JP5332707B2 - Method for producing grain-oriented electrical steel sheet with extremely excellent magnetic properties - 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, and in particular, directivity using high-temperature slab heating using AlN as a main inhibitor for secondary recrystallization. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a magnetic steel sheet, which effectively utilizes nitriding, which has heretofore been impossible because magnetic characteristics deteriorate, and obtains a grain-oriented magnetic steel sheet with extremely excellent magnetic characteristics.

  The main magnetic properties in grain-oriented electrical steel sheets are iron loss, magnetic flux density and magnetostriction. The iron loss is improved by a magnetic domain control technique (Patent Document 1, Patent Document 2, Patent Document 3, etc.) when the magnetic flux density is high (the Goss orientation integration degree is sharp). Magnetostriction also decreases (good) when the magnetic flux density is high. Furthermore, if the magnetic flux density is high, the transformer excitation current can be reduced, and the size can be reduced.

  In other words, the most fundamental magnetic property to be noted in producing grain-oriented electrical steel sheets is magnetic flux density, and there is a need for the development of a method for stably producing grain-oriented electrical steel sheets whose magnetic flux density is higher than before. Yes.

  By the way, in the grain-oriented electrical steel sheet field, an inhibitor is said to be a grain growth inhibitor and has a function of suppressing grain boundary migration. In the production of grain-oriented electrical steel sheets, the presence of an inhibitor is indispensable in order to selectively grow specific orientation grains. It is. In current industrial production, it is not surprising that a manufacturing method using AlN as the main inhibitor can achieve the highest Goss orientation integration.

  The method of manufacturing grain-oriented electrical steel sheets using AlN as the main inhibitor for secondary recrystallization is based on the metallurgical concept of slab heating in hot rolling and the presence or absence of nitridation in the post-process for reinforcing the inhibitor. 1 is classified.

That is, (a) complete solid solution non-nitriding type, (b) sufficient precipitation nitride type, (c) complete solid solution nitride type, and (d) incomplete solid solution nitride type.
Among these methods, (a) and (b) are already industrially produced, (c) is being tested by the method targeted by the present invention, and (d) is a slab or hot rolled steel strip. It is very difficult to ensure the uniformity of the inhibitor form at this point, and it has not been industrially produced.

  The inventors consider that there are two types of inhibitors. Congenital and acquired inhibitors.

The innate inhibitor is composed of an inhibitor element incorporated in the melting stage, and its function is formed by hot rolling and subsequent heat treatment, and is uniformly distributed in the thickness direction and width direction. As a function, it contributes to primary recrystallization (decarburization annealing) structure and secondary recrystallization.
On the other hand, the acquired inhibitor is an inhibitor introduced in a later step, and is AlN in the case of nitriding. Since this is introduced after the primary recrystallization, it contributes only to the secondary recrystallization. In the case of nitriding in the strip running state, since nitriding is performed from the surface, it is concentrated on the surface of the plate thickness and is not uniform in the plate thickness direction immediately after introduction. This is diffused during the secondary recrystallization annealing to make it uniform.

In addition, when classified by function, an inhibitor that contributes to primary recrystallization is called a primary inhibitor, and an inhibitor that contributes to secondary recrystallization is called a secondary inhibitor.
Innate inhibitors have the role of primary and secondary inhibitors, and acquired inhibitors have the role of secondary inhibitors. In addition, if it is possible to introduce an acquired inhibitor after the hot rolling and before the primary recrystallization annealing, it is also possible in principle that the acquired inhibitor has the role of a primary inhibitor.

Inhibitors in Al-containing grain-oriented electrical steel sheets are considered as follows according to the above classification.
(A) In the completely solid solution non-nitriding type, the innate inhibitor has a function of a primary / secondary inhibitor. (B) In the sufficiently precipitated nitriding type, the secondary inhibitor effect in the innate inhibitor is small, and AlN formed by nitrogen nitride as the acquired inhibitor has a large secondary inhibitor effect. In (c) complete solid solution nitriding type which is the subject of the present invention, AlN formed by nitrogen nitride which is an acquired inhibitor greatly contributes as a secondary inhibitor. Moreover, the primary inhibitor, which is an innate inhibitor, greatly contributes as a secondary inhibitor.

  In the case of (a) the completely solid solution non-nitriding type in Table 1, when the nitrogen content at the time of melting is about 0.008%, if it is nitrided from decarburization annealing to the start of secondary recrystallization, the Goss accumulation degree (non-patent literature) It is widely known that 1, non-patent document 2, and non-patent document 3) decrease. It is also well known that secondary recrystallization failure occurs when there is little nitrogen during melting.

  On the other hand, (c) the complete solid solution nitriding type limits the content of the inhibitor element at the time of melting in the case where the inhibitor substance is completely dissolved by medium temperature slab heating. The amount that is insufficient as an inhibitor is compensated by nitriding.

  That is, when there is little nitrogen at the time of melting, the inhibitor is composed of an innate inhibitor finely precipitated by heat treatment before decarburization annealing and an acquired inhibitor formed by nitriding by nitriding in the subsequent process. Considering the type, it becomes a multistage inhibitor state, and sharp Goss nuclei are generated on the surface layer in the thickness direction during secondary recrystallization annealing (finish annealing), and this preferentially recrystallizes.

  In other words, the inhibitors MnS, MnSe, Cu-S, Cu-Se and the like other than AlN are contained in a smaller amount than the conventional method (completely solid solution non-nitriding type), and the post-step nitriding (b) A sufficient amount of nitriding is performed from the precipitation nitriding type to make the inhibitor function multistage.

  However, in this case, in order to obtain sharp Goss orientation secondary recrystallized grains ({110} <001>), a very narrow nitrogen nitride range is required. That is, when the nitrogen after nitriding is 0.0150% or less, the secondary recrystallization is good, but the deviation from the Goss orientation is remarkably large. If the value exceeds 0.022%, the Goss orientation is deviated.

(C) In the case of the complete solid solution nitriding type, the range in which the magnetic characteristics are good with respect to nitrogen nitride is narrow. This becomes a handy cap in actual production, and because of this, it undergoes fluctuations in secondary recrystallization annealing (box annealing) in which the temperature and atmosphere are non-uniform at the coil position. Glass film formation mainly composed of magnetism and forsterite is not stable.
That is, although the magnetic characteristics are good, it is required to control the conditions of each process (for example, temperature, time, nitrogen nitride range, etc.) within a very narrow range, so the quality is not stable in actual production. For this reason, there is a difficulty in increasing the unit weight to increase the coil unit weight during secondary recrystallization annealing in order to increase productivity.

  (C) In the case of the complete solid solution nitriding type, the congenital inhibitor exhibits an appropriate function as a secondary inhibitor in order to solve a narrow range of process conditions and to cause secondary recrystallization stably at a high level. It is considered necessary to find the conditions to make it happen.

  Conventionally, various precipitates of electromagnetic steel sheets have been reported as shown below.

  In the field of non-oriented electrical steel sheets, there is an invention that defines the size of precipitates. For example, there is Patent Document 11. In this case, it does not function as an inhibitor of secondary recrystallization, but is intended to facilitate the movement of grain boundaries, and is in a direction opposite to the function of the inhibitor of grain-oriented electrical steel sheets.

The necessity of fine precipitation of inhibitors in grain-oriented electrical steel sheets has been pointed out heretofore, but its quantitative examination is not sufficient.
For example, in Patent Document 12, although defined by parameters indicating the {110} plane strength and the precipitate state, a nitriding step is not applied, which is different from the manufacturing method targeted by the present invention.
In Patent Document 13, the average diameter of copper sulfide particles is defined to be less than 100 nm, preferably 50 nm, but it is not a grain-oriented electrical steel sheet containing AlN targeted by the present invention, and a nitriding step is also given. Absent.

  Patent Document 14 stipulates that 95% or more of the nitrided precipitate remaining in the final product is 1 μm or less in diameter for the grain-oriented electrical steel sheet having no bending coating mainly composed of forsterite and having excellent bending workability. The invention does not relate to a grain-oriented electrical steel sheet having a forsterite film, which is the subject of the present invention, and does not define the state of precipitates in the middle of production immediately before secondary recrystallization.

  In Patent Document 15, although a precipitate size is presented, a specific desirable size is not defined only because it is written that miniaturization is desirable, and a nitriding step is not provided.

  In addition, as for invention about refinement | miniaturization of the precipitate of a grain-oriented electrical steel sheet, for example, as shown by patent document 16, patent document 17, etc., although many applications have been filed, those with specific sizes prescribed Absent.

JP-A-55-018566 JP 59-197520 A JP 61-117218 A Japanese Patent Publication No. 40-015644 Japanese Patent Laid-Open No. 58-023414 US 2,599,340 US 5244511 Japanese Patent Laid-Open No. 05-112817 JP 2001-152250 A JP 2000-199015 A Japanese Patent Laid-Open No. 2002-256752 JP 05-171370 A Japanese Patent Laid-Open No. 06-322443 JP 2003-049250 A Japanese Patent Laid-Open No. 11-133714 Japanese Patent Application No. 38-018337 JP 09-227941 A JP 07-252532 A JP 07-005976 A Japanese Patent Publication No. 06-051887

ISIJ, Vol. 43 (2003), No. 3, pp. 400-409 Acta Metall., 42 (1994), 2593 Kawasaki Steel Technology Vol.29 (1997) 3,129-135) Materials Science Forum Vol.204-206, Part2: pp: 631 Y. Inokuchi and F. Saito: Jpn. Inst. Met., 55 (1991), No. 11, 1167

  In the method of (c) in Table 1, although it became possible to produce a grain-oriented electrical steel sheet having extremely excellent magnetic properties, the nitriding range was narrow, and industrial production required a very narrow range of manufacturing process conditions. Another problem is that the variation in the magnetic and glass film formation over the entire length of the coil after secondary recrystallization is large.

FIG. 1 shows the relationship between nitrogen after nitridation by nitridation in a later step and magnetic characteristics (magnetic flux density: B8 (T)), and is divided into regions I, II, and III depending on the amount of nitrogen after nitridation.
In all the regions I, II and III, secondary recrystallization is good, and the secondary recrystallization texture in each region is {110} <4 4 11> in region I and in region II, Good Goss orientation ({110} <001>), {110} <229> in region III.

  However, the region where a good magnetic flux density can be obtained is the region II, and the problem is that the range is narrow as described above. An object of the present invention is to widen the range of this region II and reduce the fluctuation of the magnetic glass coating at the coil site.

As a result of repeated studies, the inventors obtained the following inferences regarding the aspect of the inhibitor.
(C) In a completely solid solution nitriding type manufacturing method, nitriding is essential to realize a multistage secondary inhibitor mode. In addition, since the primary recrystallization grain size is sufficiently smaller than the precipitation nitriding type because it is a complete solid solution type, the driving force for grain boundary movement increases and the secondary recrystallization start temperature decreases.
As a result of this weak grain growth suppression, secondary recrystallization starts near the center of the plate thickness before the nitrogen nitride diffuses throughout the plate thickness.

  In general, when a grain-oriented electrical steel sheet having an excellent Goss orientation is obtained, it is said that the secondary recrystallization generation position needs to generate nuclei at 1/5 position of the plate thickness (see, for example, Non-Patent Document 5). . However, the thickness center layer is in a texture environment in which the {110} <4 4 11> orientation, which is greatly deviated from the Goss orientation, is likely to undergo secondary recrystallization.

It is estimated that the nucleation from the thickness center layer in this way is because the heat resistance of the inhibitor in the thickness center layer is weak. That is, the heat resistance of the inhibitor (precipitate, which is a grain growth inhibitor) is small at the time of temperature increase in secondary recrystallization annealing, and the effect disappears until nitrogen nitride diffuses, and grain boundary migration cannot be suppressed. This is probably because of this.
For this reason, it is necessary to prevent secondary recrystallization from starting before nitrogen nitride diffuses into the plate thickness center layer.

That is, the inhibitors are largely divided into the innate inhibitors (1) MnS, Cu-S, MnSe fine precipitates and (2) AlN fine precipitates, and the acquired inhibitors after (3). It consists of three types of coarse AlN by process nitriding.
Since these heat resistances are different, it is considered that the time of functioning as an inhibitor in the secondary recrystallization annealing is different, and selective growth occurs sequentially.

  As a result of the study, the inventors mainly studied that the control of the form of AlN fine precipitation in (2) improves the heat resistance and stabilizes the inhibitor effect. Therefore, the balance of the inhibitors in (1) and (2) is good. As a result, it was found that the characteristics of the final product are stable even in a wide range of manufacturing conditions.

  The conceptual diagram is shown in FIG. The above-mentioned inhibitors (1) to (3) have a thermal stability that decreases in the order of (1) → (3) due to their thermodynamic properties and size. For this reason, conventionally, (2) was positioned at (2) ′. It is estimated that this is the original position (2) by the appropriate form control of AlN and is almost flat with respect to the temperature.

The present invention relates to a fully solid solution nitrided type (c) having AlN as a main inhibitor for secondary recrystallization, and a plate after primary recrystallization annealing in a method for producing a grain oriented electrical steel sheet to which a particularly high slab heating temperature is applied. The aim is to stabilize the magnetic properties by defining the innate inhibitor form of the thick center layer and improving the heat resistance.
The present invention includes the following items.

(1) By mass%, C: 0.025-0.09%, Si: 2.5-4.0%, acid-soluble Al: 0.022-0.033%, N: 0.003-0. 006%, S and Se as S equivalent Seq = S + 0.405Se, 0.010 to 0.020%, Mn: 0.03 to 0.09%, Ti ≦ 0.005%, the balance being Fe and inevitable The slab made of a general impurity is reheated at a temperature higher than the solid solution temperature of the inhibitor substance exceeding 1280 ° C. and hot-rolled to form a hot-rolled steel strip. Of N contained in the hot-rolled steel strip, AlN The hot rolled steel strip is not annealed or annealed, and is subsequently subjected to cold rolling at least twice with one or more intermediate sandwiches in between, and the final thickness of the cold rolled steel. When forming a belt, heat treatment is performed at least once before the final cold rolling, and the final cold rolling is performed. The rolling ratio is 83% to 92%, the average particle size (diameter) corresponding to the circle of primary recrystallized grains after decarburization annealing is 7 μm to less than 18 μm, and a mixed gas of hydrogen, nitrogen and ammonia under strip running conditions as .011 to .023% of the total nitrogen content in the nitriding treatment in the in the subsequent production of a grain oriented electrical steel sheet an annealing separator composed mainly of MgO was applied subjected to final finish annealing, final cold the cooling rate after the heat treatment before rolling a 15 ° C. / sec or more, oriented electrical steel sheet towards you, characterized in that the circle-equivalent mean diameter of the precipitates of the thickness center layer after decarburization annealing and 50nm or 200nm or less Manufacturing method.
(2) The method for producing a grain-oriented electrical steel sheet according to (1), wherein an average cooling rate between 900 and 550 ° C. after the heat treatment before the final cold rolling is set to 25 to 40 ° C./second.
(3) the slab further contains, by mass%, the production method of the oriented electrical steel sheet towards according to the Cu containing from 0.05 to 0.30% (1) or (2).
( 4 ) The item according to any one of (1) to (3), wherein the slab further contains 0.02 to 0.30% of at least one of Sn, Sb, and P in mass%. oriented electrical steel sheet and a manufacturing method thereof toward described.
(5) the slab further contains, by mass%, oriented electrical steel sheet towards any one of Items of Cr and characterized in that it contains from 0.02 to 0.30% (1) to (4) Manufacturing method.
(6) in at least one pass in the final cold rolling, characterized in that to keep a minute or more steel strip to a temperature range of 100 to 300 ° C. (1) ~ the direction of any one of Items (5) Method for producing an electrical steel sheet.
(7) oriented electrical steel sheet towards any one of Items of a heating rate of up to 650 ° C. from start heating in decarburization annealing, characterized in that a 100 ° C. / sec or more (1) to (6) Manufacturing method.
( 8 ) Obtained by the production method according to any one of (1) to ( 7 ), wherein the magnetic flux density B8 in the rolling direction (magnetic flux density at 800 A / m) is 1.92 T or more. Oriented electrical steel sheet.

  In the present invention, (a) the high temperature slab heating at the time of hot rolling of a completely solid solution non-nitrided directional electrical steel sheet, which is a problem of conventional grain-oriented electrical steel sheet production, (b) sufficient precipitation Without changing the primary recrystallization annealing temperature in the nitriding type, it becomes possible to produce a grain-oriented electrical steel sheet having an extremely excellent glass film with a magnetic property in a wide range of nitrogen nitride.

It is a figure which shows an example of the relationship between the conventional nitrogen amount after nitriding, and magnetic flux density. It is a conceptual diagram for demonstrating the multistage secondary inhibitor mode by inhibitor (1)-(3). It is a figure which shows the relationship between the amount of nitrogen after nitriding, and magnetic flux density with respect to hot-rolled sheet annealing conditions. It is a figure which shows the example of observation of the form of the precipitate by SEM.

  First, the reason for limiting the component range of the slab in the present invention will be described.

  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 0.09%, MnS and MnSe increase, and the degree of solid solution becomes uneven depending on the location, causing problems in stable production in actual industrial production.

  S and Se combine with Mn and Cu to precipitate finely to form an innate inhibitor and are also useful as AlN precipitation nuclei. The S equivalent (Seq = S + 0.405Se) is 0.010% or more and 0.020% or less. If the S equivalent is less than 0.010%, the absolute amount of the congenital inhibitor is insufficient and secondary recrystallization becomes unstable. On the other hand, if it exceeds 0.020%, the balance of the multistage secondary inhibitor is lost and a sharp Goss orientation secondary recrystallization texture cannot be obtained.

  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.

  As described above, in the present invention, finely precipitated sulfide, selenide and AlN play the role of primary and secondary inhibitors, so AlN contained in the slab is also very important for controlling the primary recrystallized grains. If N is less than 0.003%, the absolute amount of stage (2) of the multistage inhibitor is insufficient and secondary recrystallization failure occurs. When it exceeds 0.006%, the multistage secondary inhibitor mode is not formed, and the Goss orientation accumulation degree is lowered as described above.

  When 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 ensured and secondary recrystallization failure occurs.

  Cu heats the slab at 1280 ° C. or higher and completes hot rolling rapidly, and forms fine precipitates with S and Se early during the cooling, thereby exerting 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.

  Sn, Sb, and P are effective in improving the primary recrystallization texture. If the content of these elements is less than 0.02%, the effect of improvement is small, and if it exceeds the above range, it becomes difficult to form a stable forsterite film (primary film, glass film). Furthermore, it is well known that Sn, Sb, and P are grain boundary segregation elements and have the effect of stabilizing secondary recrystallization.

  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.

  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, the reasons for limiting other conditions in the present invention will be described.

  The average grain size of primary recrystallized grains after completion of decarburization annealing is, for example, in Patent Document 18, the average grain size of primary recrystallized grains is 18 to 35 μm. In the present invention, the average grain size of primary recrystallized grains is The diameter needs to be 7 μm or more and less than 18 μm. This is a very important point of the present invention that makes magnetic characteristics (particularly iron loss) good.

  That is, when the primary recrystallized grain size is small, the volume fraction of Goss orientation grains that become the nucleus of secondary recrystallization at the stage of primary recrystallization also increases from the viewpoint of grain growth (Non-patent Document 4). In addition, since the particle size is smaller, the number of Goss nuclei is also relatively large. As a result, the absolute number of Goss nuclei is about 5 times larger in the case of the present invention than in the case where the average radius of the primary recrystallized grains is 18 to 35 μm, so that the secondary recrystallized grain size is also relatively small. As a result, the iron loss is remarkably improved.

  In addition, (b) the fact that the average grain size of the primary recrystallized grains is small and the amount of nitriding is not small compared to the sufficiently precipitated nitriding type, the driving force of secondary recrystallization is increased, and the secondary recrystallization starts at a low temperature. Therefore, secondary recrystallization starts at an early stage (at a lower temperature) of the temperature raising stage of final finish annealing. This is matched with the formation of a multistage secondary inhibitor mode. This is because the temperature history at each point of the coil up to the maximum temperature becomes more uniform in the present situation where the final finish annealing is performed in a coil shape (the temperature rising rate at each point of the coil becomes constant) The non-uniformity of the coil part is remarkably reduced and the magnetic characteristics are extremely stable.

  As described above, the present invention is of (c) complete solid solution nitriding type, and it is essential in the present invention to subject the steel sheet to nitriding treatment after decarburization annealing and before starting secondary recrystallization. The method includes mixing a nitride (CrN, MnN, etc.) with an annealing separator during high-temperature annealing, and nitriding in an atmosphere containing ammonia under the condition that the strip is run after primary recrystallization / decarburization annealing. There is a way. Either method may be adopted, but the latter is more practical in industrial production and is limited to the latter in the present invention.

  The amount of nitriding is to secure N that binds to acid-soluble Al. When the amount is small, secondary recrystallization becomes unstable. When the amount is large, defects in the primary film (glass film) exposing the ground iron occur frequently. Is extremely deteriorated. In order to obtain a high magnetic flux density according to the present invention, the total nitrogen content after nitriding has spread to 0.011% to 0.023%. In addition, since the secondary recrystallization start temperature is low, equivalent double-sided nitriding is desired.

  The circle equivalent average diameter of the precipitate in the thickness center layer after primary recrystallization annealing is set to 50 nm or more and 200 nm or less. The lower limit value is a conventional case, and it is necessary to narrow the nitrogen nitride amount range in the nitriding step. If the upper limit is exceeded, the heat resistance of the secondary inhibitor becomes too high and the Goss orientation accumulation degree decreases. The small rectangular precipitates are mainly composed of AlN, and the large precipitates are round and non-AlN Mn. In order to obtain a precipitate having a somewhat large size within this range, it is necessary to hold at a relatively high temperature for a long time in the annealing before the final cold rolling. For example, annealing at 1130 ° C. or higher for 120 seconds or longer is required. Thereafter, it is desirable to cool to 500 ° C. at a cooling rate of 20 ° C./second or more. In this way, rectangular AlN-based precipitates grow greatly with high and long annealing.

  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 matter of fact, in these, so-called “center segregation” is observed at the time of solidification, and it is extremely difficult to obtain a complete homogeneous solution state. In order to obtain a completely uniform solution state, a solution heat treatment is strongly desired once before the hot-rolled steel strip is obtained.

  The condition of the slab heating temperature prior to hot rolling is an important point of the present invention. It is essential that the slab heating temperature be 1280 ° C. or higher to dissolve the inhibitor substance. If it is less than 1280 ° C., the precipitation state of the inhibitor substance in the slab (or hot-rolled steel strip) becomes non-uniform, and so-called skid marks are generated in the final product. Although an upper limit is not specifically limited, Actually, it is about 1420 degreeC. This complete solid solution treatment can be performed without raising the temperature to an ultrahigh temperature of 1420 ° C. due to the recent development of equipment technology such as induction heating (Patent Document 19). 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.

  If the precipitation rate of AlN in the hot-rolled steel strip exceeds 20%, the secondary recrystallization property in the steel strip varies and is not suitable for industrial production. That is, the solid solution state of the innate inhibitor becomes non-uniform, and a skid mark is generated. When it exceeds 20%, it means that hot rolling was not properly performed including slab heating.

  The annealing before the final cold rolling 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. Annealing in a hot-rolled steel strip may be used, and annealing before final cold rolling may be used. That is, it is essential to perform one or more continuous annealings in order to make the history of hot rolling uniform before the final cold rolling. The maximum temperature in this case has a great influence on the inhibitor, and this effect is important. When it is relatively low, the average diameter of the precipitate is 50 nm or less. If it is too high, it exceeds 200 nm. The cooling after annealing is desirably 15 ° C./second or more in order to secure a fine inhibitor and secure a hardened hard phase (mainly a bainite phase).

  If the final cold rolling ratio in cold rolling is less than 83%, 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.

  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 properties are improved, so that 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 Patent Document 20 that when the heating rate is increased, the Goss orientation increases in the primary recrystallization texture and the secondary recrystallization grain size decreases. In Patent Document 20, the heating rate is set to 140 ° C./sec or more. However, in the present invention, the heating rate is also effective at 100 ° C./sec, desirably 150 ° C./sec or more.

<Example 1>
C = 0.068%, Si = 3.35%, acid-soluble Al = 0.0260%, N = 0.0006%, Mn = 0.045%, S = 0.014%, Sn = 0.08% , Cu = 0.09%, Ti = 0.020, the balance is Fe and inevitable impurities are cast by a normal method, and the inhibitor substance is completely dissolved at a slab heating temperature of 1310 ° C. After rolling, the steel sheet was rapidly cooled to obtain a hot-rolled steel strip having a thickness of 2.3 mm. The precipitation ratio of AlN was 10% or less. Then, this hot-rolled steel strip was (1) annealed at 1080 ° C. for 180 seconds and then cooled to 100 ° C. hot water.
(2) After annealing at 1140 ° C. for 180 seconds, it was cooled to 100 ° C. hot water.
(3) After annealing at 1120 ° C. for 10 seconds, it was held at 900 ° C. for 120 seconds and cooled to 100 ° C. hot water.

After pickling, the steel sheet was rolled to 0.285 mm by a reverse cold rolling mill including five aging treatments at 250 ° C. to obtain a cold rolled steel strip. Thereafter, degreasing and primary recrystallization / decarburization annealing at 850 ° C. for 150 seconds are performed in a humid atmosphere of N ₂ = 25%, H ₂ = 75%, Dp = 72 ° C., followed by primary recrystallization / The second half of the decarburization annealing was performed at a temperature of 875 ° C. for 15 seconds in an atmosphere of N ₂ = 25%, H ₂ = 75%, and Dp = 30 ° C. Then, nitriding was performed in an ammonia atmosphere so that the nitrogen was about 0.0100 to 0.0260% after nitriding, and an annealing separator mainly composed of MgO was applied to the steel sheet surface.

Subsequently, in the secondary recrystallization annealing, the temperature rise at 15 ° C./hour up to 800 ° C. is as follows: (4) H ₂ % = 75%, N ₂ % = 25%, Dp = 10 ° C. ₂ % = 50%, N ₂ % = 50%, Dp = 40 ° C. (BAF condition), then up to 1200 ° C. H ₂ % = 75%, N ₂ % = 25%, Dry 15 ° C. The temperature was raised at / hour and finally purified at 1200 ° C. for 20 hours with H ₂ = 100% and cooled. After that, normally used insulating tension coating application and planarization treatment were performed.

  For each of the secondary recrystallization annealing conditions (BAF conditions) in (4) and (5) above, the post-nitriding nitrogen and magnetic flux obtained from the steel sheet using the hot rolled steel strip annealing conditions in (1) to (3) above The relationship of density (B8) is shown in FIGS. 3a and b.

  (2) is an example of the present invention, and good B8 (T) is obtained for a wide range of nitrogen after nitriding. Further, even if the secondary recrystallization annealing conditions (BAF conditions) were different, there was no significant difference in magnetic properties and the film formation was good.

  Further, FIG. 4 shows an observation example of the form of the precipitate by SEM of the precipitate in the thickness center layer after the primary recrystallization annealing, and the hot-rolled steel strip annealing conditions (1) to (3) above are used. The average equivalent circle diameters of the precipitates observed from the steel plates were (1) 29.7 nm, (2) 62.8 nm, and (3) 32.8 nm, respectively.

<Example 2>
In order to suppress precipitation of AlN as much as possible, in order to suppress the precipitation of AlN as much as possible, slabs made of molten steel shown in Table 2 were reheated in the range of 1230 to 1350 ° C., and the hot rolling was completed as much as possible. Cooled quickly. Thus, a hot-rolled steel strip having a thickness of 2.3 mm was obtained. Subsequently, continuous annealing of the hot-rolled steel strip was performed under the conditions shown in Table 2 and cooled at 20 ° C./second. Thereafter, rolling was performed three times at a temperature of 200 ° C. to 250 ° C. to a thickness of 0.285 mm.
Thereafter, annealing was performed at 850 ° C. for 150 seconds in a mixed atmosphere of H ₂ and N ₂ at a dew point of 70 ° C., which also served as decarburization and primary recrystallization. Nitriding was performed in an ammonia-containing atmosphere. Then, secondary recrystallization annealing was performed after application | coating of the annealing separation agent which has MgO as a main component. The secondary recrystallization annealing was performed by raising the temperature to 1200 ° C. at 10-20 ° C./hour in an atmosphere of N ₂ = 25%, H ₂ = 75%, Dp = 10 ° C. Thereafter, a purification treatment was performed at a temperature of 1200 ° C. for 20 hours or more and H ₂ = 100%. Thereafter, a generally used insulating tension coating was applied and planarized. The magnetic characteristics were evaluated by the magnetic characteristics at 0.0135 to 0.0150% in order to evaluate by increasing the nitrogen amount range after nitriding. The results are shown in Table 2. In the example of the present invention, good results of B ₈ ≧ 1.92T were obtained.

<Example 3>
In order to suppress precipitation of AlN as much as possible, in order to suppress the precipitation of AlN as much as possible, slabs made of molten steel shown in Table 3 were reheated in the range of 1230 to 1350 ° C., and the hot rolling was completed as much as possible. Cooled quickly. Thus, a hot-rolled steel strip having a thickness of 2.2 mm was obtained. Subsequently, continuous annealing of the hot-rolled steel strip was performed under the conditions shown in Table 2 and cooled at 20 ° C./second. Thereafter, rolling was performed three times at a temperature of 200 ° C. to 250 ° C. to a thickness of 0.220 mm.
After that, annealing was performed at 850 ° C. for 110 seconds in a mixed atmosphere of H ₂ and N ₂ at a dew point of 68 ° C., which also served as decarburization and primary recrystallization. Nitriding was performed in an ammonia-containing atmosphere. Then, secondary recrystallization annealing was performed after application | coating of the annealing separation agent which has MgO as a main component. The secondary recrystallization annealing was performed by raising the temperature to 1200 ° C. at 10-20 ° C./hour in an atmosphere of N ₂ = 25%, H ₂ = 75%, Dp = 10 ° C. Thereafter, a purification treatment was performed at a temperature of 1200 ° C. for 20 hours or more and H ₂ = 100%. Thereafter, a generally used insulating tension coating was applied and planarized. The magnetic characteristics were evaluated by the magnetic characteristics at 0.0135 to 0.0150 mass% in order to evaluate by widening of nitrogen after nitriding. The results are shown in Table 3. In the example of the present invention, good results of B ₈ ≧ 1.92T were obtained.

<Example 4>
Molten steel having the components shown in Table 4 A 250 mm slab was produced by normal continuous casting, and the slab was reheated at a temperature of 1300 ° C to 1350 ° C. And it hot-rolled at high temperature with the normal continuous hot-rolling machine, the hot-rolling after finishing hot rolling, and quenching was performed, and the 2.3 mm hot-rolling steel strip was obtained. Then, in the heat treatment before the final cold rolling, the temperature was set to 1130 ° C to 1150 ° C, the time was annealed for 120 seconds to 135 seconds, and then the average cooling rate between 900 ° C to 550 ° C was cooled to 25 ° C to 40 ° C. .

  Then, it rolled at the temperature of 200 degreeC-250 degreeC of 3 times, and was set as three types of steel plates with thickness 9mil (0.220mm), 12mil (0.285mm), and 14mil (0.335mm).

After that, 9 mil material is 850 ° C. × 110 seconds, 12 mil, 14 mil material is 850 ° C. × 150 seconds, H ₂ and N ₂ mixed atmosphere, dew point is 68 ° C., and both annealing and primary recrystallization are performed. Subsequently, nitriding was performed in an ammonia-containing atmosphere so as to be 0.014 to 0.019% while running the steel strip. Then, secondary recrystallization annealing was performed after application | coating of the annealing separation agent which has MgO as a main component. In the secondary recrystallization annealing, the temperature was raised to 1200 ° C. at 10 to 20 ° C./hour in an atmosphere of N ₂ = 25%, H ₂ = 75%, and Dp = 10 ° C. Thereafter, a purification treatment was performed at a temperature of 1200 ° C. for 20 hours or more and H ₂ = 100%, and after that treatment, a generally used insulating tension coating was applied and a planarization treatment was performed.
The results are shown in the right column of Table 4.

<Example 5>
Using a slab manufactured by a normal method having the components of steel Nos. 26 and 27 in Table 4, the slab reheating temperature in hot rolling, hot-rolled steel strip thickness, final cold rolling rate, and nitriding amount The process was changed. The thickness of the hot rolled steel strip was 1.8 mm, 2.3 mm, and 2.5 mm.

  At this time, annealing before the final cold rolling was performed at 1060 ° C. to 1150 ° C. for 110 seconds to 150 seconds, and then the average cooling rate between 900 ° C. and 550 ° C. was cooled at 25 ° C. to 40 ° C. By the way, in Examples B10 and B11, after a 2.3 mm hot-rolled steel strip was pickled, it was pre-rolled to 1.55 mm and 1.45 mm and annealed before final cold rolling.

  Then, it is rolled at a temperature of 200 ° C. to 250 ° C. three times, and the thickness is 7 mil (0.175 mm), 8 mil (0.195 mm), 9 mil (0.220 mm), 12 mil (0.285 mm), 14 mil (0 .335 mm).

Thereafter, decarburization and primary recrystallization are performed at a dew point of 68 ° C in a mixed atmosphere of H ₂ and N ₂ for 850 ° C x 110 seconds for 7,8 and 9 mil materials and 850 ° C for 150 seconds for 12 mil and 14 mil materials. Annealing was performed, followed by nitriding in an ammonia-containing atmosphere so as to be 0.0115 to 0.0245% while running the steel strip. Then, secondary recrystallization annealing was performed after application | coating of the annealing separation agent which has MgO as a main component. The secondary recrystallization annealing was performed by raising the temperature to 1200 ° C. at 10 to 20 ° C./hour in an atmosphere of N ₂ = 25%, H ₂ = 75%, Dp = 10 ° C. Thereafter, a purification treatment was performed at a temperature of 1200 ° C. for 20 hours or more and H ₂ = 100%. Thereafter, a generally used insulating tension coating was applied and planarized.
The results are shown in the right column of Table 5.

<Example 6>
Using the slab manufactured by the normal method which has the components of steel No. 26 and 27 of Table 4, slab reheating temperature in hot rolling 1335 degreeC, hot-rolled steel strip thickness 2.3mm at 1140 degreeC for 120 seconds After annealing, the average cooling rate between 900 ° C. and 550 ° C. was set to 35 ° C., and the final cold rolling rate was 90.4%, 87.6%, and held during rolling at 150 ° C. to 280 ° C. for 8 minutes or more. Cold-rolled to 9 mil and 12 mil in one pass.

After that, 9mil material was 850 ° C x 110 seconds, 12mil was 850 ° C x 150 seconds, mixed atmosphere of H ₂ and N ₂ , and decarburized at 68 ° C, annealing for both decarburization and primary recrystallization. While running the belt, nitriding was performed in an ammonia-containing atmosphere so as to be 0.0150 to 0.0205%. During the decarburization annealing, the heating rate up to 650 ° C. was set to a normal rate, 110 ° C./second, 230 ° C./second, 240 ° C./second by induction heating.

Then, secondary recrystallization annealing was performed after application | coating of the annealing separation agent which has MgO as a main component. The secondary recrystallization annealing was performed by raising the temperature to 1200 ° C. at 10 to 20 ° C./hour in an atmosphere of N ₂ = 25%, H ₂ = 75%, Dp = 10 ° C. Thereafter, a purification treatment was performed at a temperature of 1200 ° C. for 20 hours or more and H ₂ = 100%, and after that treatment, a generally used insulating tension coating was applied and a planarization treatment was performed.
The results are shown in the right column of Table 6.

Claims (8)

In mass%, C: 0.025 to 0.09%, Si: 2.5 to 4.0%, acid-soluble Al: 0.022 to 0.033%, N: 0.003 to 0.006%, S and Se, S equivalent Seq = S + 0.405Se, 0.010 to 0.020%, Mn: 0.03 to 0.09%, Ti ≦ 0.005%, the balance being Fe and inevitable impurities The slab composed of the above is reheated at a temperature higher than the solid solution temperature of the inhibitor substance exceeding 1280 ° C., hot-rolled to form a hot-rolled steel strip, and Al as NN contained in the hot-rolled steel strip. With a precipitation rate of 20% or less, this hot-rolled steel strip is annealed or not annealed, and then cold-rolled steel strip of the final thickness is obtained by performing cold rolling twice or more with one or more intermediate sandwiches in between. When the final cold rolling is performed, at least one heat treatment is performed before the final cold rolling. The rate is 83% to 92%, and the average grain size (diameter) corresponding to the circle of primary recrystallized grains after decarburization annealing of the cold-rolled steel strip is 7 μm to less than 18 μm. A grain oriented electrical steel sheet in which a total nitrogen content is set to 0.011 to 0.023% by nitriding in a mixed gas of nitrogen and ammonia, and then an annealing separator containing MgO as a main component is applied to perform final finish annealing. In the production, the cooling rate after the heat treatment before the final cold rolling is 15 ° C./second or more, and the circle-equivalent average diameter of the precipitate in the thickness center layer after the decarburization annealing is 50 nm or more and 200 nm or less. method of manufacturing oriented electrical steel sheet towards that. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein an average cooling rate between 900 and 550 ° C after the heat treatment before the final cold rolling is set to 25 to 40 ° C / second. It said slab is further method for producing oriented electrical steel sheet towards the claim 1 or 2, characterized by mass%, containing Cu 0.05 to 0.30%. It said slab further contains, by mass%, Sn, Sb, the direction of according to any one of claims 1-3, characterized in that it contains from 0.02 to 0.30% of at least one P Steel sheet and method for producing the same. It said slab further contains, by mass%, the production method of the oriented electrical steel sheet towards according to any one of claims 1-4, characterized in that it contains Cr 0.02 to 0.30%. In at least one pass of the final cold rolling, the production of oriented electrical steel sheet towards according to any one of claims 1-5, characterized in that to keep the steel strip 1 minute or more in a temperature range of 100 to 300 ° C. Method. Method for producing a grain-oriented electrical steel sheet according to any one of claims 1-6, characterized in that the heating rate up to 650 ° C. from start heating in decarburization annealing and 100 ° C. / sec or more. Obtained by the production method according to any of claims 1 to 7, grain-oriented electrical steel sheet rolling direction of the magnetic flux density B8 (magnetic flux density at 800A / m) is equal to or not less than 1.92T .