JP6524801B2 - High purity steel and its refining method - Google Patents

Description

  The present invention relates to a highly clean steel used for shipbuilding materials, line pipes, magnetic steel sheets, etc. and its refining method, and in detail, the concentration of REM sulfide in nonmetallic inclusions in steel by precisely controlling the concentration of lanthanide in steel. The present invention relates to a highly clean steel having high cleanliness by properly controlling.

  A number of techniques have been developed to reduce or render harmless nonmetallic inclusions (hereinafter referred to as inclusions) in steel for the purpose of corrosion resistance improvement, toughness improvement or workability improvement. In particular, since industrial restrictions such as treatment time extension are likely to occur for reduction of inclusions, many techniques have been developed for shape control to render inclusions harmless. For example, a technique for spheroidizing inclusions and suppressing MnS by adding Ca to molten steel to make the inclusions CaO-containing oxysulfides is well known.

  Furthermore, in recent years, a technology for improving castability and steel material performance has been developed by utilizing REM or REM oxide inclusions such as La, Ce, Nd.

  For example, in Patent Document 1, the technology for improving surface cleanliness and ridging resistance by containing 0.001 to 0.05% of REM contains 0.0001 to 0.03% of REM in Patent Document 2 A technique for improving castability by controlling the Al concentration and REM concentration in the steel to appropriate conditions when melting stainless steel has been disclosed.

  Moreover, in patent document 3, the casting property excellent in castability, surface property, and internal quality by making REM density | concentration in steel into 0.001 to 0.0044%, and controlling REM oxide density | concentration in inclusions in a suitable range Patent Document 4 and Patent Document 5 disclose a technique for improving surface properties by appropriately controlling the concentrations of CaO and REM oxide in inclusions in the technique for obtaining pieces.

  As described above, many techniques have been developed which utilize REM or REM and REM oxide inclusions.

  On the other hand, in the REM addition technology to molten steel, there are problems such as deterioration of cleanliness due to inclusions including REM and reduction of productivity due to nozzle clogging. Therefore, in order to avoid these problems, in the prior art described above, molten steel components and inclusion compositions have been investigated in detail, and appropriate conditions regarding steel components and inclusion compositions for solving the problems have been indicated.

Unexamined-Japanese-Patent No. 2001-279388 JP 2001-192723 A Unexamined-Japanese-Patent No. 2006-97110 Unexamined-Japanese-Patent No. 11-343516 JP 2000-8138 A

  However, the prior art has had two problems. The first problem is the control accuracy of the steel composition and the composition of inclusions, and the second problem is the deterioration of the cleanliness due to the REM oxysulfide.

  The first problem is described. As described in the prior art, appropriate REM concentration conditions and inclusion composition conditions are limited to a narrow range. That is, it is required to control the REM concentration in the molten steel within a narrow range. If it is an ordinary element, it is possible to control the concentration by accurately weighing the addition amount to the molten steel, but REM having strong affinity with O, S in molten steel and O, S in inclusions Then, since the REM concentration changes due to the reaction with these O and S, concentration control is difficult even if the addition amount is adjusted. Therefore, it is difficult to control the inclusion composition in a narrow range because the molten steel component can not be controlled with high accuracy.

  The second problem is described. Sufficient studies have been conducted on REM oxide inclusions and appropriate conditions have been derived, but there are many unclear points on REM sulfides formed by the reaction between REM and S, and the cleanliness by REM sulfides Of the oxide inclusions and the accuracy of the control of oxide inclusions.

  In view of the above problems, it is an object of the present invention to easily make REM sulfide concentration in steel significantly improved by making the REM sulfide concentration in inclusions, in particular, proper, in view of the above-mentioned problems, and industrially REM concentration in steel It is an object of the present invention to provide a high purity steel that is controlled with high precision and a method for refining the same.

  As a result of intensive studies to achieve the above object, the present inventors have found that controlling the concentration of REM sulfide in inclusions in a predetermined range improves the cleanliness of the steel.

The present invention has been made based on the above findings, and the summary thereof is as follows.
(1) In mass%, C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.001% to 0 Al: 0.005% or more and 1% or less, O: 0.005% or less, Ca: 0.001% or more and 0.0045% or less, and La, Ce, Nd A steel containing one or two or more in a total concentration of 0.0005% to 0.0035% and the balance being Fe and unavoidable impurities, and sulfurization of La, Ce, Nd in nonmetallic inclusions A high-purity steel characterized in that the total concentration of substances is 5% or more and 27% or less in mass%.
(2) Furthermore, it is characterized in that it contains at least one of Cu ≦ 1%, Ni ≦ 10%, Ti ≦ 0.7%, Nb ≦ 0.5%, V ≦ 0.5% by mass%. High-purity steel as described in (1).
(3) A characterized in that a flux obtained by mixing one or two or more selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ and Nd ₂ O ₃ with Ca or a Ca alloy is added to molten steel (1) Or the method for refining high purity steel according to (2).
(4) The ratio of the flux concentration R in one or more fluxes selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ , Nd ₂ O ₃ to the mass concentration Ca in Ca of the flux is: R / C is 0.5 or more and 2.0 or less, The refinement method of high purity steel as described in (3) characterized by the above-mentioned.
(5) The method of refining high purity steel according to (3) or (4), wherein the addition of the flux is carried out in an RH vacuum degassing apparatus.

  According to the present invention, highly clean and high function steel can be manufactured efficiently and stably.

Diagram showing the relationship between REM sulfide concentration in Ca-Al-REM-O-S inclusions and the number of normalized inclusions Diagram showing the relationship between R / C and inclusion A concentration

  Hereinafter, the present invention will be described in detail.

  First, steel components other than iron to be treated in the present invention were specified for the following reasons. In the present specification, "%" in steel composition and REM concentration (details will be described later) means "% by mass" unless otherwise specified.

  C: C acts as a deoxidizing element under reduced pressure, and also affects the activity of S and N. For this reason, if C is less than 0.002%, the deoxygenation effect becomes unstable, and if it exceeds 0.4%, the activity of S changes significantly, and the reaction mechanism changes. Therefore, C is 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 various steel properties. Therefore, if it is less than 0.1%, deoxidation becomes unstable, and if it becomes higher than 2%, the activity of S decreases, which makes desulfurization difficult. Therefore, the Mn concentration is 0.1% or more and 2% or less.

Si: Si as well as Mn is an element indispensable for deoxidizing stability, but if less than 0.001%, deoxidation becomes unstable, and if it exceeds 1%, the concentration of SiO _{2 in} inclusions increases, It becomes difficult to control the inclusion composition 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 it is essential to realize low O, low S and low N. 0.005% or more is required to obtain this deoxidation effect. On the other hand, if it becomes higher than 1%, it becomes difficult to realize low oxygen by increasing the concentration of dissolved oxygen again, so 1% or less is required.

  S: S is an element to be removed, but if it becomes higher than 0.005%, a large amount of MnS is generated in addition to sulfides such as REM and Ca, and the inclusion control accuracy is lowered. On the other hand, if it is less than 0.001%, the amount of desulfurizing agent used will increase significantly, resulting in an increase in cost. Therefore, in the present invention, molten steel of 0.001% or more and 0.005% or less is treated.

  O: O is an element to be removed, but when Si, Al and Mn are in the above concentration range, a large amount of non-metallic inclusions (hereinafter referred to as “when the O concentration is higher than 0.005%”) Inclusions ") will be present in the molten steel. Therefore, the O concentration is set to 0.005% or less.

  Ca: Ca is an element effective for deoxidation and desulfurization, and at the same time is also effective for inclusion type control. If the Ca concentration is less than 0.001%, deoxidation will be insufficient, and the REM required for deoxidation will increase, resulting in an increase in the amount of REM added and cost. When the Ca concentration increases to more than 0.0045%, the formation of CaS inclusions becomes active, and the formation of REM sulfide intended by the present invention is suppressed. Therefore, in the present invention, Ca is set to 0.001% to 0.0045%.

  In addition, one or more components of Cu 1% 1%, Ni 10 10%, Ti 0.7 0.7%, Nb 0.5 0.5%, V 0.5 0.5% as necessary for the purpose of securing strength and corrosion resistance May be added.

  Next, the concentrations of La, Ce, and Nd will be described.

  La, Ce, Nd: These REMs are constituent elements of inclusions for obtaining the clean steel of the present invention, and one or more of them are added. If these concentrations are less than 0.0005% in total, REM compounds can not be formed in inclusions. On the other hand, when it becomes higher than 0.0035%, the activity of S is reduced, and it is predicted that the reaction rate between S and inclusions decreases. Therefore, in the present invention, the total concentration of La, Ce and Nd is made 0.0005% or more and 0.0035% or less.

  Next, the reason why the total concentration of La, Ce, and Nd sulfide in inclusions is 5% or more and 27% or less will be described.

  In REM-added molten steel, REM such as La and Ce combines with O and S in molten steel to form oxides and sulfides, but in the case of Al deoxidation as in the present invention, sulfides are mainly It is thought that the behavior affects the number of inclusions.

Therefore, 50 kg molten steel is adjusted to the above-mentioned component range with a vacuum melting furnace, and La, Ce, Nd are added singly or in a predetermined amount, mixed and cast in a mold after 5 minutes to solidify the inclusion composition in steel EPMA It measured by. Moreover, the number of inclusions observed the same sample with the optical microscope, and measured the number of inclusions of 5 micrometers or more. The observation area is 400 mm ² . The inclusions were mainly Ca-Al-REM-O-S inclusions, and the ratio of Ca-Al-REM-O-S inclusions in all the inclusions measured was 91.5% or more. . From the concentrations of S, O and REM in the inclusions, the concentration of REM sulfide in the inclusions (hereinafter referred to as A concentration) was measured. The inclusion composition was analyzed by EPMA, the area of the region in which only REM and S coexisted was measured, and the A concentration was calculated from the concentration of each element of the entire inclusion using the ratio of the area area to the inclusion area. The relationship between the A concentration and the number of inclusions is shown in FIG .

  It can be understood from FIG. 1 that the number of inclusions is increased when the concentration of A (concentration of REM sulfide in inclusions) is less than 5%. This is because CaS is generated as a result of a decrease in S capture and absorption capacity by Ca-Al-REM-O-S inclusions due to a shortage of REM in molten steel. On the other hand, when the A concentration exceeds 27%, the number of inclusions increases. When the A concentration increases, the specific gravity of inclusions approaches the molten steel specific gravity, which is considered to be because the floating and separation of inclusions is suppressed. On the other hand, by setting the A concentration to 5% or more and 27% or less, both the CaS formation suppression and the inclusion floating separation are satisfied, so that the number of inclusions can be reduced.

  As described above, when the sum of the sulfide concentrations of La, Ce, and Nd in the nonmetallic inclusions is 5% or more and 27% or less, the cleanliness can be improved more than in the case of molten steel to which REM is not added.

  Next, a refining method for setting the A concentration (the concentration of REM sulfide in inclusions) to 5% or more and 27% or less will be described.

  As described above, it is possible to obtain a steel excellent in cleanliness by adjusting the A concentration, but using REM having a strong affinity for S and making the REM sulfide concentration in inclusions as low as 27% or less It is not easy to control stably. REM concentration in molten steel to be in equilibrium with inclusions of low A concentration is as low as several tens ppm or less, and since there is an appropriate lower limit for A concentration, it is necessary to control REM concentration in molten steel within a narrow range of ppm unit is there. It is for such a low concentration range. When metal Ce or misch metal alloy is directly added to the molten steel, the reaction between REM and inclusions and S, O is started before the REM concentration is mixed to a uniform concentration in the ladle in the molten steel, The concentration after uniform mixing does not reach the target concentration, and as a result, it becomes difficult to control the inclusion composition accurately within the target range.

  Therefore, the present inventors examined a method of supplying REM to molten steel using a chemical reaction, not a method of adding metal REM directly to molten steel. As described above, when REM is added to a metal or alloy and it melts before uniform mixing and an unintended reaction proceeds, as described above, it was thought that the following two points should be taken to avoid this. The first point is that REM addition is performed using a chemical reaction defined by chemical equilibrium. When a chemical reaction defined by chemical equilibrium is used, the concentration is uniquely defined by the equilibrium constant, so that nonuniformity, excessive decrease in concentration, or increase in concentration does not occur as in the dissolution at the time of metal addition. The second is to suppress the reaction rate which defines this equilibrium. Although the REM concentration is uniquely defined by chemical equilibrium, it is not appropriate to use an extremely fast chemical reaction to avoid stabilization or local equilibrium and achieve more stabilization.

  As a result of earnestly examining techniques to satisfy the above two points, we devised a technique to mix and add Ca or a Ca alloy and REM oxide.

When chemical equilibrium is used to define REM concentration, deoxidation equilibrium is dominant, but this requires precise control of the oxygen concentration. Therefore, for example, the redox reaction represented by the following formula is used.
Ce ₂ O ₃ + 3Ca = 3CaO + 2Ce (1)
Here, Ce is the target concentration, and it is the Ce oxide activity and the Ca concentration that define the target concentration. Therefore, it was considered that both regulatory factors can be controlled by mixing and adding Ce oxide and Ca. In addition, in order to suppress excessive rise of Ca, it was considered that mixed addition under reduced pressure was appropriate, taking advantage of the low boiling point of Ca. Next, this phenomenon was verified experimentally.

A 50 kg molten steel is adjusted to the above-mentioned component range, a single or mixed oxide of La, Ce, Nd under reduced pressure (Ar 133 to 8000 Pa) or atmospheric pressure (Ar 101 kPa) and a CaSi alloy (Ca concentration 30 A predetermined amount of a flux obtained by mixing%) and an arbitrary ratio was added, and the inclusion composition in molten steel was measured by EPMA. The mass ratio of the mass concentration R in the flux of La ₂ O ₃ , Ce ₂ O ₃ and Nd ₂ O _{3 to} the concentration C of Ca in the flux is defined as R / C. The sulfide concentration A concentration in the inclusions of REM was measured from the concentration of S, O, and REM in the inclusions. The results are shown in FIG.

  It was confirmed that the A concentration was controlled to be 5% or more and 27% or less which is the appropriate composition except in the case where R / C = 0, that is, when the REM oxide was not added. Furthermore, according to FIG. 2, inclusions are controlled near the median value of the appropriate composition in the range of R / C (concentration of REM oxide in flux / concentration of Ca in flux) from 0.5 to 2.0. It was confirmed that the inclusion control accuracy is further improved by setting R / C to 0.5 or more and 2.0 or less.

  Next, focusing on the results in the normal pressure atmosphere and the reduced pressure atmosphere, the A concentration (rem sulfide concentration in inclusions) obtained in the reduced pressure atmosphere is distributed around the appropriate range central value. From this fact, the effect can be further enhanced by adding the present flux in a reduced pressure atmosphere such as RH.

  An embodiment in which the present invention is implemented using a converter and an RH will be described. After the converter processing is finished, the molten steel is discharged to a ladle. An alloy such as Si or Mn may be added at the time of tapping, or an iron-forming agent such as CaO may be added. Moreover, you may use a slag modifier and Al for the purpose of reducing the low-grade oxide in slag at the time of tapping. At this time, the amount of slag is preferably 10 kg / ton or more. This is because if the amount of slag is small, the effect of covering the surface of the molten steel will be small, and it will be susceptible to reoxidation from the atmosphere and nitrogen adsorption. Further, in the slag composition, the total of FeO and MnO in the slag is preferably 3% by mass or less, more preferably 1.5% by mass or less. When the FeO and MnO concentrations in the slag are high, the deterioration in cleanliness due to reoxidation after refining treatment to the end of casting is likely to progress.

  Immediately after transferring the ladle to the RH, start processing. The treatment with RH involves vacuum degassing such as dehydrogenation, molten steel temperature control, component control, and REM addition according to the present invention. These treatments may be performed in any order, but preferably in the order of temperature control, component adjustment, vacuum degassing, and the present invention. This is due to the following reasons. The temperature adjustment is performed by adding oxygen to the molten steel and supplying oxygen to the molten steel to utilize the oxidation heat of Al, but in this process, since alumina-based inclusions are formed, the cleanliness may be slightly deteriorated. is there. In order to reduce these aluminas, it is necessary to adjust the temperature earlier.

The addition time of the flux (hereinafter referred to as “flux”) in which one or more selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ and Nd ₂ O ₃ (hereinafter referred to as REM flux) and Ca or Ca alloy are mixed It is the same as the addition of La, Ce, Nd metal or alloy. It is preferable to adjust the components other than Ca, La, Ce and Nd to the steel components of the present invention as molten steel components at the time of flux addition. Further, it is desirable that the flux composition satisfies the fourth aspect. The amount of flux addition is preferably 0.3 kg / t or more and 0.7 kg / t or less in terms of REM flux. If it is less than 0.3 kg / t, it may be influenced by the amount of inclusions in the molten steel, and if it is more than 0.7 kg / t, the amount of slag becomes excessive and RH treatment becomes difficult.

  The flux is added by spraying onto the surface of the molten steel in the vacuum tank through the upper blowing lance placed in the RH vacuum tank, or in the ladle or in the molten steel in the ladle during the RH processing. The method may be any method such as a method of blowing and adding, or a wire feeder method of feeding and adding a wire to a molten steel in a ladle as an iron-coated wire. The following is an example of a method of spray addition to the surface of molten steel in a vacuum tank through an upper blowing lance disposed in an RH vacuum tank.

  The flux supply rate is preferably 0.05 to 0.2 kg / (ton · min), more preferably 0.8 to 1.5 kg / (ton · min), at a rate of 1 ton of molten steel. If the powder feed rate is too slow, the total processing time will be long, causing operational challenges such as temperature drop. On the other hand, if it is excessively early, flux is deposited in the vacuum chamber, and the reaction between REM flux and Ca in the intended molten steel is inhibited.

  The atmosphere pressure in the vacuum tank at the time of flux addition may be a pressure that can maintain the reflux of RH, that is, 13 kPa or less, preferably 6.5 kPa or less and 3.9 kPa or more. If the pressure is less than 3.9 kPa, the REM may be consumed by scattering or evaporation of the REM. If the pressure is higher than 6.5 kPa, the reflux rate may be slow, and the time required for uniform mixing may be long.

In order to further enhance the effect of one or two or more elements selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ , Nd ₂ O ₃ and Ca or a Ca alloy as the flux, Al or Mg metal or These alloys may be mixed. The preferred range of the metals Al and Mg is 5 to 15% by mass concentration in the flux. If it is less than 5%, the effect is small, and if it exceeds 15%, the wear of the refractory in the vacuum chamber proceeds.

Further, the flux may be mixed with oxides other than REM such as Al ₂ O ₃ and CaF ₂ or compounds such as fluoride of Ca for the purpose of lowering the melting point of the flux, but in the present invention, high refining You do not need these, as you can fully demonstrate your ability.

The speed and amount of the flux top blowing are preferable, but the vertical distance between the bottom end of the top blowing lance and the surface of the molten steel in the vacuum tank is preferably 1 m to 3 m. If it is less than 1 m, the wear of the lance by the splash becomes severe, and if it is higher than 3 m, the landing efficiency of the flux powder may be reduced. The carrier gas is preferably an inert gas such as Ar, and the flow rate is preferably ² Nm ³ / min or more and 10 Nm ³ / min or less. 2Nm conveyance of flux powder becomes unstable in less than ³ / min, the splash or the like becomes severe becomes higher beyond 10 Nm ³ / min.

  The shape of the lance nozzle may be any shape such as a laval, straight or tapered, but a laval nozzle is desirable for powder acceleration.

  In addition, when blowing and adding a flux into molten steel in a ladle during RH processing, the same conditions as the upper blowing addition may be used. However, it is desirable that the tip of the blowing lance be in the vertical lower region of the RH rising and dipping pipe. When the blowing is performed from other than this region, the RH reflux and the stirring flow by the blowing gas may interfere with each other, and the RH reflux may be stagnated.

300t of molten metal is charged into the upper and lower blowing converter and rough decarburizing blowing is performed until the C content in the molten iron becomes 0.03 to 0.2%, and the end point temperature is 1630 to 1650 ° C. The steel was extracted into a ladle, and various deoxidizers and alloys were added at the time of steel extraction to adjust the molten steel components in the ladle to the concentrations shown in Table 1, C, Si, and Mn, and the P concentration was adjusted to 0. It was referred to as 005 to 0.013% and Al: 0.007 to 0.05%. Furthermore, CaO was added at the time of tapping, the CaO / Al ₂ O ₃ weight ratio in the slag was adjusted to 2 to 2.5, and the total concentration of FeO and MnO in the slag was adjusted to 5% or less.

  Thereafter, the ladle was transferred to the RH, and the RH treatment was started promptly. At RH, the temperature was adjusted first, followed by adjustment of molten steel components such as Ni, Nb and Cu (alloy addition). Thereafter, a flux consisting of REM oxide, Ca alloy, and metal REM was added under the conditions shown in Table 1. For comparison, REM was not added in Test No. 1. In addition, in Table 1, "the REM addition amount" is the addition amount which converted REM oxide in a flux, or REM into metal REM. Also, “R / C” is a mass ratio of REM oxide concentration in flux / Ca concentration in flux.

  After the addition, RH refluxing was performed for 3 minutes to complete the RH treatment, and casting was performed using a continuous casting machine. After casting, a sample was cut out from the slab, and the A concentration (the concentration of REM sulfide in inclusions) and the number of inclusions were measured by the method described above.

  The results are shown in Table 1. Table 1 shows the cleanliness after normalization with A concentration and the number of inclusions in test No. 1 as 1 (= number of inclusions in each test No./number of inclusions in test No. 1).

  The test numbers 2 and 3 were those to which metal Ce was added, and the A concentration did not satisfy the range of the present invention, and the cleanliness did not improve.

In the test numbers 4 to 7, a mixed flux of Ce ₂ O ₃ oxide having a R / C ratio of 2.5 and a CaSi alloy (30 mass% Ca content) was blown into and added to the molten steel in a ladle. The A concentration was within the scope of the present invention, and the cleanliness was also improved. In addition, the REM addition amount of test numbers 4-18 is a Ce conversion addition amount of Ce ₂ O ₃ .

In test numbers 8 to 11, a mixed flux of Ce ₂ O ₃ oxide and CaSi alloy with R / C = 0.7 or 1.8 was blown into and added to the molten steel in the ladle. As compared with the test numbers 4 to 7, the inclusion composition was higher than the median value, the controllability was improved, and the cleanliness was further improved accordingly.

In the test numbers 12 to 15, a mixed flux of Ce ₂ O ₃ oxide and CaSi alloy with R / C = 0.7 or 1.8 was sprayed and added to the surface of the molten steel in the RH vacuum tank. Further improvement in controllability and improvement in cleanliness were observed as compared to the test numbers 8-11.

In the test numbers 16 to 18, a mixed flux of Ce ₂ O ₃ oxide and CaSi alloy of R / C = 1.0 was blown into and added to the molten steel in the ladle. The C, Si, and Mn concentrations in the molten steel differ greatly from Test Nos. 4 to 15, but the same effects as Test Nos. 8 to 11 are obtained, and even when the steel components are different, the same effects can be obtained by the present invention I understand that.

In test number 19, mixed flux of La ₂ O ₃ oxide and CaSi alloy with R / C = 1.8, in test number 20, Ce ₂ O ₃ oxide with R / C = 0.7, Nd ₂ O ₃ oxide And mixed flux of CaSi alloy were blown and added to the molten steel in the ladle respectively. Even when the La ₂ O ₃ oxide or Nd ₂ O ₃ oxide is used, the same effect as the case where the above-mentioned Ce ₂ O ₃ oxide is used can be obtained.

  As described above, according to the present invention, the cleanliness of molten steel can be stably improved.

Claims (5)

  C: 0.002% to 0.4% by mass, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.001% to 0.005% Al: 0.005% or more and 1% or less, O: 0.005% or less, Ca: 0.001% or more and 0.0045% or less, and one of La, Ce, Nd or A steel that contains two or more in total concentration of 0.0005% to 0.0035% and the balance is Fe and unavoidable impurities, and the sulfide concentration of La, Ce, Nd in nonmetallic inclusions A high-purity steel characterized in that the total is 5% or more and 27% or less in mass%.   Furthermore, it is characterized in that it contains one or more of Cu ≦ 1%, Ni ≦ 10%, Ti ≦ 0.7%, Nb ≦ 0.5%, V ≦ 0.5% by mass%. High clean steel as described in. A flux obtained by mixing one or two or more selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ and Nd ₂ O ₃ with Ca or a Ca alloy is added to the molten steel. The refinement | purification method of high purity steel as described in 2. The flux is selected from the group consisting of La ₂ O ₃ , Ce ₂ O ₃ and Nd ₂ O ₃ and the ratio R / C of mass concentration R in the flux to mass concentration Ca of Ca in the flux is 0 It is 5 or more and 2.0 or less, The refinement method of the high purity steel of Claim 3 characterized by the above-mentioned.   5. The method according to claim 3, wherein the addition of the flux is carried out in an RH vacuum degassing apparatus.

Images