JPH07122089B2 - Method for manufacturing slabs for grain-oriented silicon steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a slab of grain-oriented silicon steel sheet useful as an iron core material of a transformer, and a slab containing Al and N as main inhibitor components. With respect to the above, the present invention intends to propose a manufacturing method which enables advantageous improvement of the surface properties of the grain-oriented silicon steel sheet to be produced without deterioration of magnetic properties.

[0002]

2. Description of the Related Art Grained silicon steel sheets have magnetic properties
High magnetic flux density and low iron loss are required.
In recent years, due to the progress of manufacturing technology, for example, in a steel plate having a plate thickness of 0.23 mm, the magnetic flux density B ₈ (value at a magnetizing force of 800 A / m): 1.
92T was obtained, and iron loss characteristics W _17/50 (50Hz, 1.7
It is also possible to produce excellent products on an industrial scale such that the value of T at the maximum magnetization) is 0.90 W / kg. Materials with such excellent magnetic properties are easy to magnetize iron <001
〉 Orientation is composed of crystal orientations that are highly aligned with the rolling direction of the steel sheet, and such a texture is called the so-called Goss orientation during the final finish annealing during the manufacturing process of grain-oriented silicon steel sheet. It is formed through a phenomenon called secondary recrystallization in which crystal grains having a {110} <001> orientation are preferentially grown hugely. The basic requirement for sufficiently growing the secondary recrystallized grains having the {110} <001> orientation is that crystal grains having an unfavorable orientation other than the {110} <001> orientation in the two-hour recrystallization process. It is a well-known fact that the existence of an inhibitor that suppresses the growth of Al and the formation of a primary recrystallized structure suitable for sufficiently developing the secondary recrystallized grains in the {110} <001> orientation are essential. .

As the inhibitor, MnS,
Fine precipitates such as MnSe and AlN are utilized, and in addition to these, grain boundary segregation type components such as Sb and Sn as disclosed in JP-B-51-13469 and JP-B-54-32412 are combined. Addition is performed to reinforce the effect of the inhibitor. By the way, generally, those using MnS or MnSe as the main inhibitor have been advantageous in reducing iron loss because the secondary recrystallized grain size is small, but in recent years, laser irradiation method and plasma jet method have been used. Since it became possible to miniaturize magnetic domains by artificially introducing pseudo grain boundaries,
The advantage due to the small secondary recrystallized grain size decreased, and the advantage due to the high magnetic flux density increased. A method for obtaining a grain-oriented silicon steel sheet having a high magnetic flux density has been known for a long time.For example, as described in Japanese Patent Publication No. Sho 46-23820, final cold rolling is performed by adding AlN as an inhibitor component to the steel. It is said that it can be manufactured by the above three-point bonding, in which the cooling of the previous annealing is rapidly cooled to precipitate AlN, and the final cold rolling reduction rate is set to 80 to 95% and the high pressure reduction rate.

However, in the production of a steel sheet containing AlN as an inhibitor component, there was a problem that surface defects, specifically blisters, frequently occurred. This surface defect tends to increase more and more as the product plate thickness decreases, and has become a big problem when iron loss is reduced by reducing the plate thickness of the steel plate as in recent years.

The cause of this surface defect is considered to be that N (nitrogen), which is present in a large amount in the molten steel before casting, forms bubbles in the steel as a gas during solidification. On the other hand, some preventive measures have been proposed. For example, in Japanese Examined Patent Publication No. Sho 49-42208, it is necessary to set H in the molten steel to 3 ppm or less and N to [Al (%) × 10 ³ +40] ppm or less in order to prevent blisters from being generated in the final product of silicon steel. It is disclosed that there is. That is, in the Al-containing silicon steel, the occurrence of blisters is observed when the N content is high, and therefore it is necessary to limit the N content depending on the Al content. In addition,
49-42211 discloses that the content of N in steel is {Al (%)
If it is higher than × 10 ³ +50} ppm, blistering occurs, so it is disclosed that the amount is suppressed below this value.

From such a viewpoint, in the production of a silicon steel sheet containing AlN as an inhibitor, a steel slab having an N content as industrially contained is used without special nitrification treatment, or In order to obtain a higher N content, a steel slab having an N content that is usually contained industrially is used and subjected to a nitriding annealing nitrification treatment in a cold rolling process (special (See Japanese Patent Publication No. 54-19189),
As shown in Examples 1 to 3 of JP-A-2-200731, a blister-free slab containing about 60 ppm N is used as a material, and N is introduced into the steel sheet during the final finish annealing to perform nitrification. Has been adopted.

However, as in the above-mentioned blister suppression measures, the technique of suppressing the N content or the technique of nitrifying during the manufacturing process causes insufficient AlN precipitation amount, or the magnetic properties are not constant. Prone to instability. Particularly with a steel composition containing Sb, it is extremely difficult to perform nitrification in the annealing process. Therefore, it has been necessary to accurately control a sufficient N content from the slab material.

Therefore, as a method of increasing the N content of molten steel of silicon steel without the occurrence of blister, a technique of controlling the N content of the steel ingot by placing calcium nitride in front of the ingot has been used. .

[0009]

In accordance with the change in casting technology for slab production from ingot-casting to continuous casting, even in the continuous casting method, the slab production for grain-oriented silicon steel sheet containing Al is performed. Therefore, it has been attempted to cast calcium nitride into molten steel and then perform continuous casting, but it has been found that the number of blisters increases sharply by such a method.
Therefore, a technique for regulating the N content according to the Al content has been developed.

The inventors have found that the increase in blisters that occurs when calcium nitride is added and continuous casting increases Ca-based inclusions near the slab surface, and from that, the generation of N ₂ gas bubbles increases. I found that it was due to doing. That is, when molten steel is solidified with an ingot, Ca-based inclusions are sufficiently discharged out of the steel system because it takes a long time to solidify, whereas when solidified by continuous casting, The Ca-based inclusions are trapped near the surface of the slab because they solidify in an extremely short time, and therefore blisters are generated starting from the Ca-based inclusions. From this mechanism, it is also possible to understand the phenomenon that the amount of blisters generated increases as the product plate thickness decreases. That is, it is understood that the defects that were latent under the surface layer of the steel sheet become more apparent as the reduction rate of hot rolling and cold rolling increases.

In order to avoid these problems, the inventors tried to increase the N content in the molten steel by blowing N ₂ gas in the secondary refining in the vacuum degassing equipment. However, when this technology is actually applied,
Blistering can be prevented, but the product often swells, and in extreme cases, slab heating, which is the next step of continuous casting, causes a problem of blistering on the slab surface from an early stage. This is a phenomenon that appears uniquely during the production of grain-oriented silicon steel sheets that are heated to a high temperature of 1350 to 1460 ° C. For example, when this slab is heated at 1250 ° C,
Although such defects do not appear, the magnetic properties are naturally inferior.

The present invention is directed to a slab containing high N, which is used for grain-oriented silicon which has no surface defects such as blisters and blisters on the grain-oriented silicon steel sheet to be produced and has good surface properties and excellent magnetic properties. The aim is to propose a method for advantageously manufacturing slabs.

[0013]

SUMMARY OF THE INVENTION The present invention provides molten steel for directional silicon steel by primary refining in a converter and secondary refining in a ladle, Si: 2.5-4.5 wt%, Al: 0.010- 0.040 wt
%, N: 0.0050 to 0.0130 wt% After being melted to a component composition, when cast into a continuous casting machine to manufacture a slab,
Performed blown by pressurized nitrogen treatment of N ₂ gas into the molten steel until prior to secondary refining in the late of the primary refining, and at the secondary refining, the oxide inclusions by blowing inert gas A method for producing a slab for grain-oriented silicon steel, characterized in that the content of molten steel N is adjusted to the above range by performing a removal treatment.

Here, it is advantageous to carry out deoxidation with a silicon alloy in a ladle after tapping from the converter and before secondary refining.

The process of clarifying the present invention will be described below. We have When checking products generated bulging details, check the fine inclusions SiO ₂ and Al ₂ 0 _3, the oxygen content as compared with other products not subjected to nitriding treatment Ten
The result is that the value is increased by about ppm. Such fine oxides are peculiar to grain-oriented silicon steel sheets, and even if the amount of oxygen increases by about 10 ppm, the number of them present in the steel becomes large. Therefore, N gathers in the slab heating with this oxide as a nucleus,
It seems that N ₂ bubbles were generated in the steel.

From this, the inventors have realized that the removal of such oxides is important for preventing blisters. Since the oxide in the molten steel of silicon steel is extremely fine, it is difficult to remove it, and a sufficient stirring force and stirring time of the molten steel in the secondary refining are required. However, it has been found that it is difficult to remove such oxide inclusions when only N ₂ gas is blown in during secondary refining as has been attempted conventionally.

Since it is more advantageous in terms of magnetic properties to increase the N content from the beginning, in the present invention, prior to secondary refining, for example, a nitrification treatment by blowing N ₂ gas during primary refining is adopted. By this, it is possible to perform nitrification while effectively suppressing the occurrence of blisters in products manufactured by continuous casting.
Further, the present invention has been completed by newly discovering that blowing Ar in the secondary refining is effective in preventing the occurrence of swelling of the product.

[0018]

The preferred component composition of the grain-oriented silicon steel slab of the present invention will be described. If Si is too small, the electric resistance becomes small and good iron loss characteristics of the product cannot be obtained. On the other hand, if it is too large, cold rolling becomes difficult, so the range is 2.5 to 4.5%.

Next, regarding the inhibitor, since AlN is particularly advantageous for the product to obtain a high magnetic flux density,
In this invention, AlN is used as the main inhibitor, but if it is too much, fine precipitation is rather difficult. Therefore, 0.01 ≦ acid-soluble Al ≦ 0.04%, and N is 0.0050 to 0.01.
The range is 30%. Acid-soluble Al less than 0.01%, N
When the content is less than 0.0050%, the amount of the inhibitor is insufficient and the magnetic properties are deteriorated. On the contrary, when the acid-soluble Al exceeds 0.04% or N exceeds 0.0130%, the precipitated AlN is coarse. And the function as an inhibitor deteriorates, and the magnetic properties deteriorate.

The term "primary inhibitor" as used herein means that the secondary recrystallized grains cannot be expressed if it is lacking. In the present invention, a secondary inhibitor may be used in addition to the major inhibitor. . For example, S and Se may be supplementarily contained as an inhibitor-forming component. S,
Se precipitates as MnS 2 or MnSe and is effective as an inhibitor. Of these, MnSe is particularly preferable because it has a strong suppressing effect even when the final finished plate thickness becomes thin. Such MnS, Mn
The range of S and Se suitable for finely depositing Se is about 0.01 to 0.04% in both cases of single and combined use.

Mn is an effective component for preventing embrittlement, and in order to exert its effect, it is preferably contained in an amount of 0.05% or more. On the other hand, the upper limit is preferably about 0.15% when S and Se are not supplementarily contained as an inhibitor-forming component, and about 0.10% when S or Se is contained.

Inhibitor-reinforcing components such as Sb, Cu, Cr, Bi, Sn, B, Ge, and P can also be added as appropriate, and the range is 0.015 to 0.060% for Sb and 0.03 to 0.03 for Cu.
0.30%, Cr 0.02-0.10%, Bi 0.005-0.020%, Sn
Is 0.03 to 0.20%, B is 5 to 25 ppm, and Ge is 0.005 to 0.060.
% And P are preferably 0.010 to 0.090%. The lower limits of the above components are all minimums for exhibiting the effect as an inhibitor reinforcing component, and the upper limits are from the viewpoint of avoiding embrittlement of the steel sheet and ensuring workability.

C is a component that effectively contributes to the improvement of the hot rolled structure, but if it is too much, it becomes difficult to decarburize, so 0.035-
It is preferably about 0.090%.

Further, in order to prevent surface defects due to hot embrittlement, Mo is preferably contained in the range of 0.005 to 0.020%.

In order to produce a slab having such a steel composition, the hot metal is first refined in a converter. At this time, the first technical point of the present invention is to subject the molten steel to nitrification treatment using a gas containing N ₂ between the latter stage of the primary refining in the converter and the second refining. That is, since the N content of molten steel is 20 to 40 ppm in normal converter blowing, it is necessary to blow only N _{2 in the} secondary refining in the next step in order to obtain the desired N content. This is because it is impossible to carry out the second refining, which is the second technical point of the present invention, by blowing Ar gas, and there is a joy that blistering occurs. Therefore, for this purpose, N in molten steel before secondary refining is
The content should be increased, preferably to 90 ppm or higher. If it exceeds 160 ppm, denitrification in secondary refining becomes difficult, so it is desirable to set the N content to 160 ppm.

The nitriding treatment in the converter may be at the end of blowing or at the time of rinsing after the blowing, but at the beginning of blowing, the N yield is extremely high due to bubbling of C0 gas. I don't use it because it's bad. It is also possible to blow the N ₂ -containing gas by blowing the gas into the molten steel in a ladle after tapping other than in the converter.

The gas used for the nitriding treatment may be pure N ₂ gas,
A mixed gas of propane or the like and N ₂ may also be used.When blowing into the converter, the nozzle to be blown includes a sublance in the upper blowing converter, and a bottom blowing nozzle in the lower blowing and up / down blowing converters. Blowing from a blowing nozzle is suitable for promoting nitrification.

Next, the molten steel discharged from the converter is subjected to secondary refining, that is, out-furnace refining. Prior to this secondary refining, it is desirable to perform deoxidation treatment with a silicon alloy. This is because the time from the deoxidizing treatment to the casting is taken as long as possible to separate and float the oxide-based inclusions from the molten steel.

In the secondary refining, various alloy additives are added to adjust the component composition. At this time, the oxide inclusions are separated and floated by using an inert gas to adjust the N content in the molten steel. Since denitrification is performed by injecting an inert gas, the amount of N component can be adjusted.

Since the effect of removing inclusions and the effect of denitrification vary depending on the flow rate of gas blown in, the temperature of molten steel, the stirring time, the degree of vacuum of equipment, etc., the treatment is carried out in consideration of these factors.

As the inert gas, Ar is mentioned as a representative, but needless to say, He, Ne, CO or the like may be used.

Equipment for secondary refining includes vacuum or reduced pressure,
Alternatively, a so-called vacuum or reduced pressure ladle refining equipment that repeats depressurization and pressurization and uses gas, atmospheric pressure, electromagnetic force, etc. as a molten steel stirring force is suitable.
H, PM (pulse mixing), VOD, TD, TN, LF,
VAD, VC, VODC, ASEA-SKF, etc. correspond. Among them, the vacuum degassing equipment is suitable for efficient inclusion removal and denitrification.

The molten steel melted through the above steps is made into a slab by a continuous casting machine. A known technique may be applied during continuous casting. After that, the slab is repressed if necessary, and the size is adjusted, followed by heating and hot rolling. The steel strip after hot rolling is made into a final strip thickness by one cold rolling or two or more cold rollings with intermediate annealing.

The annealing before final cold rolling requires a high temperature of 850 to 1200 ° C. for solution treatment of AlN, and after annealing, after annealing
A quenching process up to 500 ° C is required for N precipitation. As for the final cold rolling reduction rate, it is necessary to use a high pressure reduction rate in order to obtain a high magnetic flux density as known in the art. Therefore, the single reduction rolling reduction and the final cold rolling reduction reduction in the two-pass rolling method are performed. Is in the range of 80 to 95%. This is because if the rolling reduction is less than 80%, a high magnetic flux density cannot be obtained, while if it exceeds 95%, secondary recrystallization becomes difficult.

Further, in order to replenish the N in the steel which is lost by the oxidation of the surface during annealing in the cold rolling process, it is necessary to apply a nitriding accelerator before annealing to prevent excessive denitrification. Is preferable in terms of stability of magnetic properties.

The aging treatment performed during the final cold rolling is advantageous in reducing the iron loss of the product. In particular, the component system containing Sb has an excellent feature in that the magnetic flux density can be remarkably improved by only one aging treatment for a short time. The steel sheet after the final rolling is subjected to degreasing treatment, and then subjected to decarburization and primary recrystallization annealing.

Next, an annealing separating agent containing MgO as a main component is applied, and then wound in a coil shape and subjected to final finishing annealing, and then an insulating coating is applied if necessary. It goes without saying that the magnetic domain subdivision processing can be performed by other methods.

[0038]

[Example] Example 1 Blowing was carried out using a bottom blowing converter, at this time, before completion of blowing 3
N ₂ gas was blown in at a flow rate of 150 Nl / min using a bottom blowing nozzle for 1 minute. At this time, the N content of the molten steel was 123 ppm. After that, place 200 t in the ladle where FeSi alloy was placed in front.
Injecting tapped steel, Ar blowing treatment at RH equipment at 100 Nl / min
I went for 35 minutes. Further, various alloy additives were put into the RH treatment to melt various molten steels shown by A to N in Table 1.

[0039]

[Table 1]

These molten steels were continuously cast to a thickness of 250 mm.
Slab and after re-pressing to a thickness of 220 mm,
This slab was heated to 1430 ° C and hot-rolled to a thickness of 2.0 mm. The hot-rolled coil thus obtained was annealed at 1000 ° C., then cold-rolled to a thickness of 1.50 mm, intermediate-annealed at 1100 ° C. and a subsequent quenching treatment, and then cold-rolled again to obtain a thickness. 0.75 mm, then heat treated in a continuous annealing furnace at 300 ° C for 1 minute, then cold rolled to a thickness of 0.23 mm.
And the final plate thickness. Then, after degreasing treatment, decarburization / primary recrystallization annealing is performed at 850 ° C for 2 minutes, an annealing separating agent containing MgO as a main component is applied, and then the material is coiled and wound 1200
It was subjected to final finishing annealing at ℃ for 10 hours, and then applied with a tension-imparting insulating coating. The magnetic properties and surface defect rate of the product thus obtained were examined. The surface defect rate was measured by an optical surface defect continuous measuring machine. The obtained results are shown in Table 2.

[0041]

Example 2 Steels were melted by primary refining and secondary refining by the following three types of methods, and tapped. After the blasting was performed using a vertical blowing converter, N ₂ was blown into the molten steel in the converter by a bottom blowing nozzle to perform rinsing for 1 minute. After that, the FeSi alloy was poured into the ladle in which the FeSi alloy was placed in advance. The N content in the molten steel at this time was 108 ppm. After that, secondary refining is performed in the RH equipment, and Ar gas is flown at 40 Nl / min.
It was blown in for a minute. N content after secondary refining is 86 pp
m (Example). After performing normal blowing using a vertical blowing converter, steel was poured into a ladle in which FeSi alloy and CaN ₂ were placed in advance. The N content in the molten steel at this time was 106 ppm. Then, Ar was blown in the RH facility at a flow rate of 150 Nl / min for 40 minutes to complete the secondary refinement. The N content of the molten steel at this time was 88 ppm (comparative example). After performing normal blowing using a vertical blowing converter, N ₂ was blown into the molten steel in the converter by a bottom blowing nozzle to perform rinsing for 1 minute. After that, FeSi alloy was poured into the ladle which was placed in front. The N content in the molten steel at this time is 82 ppm.
Met. Next, N ₂ was blown into the RH facility at a flow rate of 80 Nl / min for 40 minutes to complete the secondary refining. The N content of the molten steel at this time is
It was 94 ppm (comparative example).・ ・ ・ In all cases, the contents of Si, Mn, and C are adjusted during secondary refining, Se, Sb, and Al are added, and at the end of secondary refining, C: 0.068 to 0.070%, Si: 3.30 to 3.32
%, Mn: 0.068 to 0.070%, Al: 0.027 to 0.030%,
S: 0.003%, Se: 0.019 to 0.022%, Sb: 0.022 to 0.
A composition of 026%, P: 0.004% and the balance of Fe was obtained.
These molten steels were made into products by the same treatment as in Example 1 after continuous casting. The magnetic properties and surface defect rate of the product thus obtained are shown in Table 3.

[0043]

Example 3 Primary refining and secondary refining were performed under the following conditions. After blowing using a vertical blowing converter, N ₂ gas was blown into the molten steel in the converter at a flow rate of 190 Nl / min by a bottom blowing nozzle to perform rinsing for 1 minute. The N content in the molten steel at this time is
It was 93 ppm. After that, when the FeSi alloy is poured into the ladle that has been placed in front, and then secondary refining is performed in the RH equipment,
The secondary refining was completed by blowing Ar gas at a flow rate of 40 Nl / min for 30 minutes. The N content in the molten steel at the end was 85 ppm (condition I). N ₂ gas was blown into the molten steel in the converter under the same conditions, and then the FeSi alloy was poured into the ladle that was placed in front. At this time, the N content was 102 ppm. Then RH
When the secondary refining was performed in the facility, Ar was blown at a flow rate of 80 Nl / min for 30 minutes to complete the secondary refining. The N content in the molten steel at the end was 90 ppm (condition II). N ₂ gas was blown into the molten steel in the converter under the same conditions, and then the FeSi alloy was poured into the ladle that was placed in front. At this time, the N content was 99 ppm. Then PM
When carrying out the secondary refining in the facility, Ar was blown at 70 Nl / min for 30 minutes to complete the secondary refining. The N content in the molten steel at this time was 89 ppm (condition III). After N ₂ gas was blown into the molten steel in the converter under the same conditions, FeSi alloy was added to the tapping flow at tapping. At this time, the N content was 105 ppm. After that, secondary refining was performed in the RH facility under the same conditions as Condition I. The N content in the molten steel at the end was 97 ppm (condition IV). Under the same conditions, N ₂ gas was blown into the molten steel in the converter, and then the steel was tapped. At this time, the N content in the molten steel was 98 ppm. After that, the FeSi alloy was added at the initial stage of the secondary refining in the RH facility to perform deoxidation treatment and adjustment of the Si content. The gas for secondary refining was blown in the same manner as in Condition I. The N content in the molten steel at the end of secondary refining was 90 ppm (condition V). After blowing using a vertical blowing converter, FeSi was poured into a ladle in which FeSi was placed in advance. The N content of the molten steel at this time is
It was 25 ppm. Next, N ₂ gas was blown into the molten steel using a flushing equipment to perform a nitrification treatment, and the N content in the molten steel was increased to 113 ppm. After that, when performing secondary refining in the RH facility, Ar gas was blown thereinto at a flow rate of 100 Nl / min for 30 minutes to complete the secondary refining. In this case, the N content in the molten steel after the secondary refining was 90 ppm (condition VI). After blowing using a vertical blowing converter, FeSi alloy was poured into a ladle that had been placed in front. The N content in the molten steel at this time was 35 ppm. Next, when performing secondary refining in the RH facility, Ar gas was blown thereinto at a flow rate of 60 Nl / min for 30 minutes to complete the secondary refining, which was used as a comparative example. The N content in the molten steel at the end was 32 ppm (condition VII).

Conditions I to VII are all S during the secondary refining.
After adjusting the contents of i, Mn and C, adding Se, Sb and Al, C: 0.067 to 0.07 as a molten steel component at the end of secondary refining.
1%, Si: 3.31 to 3.33%, Mn: 0.070 to 0.073%, Al:
0.025 to 0.029%, S: 0.003 to 0.005%, Se: 0.020
Up to 0.022%, Sb: 0.025 to 0.027%, P: 0.002 to 0.00
A composition of 4% with the balance being substantially Fe was obtained. Further conditions I to VII
After the continuous casting, the molten steel of No. 2 was manufactured by the same treatment as in Example 1. Further, the molten steel under the condition VII was made into a product by almost the same treatment as in Example 1, but was subjected to nitrification by intermediate annealing and decarburization annealing to increase the N content in the steel to 95 ppm. Table 4 shows the magnetic properties and surface defect rates of these products.

[0046]

[0047]

EFFECT OF THE INVENTION The method for producing a slab for directional silicon steel of the present invention is to perform a nitrification treatment by blowing N ₂ gas into molten steel between the latter stage of primary refining and prior to secondary refining, and In this secondary refining, an inert gas is blown in to remove oxide inclusions to adjust the content of the molten steel N within the above range, thereby preventing the occurrence of blister in the product manufactured by continuous casting. After effectively suppressing, the nitrification treatment can be performed without the product swelling.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takahiro Suga 1 Kawasaki-cho, Chiba-shi, Chiba Inside the Technical Research Division, Kawasaki Steel Co., Ltd. ) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (2)

[Claims] 1. Si: 2.5 to 4.5 wt% of molten steel for grain-oriented silicon steel is subjected to primary refining in a converter and secondary refining in a ladle.
%, Al: 0.010 to 0.040 wt%, N: 0.0050 to 0.0130 wt%
After melting the component composition comprising, in manufacturing a slab cast in a continuous casting machine, blowing in pressurized nitrogen treatment of N ₂ gas into the molten steel during the late of the primary refining until prior to the secondary refining And in the secondary refining, the content of the molten steel N is adjusted to the above range by blowing an inert gas to remove oxide inclusions. Manufacturing method of slab for raw steel. 2. The method for producing a slab for grain-oriented silicon steel according to claim 1, wherein deoxidation with a silicon alloy is carried out in a ladle after tapping from the converter and before secondary refining.