JP5332946B2 - Coil winding method after nitriding of nitriding grain-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a method for producing a grain-oriented electrical steel sheet mainly used as an iron core such as a transformer.

The grain-oriented electrical steel sheet naturally has good magnetic properties, particularly iron loss, magnetic flux density, and magnetostriction, but its production requires high yield in industrial production.
In the manufacture of grain-oriented electrical steel sheets, a flattening process is performed after secondary recrystallization annealing. As a result, the grain-oriented electrical steel sheet has a glass film mainly composed of forsterite on the secondary recrystallized steel strip in the Goss orientation, and further has a phosphoric acid-based tensile insulating film applied thereon.
At that time, not only the appearance is poor if the glass coating mainly composed of forsterite is insufficient, but in the grain-oriented electrical steel sheet used by laminating, a short circuit occurs at a location where the coating is insufficient, It becomes a defective product as a transformer, and the commercial value as a grain-oriented electrical steel sheet product is lost. For this reason, it is required that the glass film formation is uniform and sufficient.

  The secondary recrystallization annealing treatment of grain-oriented electrical steel sheets is performed by applying an annealing separator mainly composed of MgO to the steel strip after primary recrystallization / decarburization annealing and winding it into a coil shape. Level the high-temperature annealing in the furnace. In this box-type annealing, secondary recrystallization, glass film formation, and purification are performed.

  This box-type annealing is carried out with a steel strip wound in a coil shape in a vertical state with the steel plate width direction set up vertically, so that the steel strip has an inner surface and an outer surface. Here, as shown in FIG. 1, the inner surface of the coil is a surface facing the center of the coil.

  Conventionally, there has been a case where the glass coating is insufficiently formed on the inner surface of the coil regardless of the manufacturing method. This is because the inner surface of the coil is pulled because tension acts during the subsequent flattening treatment, and peels off when the glass film formation is insufficient. That is, since the formation of the root of the glass coating on the steel plate body is insufficient, it is easy to peel off due to the tension.

  This phenomenon is particularly noticeable in the manufacture of nitride-type grain-oriented electrical steel sheets. In the production of a nitriding-type grain-oriented electrical steel sheet using AlN as a main inhibitor for secondary recrystallization, as exemplified in Patent Documents 1 to 5, nitriding is performed before secondary recrystallization annealing. For this reason, since the nitrogen content of the steel sheet increases in the nitriding type production, nitrogen is released to the outside of the steel strip during purification in the final stage of box annealing, and the glass film is formed because the outermost glass film is damaged. Is estimated to be insufficient.

In the first place, nitriding is performed in an atmosphere containing ammonia by running the strip after primary recrystallization and decarburization. This ammonia gas is finally decomposed into nitrogen and hydrogen using iron as a catalyst, so that the atmosphere on the surface of the steel sheet is extremely dry (reducing atmosphere). In this extremely dry atmosphere (reducing atmosphere), reduction of the oxide layer (main component is SiO ₂ ) that becomes the material of the folded forsterite occurs, and the glass film formation becomes insufficient.

If the properties of the surface in contact with the reducing atmosphere are uniform on the front and back of the steel strip (inner surface and outer surface), it can be improved by optimizing the annealing conditions before nitriding. Uniform nitriding on the front and back of the steel strip (inner and outer surfaces) due to restrictions on the arrangement of various gas pipes and control system wiring in the annealing furnace to reduce capital investment and secure space for furnace maintenance It is not guaranteed or realized.
For this reason, it is considered that in the case of manufacturing a nitriding-type grain-oriented electrical steel sheet in which an inhibitor is formed by post-nitridation, a coating defect becomes conspicuous.

  By the way, in the manufacture of nitriding-type grain-oriented electrical steel sheets, the secondary recrystallization temperature is generally high even if the nitriding amount on both sides of the steel strip (inner surface and outer surface) is different. In addition, there is no problem from the viewpoint of secondary recrystallization (magnetic properties) because sufficient diffusion of nitrogen to the entire plate thickness is performed.

Japanese Patent Laid-Open No. 05-112827 JP 2001-152250 A JP 2000-199015 A Japanese Patent Application Laid-Open No. 07-252523 JP 09-227941 A

  Therefore, the present invention makes it possible to make the glass film formation uniform on the front and back of the steel strip in the production of a nitride-type grain-oriented electrical steel sheet that uses AlN as a main inhibitor for secondary recrystallization and performs nitriding before secondary recrystallization annealing. It is an object of the present invention to provide manufacturing conditions that can be produced.

  As a result of intensive studies, the inventors have clarified that in the production of nitride-type grain-oriented electrical steel sheets, there is a difference between the amount of nitriding on the front and back of the steel strip (inner surface and outer surface) and the winding method when forming the coil after nitriding. It has been found that there is a relationship with respect to film formation. That is, it has been found that the glass film formation is improved by changing the coil winding method depending on the ratio of the nitriding amount on the front and back of the steel strip (inner surface and outer surface).

  As a result of such studies, the present invention provides an inner surface and an outer surface of a coil in the production of a nitride-type grain-oriented electrical steel sheet that uses AlN as a main inhibitor of secondary recrystallization and performs nitriding before secondary recrystallization annealing. The winding direction after nitriding is defined by the ratio of nitriding, and has the following configuration.

(1) After decarburization annealing of the steel strip, primary recrystallization annealing is performed, nitriding treatment is performed in a mixed gas of hydrogen, nitrogen and ammonia under the strip running condition, and then an annealing separator mainly composed of MgO is applied. In the method of manufacturing a grain-oriented electrical steel sheet in which the final finish annealing is performed in a state where the steel strip is wound in a coil shape, when the difference in the increase in nitrogen nitride on the front and back surfaces of the steel strip satisfies (Equation 1), the nitriding amount A method for winding a steel strip, characterized in that a surface having a large amount of coil is formed in a coil shape on the coil outer surface side when final finishing annealing is performed.
│ (front side nitridation amount-back side nitridation amount) / ΔN│ × 100 ≧ 15% (Formula 1)
here,
Front side nitridation amount: Value obtained by subtracting the nitrogen content in melting from the nitrogen content of the portion from the front side surface to the plate thickness (1/10) t Back side nitridation amount: Plate thickness from the back side surface (plate thickness ( 1/10) Value obtained by subtracting the nitrogen content of the melt from the nitrogen content of the portion up to t ΔN: Total nitriding amount, subtracting the nitrogen content before nitriding from the nitrogen content after nitriding at the total thickness Value.
The outer surface of the coil is as shown in FIG. 1, and the plate thickness (1/10) t is as shown in FIG.

  By producing the grain-oriented electrical steel sheet according to the winding method of the present invention, it is possible to improve the glass film formation of the grain-oriented electrical steel sheet in the nitriding type production method.

It is the figure which showed the inner surface of a coil, and the outer surface. It is the figure which showed the board thickness position which prescribes | regulates the nitrogen amount in a steel strip. It is the figure which showed the relationship between the nitriding amount difference of a coil inner surface, and a film | membrane defect rate.

  In the manufacture of a nitride-type grain-oriented electrical steel sheet that uses AlN as a main inhibitor for secondary recrystallization and performs nitriding before secondary recrystallization annealing, the strip is run after primary recrystallization and decarburization annealing. When nitriding in an atmosphere containing ammonia under wet conditions, the difference in the amount of nitriding between the front and back of the steel strip (inner surface and outer surface) and the winding method when forming the coil after nitriding are related to the formation of the glass coating I found.

In the manufacture of a nitride-type grain-oriented electrical steel sheet, nitriding is performed using ammonia decomposition, so that the steel sheet surface is in an extremely reducing atmosphere. Therefore, since the SiO ₂ as a raw material for glass film composed mainly of forsterite formed on the surface of the steel sheet during decarburization annealing is reduced, glass film formation is insufficient.

  On the other hand, even in a grain-oriented electrical steel sheet that does not nitride in the first place, a glass coating defect tends to occur on the inner surface. This is a coil-like secondary recrystallization annealing and then smoothed, so the inner surface is tensioned, peeling off if the formation of the root of the internal oxide layer is insufficient and the metal surface exposed To do. That is, in the first place, the glass film forming property is inferior on the inner surface.

In addition to this, in the nitriding type, if the nitriding amount is different on both sides, the formation of SiO ₂ after nitriding is different as described above.

As a result of the study by the present inventors, if the difference ratio of the nitridation amounts of the double-sided layers is 15% or less, it is regarded as almost equal nitridation, and there is almost no difference in the formation of the glass film on the inner surface and the outer surface of the coil. Furthermore, when the difference ratio exceeds 15% and the surface with much nitriding is used as the inner surface, many glass coating defects (peeling) occur, and particularly severe defects occur in the portion where the radius of curvature of the inner winding portion of the coil is small. It was found to occur.
For this reason, it has also been found that when this difference ratio exceeds 15%, the surface with much nitridation is used as the outer surface to suppress the occurrence of glass coating defects on that surface.

If this method is applied to actual production, nitrogen nitridated by the ammonia method is localized on the double-sided surface layer at that time, so this quantification is extremely difficult even with modern analytical instruments. Accompany.
Therefore, as a practical index of operation, as shown in FIG. 2, an average nitrogen content from the inner surface to a thickness of 1/10 of the plate thickness t (1 / 10t position) may be used. I found out.

That is, on the front side surface and back side surface of the steel sheet after nitriding, the value obtained by subtracting the nitrogen content in the melt from the nitrogen content of the portion from the respective surface to the plate thickness of 1/10 t, respectively, When the following (Equation 1) is satisfied when ΔN is a value obtained by subtracting the nitrogen content before nitriding from the nitrogen content after nitriding at the total plate thickness as the back side nitridation amount, The surface with a large amount of nitridation is the coil outer surface side when secondary recrystallization annealing is performed in a coil shape. | (Front side nitridation amount−back side nitridation amount) / ΔN | × 100 ≧ 15% (Formula 1)
Here, the outer surface of the coil is as shown in FIG. 1, and the plate thickness (1/10) t is as shown at the plate thickness position in the steel strip of FIG.

  The method of making the surface having a large amount of nitriding the outer surface is possible and easy by changing the rotating direction of winding when winding the steel strip after nitriding in a continuous nitriding furnace. For example, when there is much nitriding on the upper surface of a continuous nitriding furnace during nitriding in a horizontal furnace, winding is performed clockwise when viewed from the right side in the direction of steel strip travel. The winding method may be a normal reel or a carousel reel, and is determined by the geometric arrangement of the device in the vicinity of winding.

  The analysis of the nitrogen amount is performed by a chemical analysis that is generally performed. The amount of nitridation on the surface side is polished from the back side, leaving a portion from the surface on the surface side to the plate thickness (1/10) t, and a facet is collected therefrom, and the amount of nitrogen on the surface side is analyzed by chemical analysis. . The backside nitridation amount is similarly analyzed.

  As described above, the present invention changes the winding direction of the steel sheet to be subjected to final finish annealing based on the difference in the nitriding amounts of the front and back sides of the steel sheet after nitriding, thereby making the glass coating a directional electromagnetic wave. It can be satisfactorily formed on both the front and back sides of the steel sheet.

  The present invention as described above can be applied as it is to a method for producing a normal nitride-type grain-oriented electrical steel sheet. Therefore, although it does not restrict | limit especially about the raw material for grain-oriented electrical steel sheets and manufacturing conditions, a preferable aspect is demonstrated, respectively.

In the present invention, steel that is generally known for nitride-oriented grain-oriented electrical steel sheets can be used as the material for grain-oriented electrical steel sheets.
Preferable chemical composition of steel is C: 0.025 to 0.09%, Si: 2.5 to 4.0%, Mn: 0.03 to 0.15%, S + 0.405Se: 0.005 by mass%. -0.020%, acid-soluble Al: 0.022-0.033%, N: 0.003-0.009%, the balance consisting of Fe and unavoidable impurities, or Sb One or more of Sn, P: 0.02 to 0.30%, Cu: 0.05 to 0.15%, Cr: 0.02 to 0.15%, if necessary .
The reasons for selecting each component are as follows.

When C is less than 0.025%, the primary recrystallization texture becomes unsuitable, and when it exceeds 0.09%, decarburization becomes difficult and is not suitable for industrial production.
If Si is less than 2.5%, good iron loss cannot be obtained, and if it exceeds 4.0%, cold rolling becomes extremely difficult and is not suitable for industrial production.

  Mn tends to crack in a hot-rolled steel strip of less than 0.03%, yield decreases, and secondary recrystallization is not stable. On the other hand, if it exceeds 0.15%, the amount of MnS and MnSe becomes large and coarse, and the degree of solid solution / precipitation becomes uneven depending on the location, causing problems in stable production in actual industrial production.

  S and Se combine with Mn and Cu to form an innate inhibitor and are also useful as precipitation nuclei for AlN. The S equivalent (Seq = S + 0.405Se) is 0.005% or more and 0.020% or less. If the S equivalent is less than 0.005%, the absolute amount of the congenital inhibitor is insufficient and secondary recrystallization becomes unstable. Further, if it exceeds 0.020%, the degree of solid solution / precipitation is uneven depending on the steel strip part, and there is a problem in stable production in actual industrial production. In addition, if nitriding at this time, the inhibitor strength becomes too large, and the Goss orientation sharpness is increased. Inferior magnetic flux density decreases.

  Acid-soluble Al combines with N to form AlN, and functions mainly as a primary and secondary inhibitor. This AlN includes those formed before nitriding and those formed at the time of high-temperature annealing after nitriding, and 0.022 to 0.033% is necessary for securing the amount of both AlN. Outside this upper limit, secondary recrystallization failure occurs. Further, if the lower limit is exceeded, the Goss orientation integration degree is significantly degraded.

  Similarly, AlN contained in the slab is very important for controlling the primary recrystallized grains. When N is less than 0.003%, the absolute amount of the primary inhibitor is insufficient and secondary recrystallization failure occurs. When it exceeds 0.009%, blisters called blisters often occur and surface defects occur.

  Sn, Sb, and P are effective in improving the primary recrystallization texture. If the content of these elements is less than 0.02%, the improvement effect is small, and if it exceeds 0.30%, it is difficult to form a stable forsterite film (primary film or glass film) in the first place. Furthermore, it is well known that Sn, Sb, and P are grain boundary segregation elements and have an atmosphere blocking effect at the time of secondary recrystallization annealing to stabilize secondary recrystallization.

  Cu, along with S and Se, forms fine precipitates by annealing before the final cold rolling regardless of the hot rolling conditions, and exhibits primary and secondary inhibitor effects. The precipitates also serve as precipitation nuclei that make the dispersion of AlN more uniform and also play a role of secondary inhibitors, and this effect makes secondary recrystallization good. If it is less than 0.05%, the above effects may be reduced and the stability of industrial production may be inferior. If it exceeds 0.30%, the above effects will be saturated, and it will cause surface flaws such as “copper hege” during hot rolling.

  Since Cr is effective for forming a forsterite film (primary film, glass film), it is desirable to contain 0.02 to 0.30%. If it is less than 0.03%, it is difficult to ensure oxygen, and if it exceeds 0.30%, no film is formed.

  If Ti is contained in excess of 0.005%, N becomes TiN and becomes a substantially low N-containing steel, and the inhibitor strength is not secured and secondary recrystallization failure may occur.

  In addition, Ni, Mo, and Cd are not prevented from being added. Moreover, in the case of electric furnace melting, it is inevitably mixed. Since Ni has a remarkable effect on the uniform dispersion of precipitates as primary and secondary inhibitors, the magnetic properties are further improved and stabilized when Ni is added. If it is less than 0.02%, there is no effect, and if it exceeds 0.3%, it becomes difficult to enrich oxygen after decarburization annealing and it becomes difficult to form a forsterite film. Mo and Cd form sulfides or selenides and contribute to strengthening of the inhibitor. If it is less than 0.008%, there is no effect, and if it exceeds 0.3%, precipitates are coarsened and the function of the inhibitor cannot be obtained, and the magnetic properties are not stable.

Next, production conditions other than the components suitable for carrying out the present invention will be described.
The present invention is directed to a manufacturing method in which nitriding is essential in the manufacture of grain-oriented electrical steel sheets containing aluminum. In the manufacturing method that requires nitriding, slab heating is performed at a temperature lower than 1280 ° C. (a) a sufficiently precipitated type and slab heating is performed at a higher temperature as exemplified in Patent Document 1 to completely dissolve the inhibitor substance. There is (b) complete solid solution type exemplified in Patent Document 2. The present invention can be applied to any of these methods.

  The casting for obtaining the slab may be a conventional continuous casting. Furthermore, in order to make the slab heating easy, it is possible to apply the lump method. In this case, it is well known that the carbon content can be reduced. Specifically, a slab having an initial thickness in the range of 150 mm to 300 mm, preferably in the range of 200 mm to 250 mm, is manufactured by a known continuous casting method.

  Instead, as a supplement to normal continuous hot rolling in recent years, thin slab casting with a thickness of 30 mm to 100 mm and steel strip casting (strip caster) to obtain a direct steel strip have been put into practical use. The application is not hindered. However, as a practical matter, in these cases, precipitates for so-called “center segregation” and the like are not uniform during solidification, and it is extremely difficult to obtain a completely uniform solid solution / precipitation state. In order to obtain a completely uniform solution / precipitation state, slab reheating with a solution heat treatment or sufficient precipitation treatment at a low temperature of 1200 ° C. or lower is strongly desired once before obtaining a hot-rolled steel strip.

  In the case of normal hot rolling, the conditions of the slab heating temperature preceding are different depending on whether (a) a sufficient precipitation type or (b) a complete solid solution type. In the former, 1200 ° C. or lower is desirable, but in the latter, it is desirable to exceed 1300 ° C. Of course, in addition to the usual gas heating method for industrial production, in addition to the usual gas heating method, induction heating, direct current heating may be used, and in order to ensure the shape for these special heating methods, There is no problem even if breakdown is applied to the cast slab. When the heating temperature is higher than 1300 ° C., the texture may be improved by this breakdown to reduce the amount of C. These are within the scope of conventional known techniques.

(A) In the case of a sufficient precipitation type, the finishing exit temperature is desirably low 930 ° C. or lower, and the winding temperature is desirably 600 ° C. or lower. (B) In the complete solid solution type, the finishing entrance temperature is preferably as high as 1100 ° C. or higher, and the winding temperature is preferably 600 ° C. or lower.

After hot rolling, annealing is performed to stably obtain a good Goss orientation. This annealing is performed mainly for the homogenization of the structure in the steel strip generated during hot rolling and the fine dispersion precipitation of the inhibitor. It may be annealed in the hot rolled steel strip or cold rolled once. It may be annealed before the final cold rolling. That is, at least one continuous annealing is performed in order to uniformize and optimize the inhibitor and metal structure in hot rolling before the final cold rolling.
As an annealing condition, the aspect cooled to room temperature with the cooling rate of 15 degrees C / second or more from 750 to 900 degreeC after 90 second to 180 second annealing at 1060 degreeC to 1150 degreeC is illustrated, for example.

  The cooling after annealing is desirably 15 ° C./second or more in order to secure a uniform inhibitor distribution state and secure a hardened hard phase (mainly bainite phase). For example, the method disclosed in Patent Document 5 may be used.

  If the final cold rolling ratio in cold rolling is less than 80%, the {110} <001> texture becomes broad and a high magnetic flux density cannot be obtained, and if it exceeds 92%, the {110} <001> texture is extreme. The secondary recrystallization becomes unstable.

  The final cold rolling may be carried out at room temperature, but if at least one pass is kept in the temperature range of 100 to 300 ° C. for 1 minute or longer, the primary recrystallization texture is improved and the magnetic properties become extremely good. This is known. The holding time may be 1 minute or more, but since actual cold rolling is performed by a reverse mill, the holding time at a certain temperature is generally 10 minutes or more. Increasing the length does not hinder the present invention, but rather is a measure for obtaining good magnetic properties.

  Decarburization annealing is performed after cold rolling. The decarburization annealing is performed under known conditions, that is, at 650 to 950 ° C. for 60 to 500 seconds, preferably 80 to 300 seconds in a mixed wet atmosphere of nitrogen and hydrogen, depending on the plate thickness.

  For example, in Patent Document 4, the average particle size of the primary recrystallized grains after completion of decarburization annealing is set to 18 to 35 μm. On the other hand, in patent document 1, the average particle diameter of a primary recrystallized grain is 7 micrometers or more and less than 18 micrometers.

  When the heating rate from room temperature to 650 to 850 ° C. in the decarburization annealing is set to 100 ° C./sec or more, the primary recrystallization texture is improved and the magnetic characteristics are improved, so that the application is not hindered. Various methods are conceivable for securing the heating rate. That is, there are resistance heating, induction heating, direct energy application heating, and the like. It is known in Japanese Patent Publication No. 6-51887 that the Goss orientation increases in the primary recrystallization texture and the secondary recrystallization grain size decreases when the heating rate is increased. In Japanese Examined Patent Publication No. 6-51887, the heating rate is 140 ° C./sec or more. However, the present invention is effective even when the heating rate is 100 ° C./sec, preferably 150 ° C./sec or more.

  After primary recrystallization and decarburization annealing, nitriding is performed in an atmosphere containing ammonia under the condition of running the strip. When the total amount of nitriding is large, defects in the primary coating (glass coating) exposing the base iron occur frequently, the Goss orientation integration degree is extremely deteriorated, and when the amount is small, secondary recrystallization becomes poor. In order to obtain a high magnetic flux density according to the present invention, the total nitrogen content after nitriding is desirably 0.011% to 0.023%.

After the nitriding treatment, a final finish annealing is performed by applying an annealing separator mainly composed of MgO according to a known method. In the present invention, as described above, since the amount of nitriding of each of the front and back side surfaces of the steel sheet after nitriding is defined on the structure of the furnace, the winding direction of the steel sheet to be subjected to final finish annealing based on this Decide.
Usually, after annealing, an insulating tension coating is applied and planarized to obtain a product.

In mass%, C is 0.065%, Si is 3.37%, acid-soluble Al is 0.026%, N is 0.0081%, S is 0.0065%, Mn is 0.098%, Sn is The slab obtained from molten steel with 0.065%, Cr 0.12%, balance Fe and inevitable impurities is reheated at 1150 ° C, then hot-rolled by a normal method and finished at 890 ° C The hot rolling was finished to obtain a 2.6 mm hot-rolled sheet, which was wound up at 560 ° C. Then, it hold | maintained at 1120 degreeC for 10 second and 900 degreeC for 100 second, it cooled to room temperature by cooling at 750 degreeC20 degreeC / second, and deskeling was performed by pickling. Then, it was set as the 0.22 mm cold-rolled sheet by warm rolling 3 times at 250 degreeC. Thereafter, heating was performed at 150 ° C./second up to 700 ° C., and primary recrystallization / decarburization annealing was performed at 850 ° C. in 10 seconds in a wet atmosphere of 75% hydrogen, 25% nitrogen, and 70 ° C. dew point. Thereafter, the amount of nitriding was changed on each of the front side and the back side of the steel strip so that the total nitrogen amount was in the range of 195 ppm to 225 ppm in the ammonia atmosphere, and the value of (Equation 1) was 2.5% to 27%. %, The ammonia flow rate at the upper and lower surfaces of the steel strip of the nitriding furnace was changed, and then an annealing separator mainly composed of MgO was applied, and the winding direction was changed to wind up.
After that, the box-type annealing was heated to 1200 ° C. at 15 ° C./hour with 75% hydrogen and 25% nitrogen, and then subjected to purification annealing at 1200 ° C. for 20 hours. Thereafter, a flattening treatment was performed, and a tension insulating film mainly composed of aluminum phosphate was applied. The glass film defect rate in that case is shown.

For each nitridation difference between the inner and outer surfaces of the coil determined from (Equation 1), the glass film defect rate when the winding direction was changed was examined, and the obtained results are shown in FIG.
As shown in FIG. 3, in the example (◯) in which the surface having a large amount of nitriding is coiled and the outer surface of the coil is subjected to final finish annealing, the glass film defect rate is 5 regardless of the difference in the amount of nitriding. In the example (●) in which the surface with a large amount of nitriding is the inner surface side, the nitriding difference value obtained from (Equation 1) exceeds 15%, and the lath film defect rate is 5%. Increased beyond.
The glass film defects include poor adhesion, metallic gloss exposure, poor glass formation, and discoloration. In this example, the poor adhesion and metallic gloss exposure were defined as glass coating defects.

Claims (1)

After decarburization annealing of the steel strip, it is subjected to primary recrystallization annealing, nitriding in a mixed gas of hydrogen, nitrogen and ammonia under the strip running condition, and then applying an annealing separator mainly composed of MgO, In the method of manufacturing a grain-oriented electrical steel sheet in which the final finish annealing is performed in a state where the strip is wound in a coil shape, when the difference in the increase in nitrogen nitride on the front and back surfaces of the steel strip satisfies (Equation 1), the surface having a large amount of nitriding Is a coil outer surface side when final finishing annealing in a coil shape.
│ (front side nitridation amount-back side nitridation amount) / ΔN│ × 100 ≧ 15% (Formula 1)
here,
Front side nitridation amount: Value obtained by subtracting the nitrogen content in melting from the nitrogen content of the portion from the front side surface to the plate thickness (1/10) t Back side nitridation amount: Plate thickness from the back side surface (plate thickness ( 1/10) Value obtained by subtracting the nitrogen content of the melt from the nitrogen content of the portion up to t ΔN: Total nitriding amount, subtracting the nitrogen content before nitriding from the nitrogen content after nitriding at the total thickness Value.

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