

Using Al deoxidized steel and Al-Si deoxidized steel having the components shown in Table 3, the REM addition amount and REM addition time were variously changed to produce steel. Those having the same numerical value in the “steel type” column of Table 3 mean that they were manufactured with the same component target. In order to change the uniform mixing time τ, the argon gas flow rate V _g in CAS was changed in the range of 0.23 to 0.32 Nm ³ / min.

  The same Table 3 shows the result of the product defect caused by the nozzle clogging situation and inclusions. As a nozzle clogging evaluation method, it was determined from the amount of change in the stopper opening of the continuous casting equipment. ◯: Change margin ≦ 3 mm, Δ: Change margin ≦ 5 mm, ×: Change margin> 5 mm As a surface defect evaluation method, the presence or absence of defects caused by inclusions detected in the rolling process was evaluated.

  For the inclusion composition evaluation, a sample is taken from the molten steel in the continuous casting tundish, the inclusion is extracted by the cold crucible method, the inclusion composition analysis is performed by SEM-EDX, and the average value of the four locations in the sample is used. Calculated. FIG. 8 shows the inclusion composition.

  In the present invention example, the inclusion composition was within the scope of the present invention, and it was confirmed that nozzle clogging and product defects were greatly improved as compared with the comparative example.

  On the other hand, for Comparative Examples 1 to 19 in which REM was not added and Comparative Examples 20 and 21 in which the stirring time from the addition of REM to the end of treatment exceeded the uniform mixing time, the inclusion composition was outside the scope of the present invention, and the nozzle was clogged. The product defects were poor in comparison with the examples of the present invention.

Claims (3)

In the refining method of the molten steel using the simple ladle refining method of refining the molten steel in the ladle while stirring Ar gas in an atmospheric pressure atmosphere,
The amount of REM added to the molten steel by adding rare earth elements (REM) to the Al deoxidized or Al-Si deoxidized molten steel is in the range of 5 to 20 ppm with respect to the molten steel mass. After the final component adjustment in the simple ladle refining method, and the time from the addition to the end of processing is equal to or less than the uniform mixing time,
Average inclusion composition in steel by mass ratio, CaO% = 1-25%, Al ₂ O ₃ % = 8-95%, REM oxide% = 3-90%, trace element oxide as the residue A method for refining molten steel, comprising:   The molten steel is C: 0.01-1.5%, Si: 0.001-3.0%, Mn: 0.01-3.0%, P: 0.001-0.1% by mass%. The method for refining molten steel according to claim 1, wherein S: 0.0001 to 0.05%, Al: 0.001 to 2.0%, and the balance is Fe and inevitable impurities.   The molten steel further contains 1% or more of Nb: 0.001 to 0.1%, Ti: 0.001 to 0.2%, and B: 0.0005 to 0.005% in mass%. The method for refining molten steel according to claim 2.