Process for manufacturing double oriented electrical steel having high magnetic flux density

Abstract: The present invention is a process for manufacturing a double oriented electrical steel sheet having a high magnetic flux density by carrying out a low temperature annealing for the secondary recrystallization by the use of AℓN as inhibitor, characterized by hot-rolling a silicon steel slab comprising 1.8-4.8% by weight of Si, 0.008-0.048% by weight of acid soluble Aℓ, totally 0.0028-0.0100% by weight of N, not more than 0.016% by weight of S and the balance being Fe and unavoidable impurities into a hot-rolled sheet, subjecting the sheet to cold-rolling at a reduction rate of 40-80%, subsequently subjecting the sheet to another cold-rolling at a reduction rate of 30-70% in the direction crossing the cold-rolled direction, annealing at 750-950°C in a wet hydrogen atmosphere for decarburization, and carrying out the final finishing annealing which comprises a stage for completing the secondary recrystallizing at a temperature of 920-1100°C, followed by a stage for purification.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to a process for manufacturing a double oriented electrical steel sheet including crystallized grains whose easy axis <001> of magnetization is oriented both in the longitudinal orientation and in the direction vertical thereto, together with the rolled surfaces exhibiting {100} planes (those crystallographic orientations can be represented as {100} <001> in the Miller indices). (2) Description of the Related Art Since the double oriented electrical steel sheet has excellent magnetic properties in the two different directions (e.g. B 8 values both in the rolled direction and in the direction vertical thereto: 1.92 Tesla), because of its easy axis (<001> axis) in the rolled direction and in the direction vertical thereto, it can be more advantageously used for a magnetic core material of a specific apparatus, e.g. a large-scale rotating machine, where the magnetic flux flows in two different directions in comparison with a grain oriented electrical steel sheet which exhibits excellent magnetic properties in only one rolled direction. Non-oriented electrical steel sheet, whose easy axis is not greatly accumulated, are generally been used for a small stationary machine or installation. The use of a double oriented electrical steel sheet, therefore, makes it a possible to miniaturize the machine with an increased efficiency. The double oriented electrical steel sheet, which has excellent magnetic properties as described above, has long been expected to be put into mass production, but the general use of such a type of sheet as an industrial product is still limited at present. Although various methods have been suggested, these are all only on a laboratory scale and have problems in terms of the industrial scale of the process. As a prior art technique, a method wherein an initial steel sheet is annealed at a high temperature in an atmosphere containing a polar gas, e.g., hydrogen sulfide, to secondarily recrystallize out {100} <001> oriented grains with the aid of surface energy is described in Japanese Examined Patent Publication No.37-7110. Nevertheless, this method is inadequate for mass production, because it requires a very accurate control of the surface energy of the sheet. The other method is that wherein a steel sheet is cold-rolled in the direction and further cold-rolled in the direction vertical thereto, i.e. a "cross cold-rolling method", as described in Japanese Examined Patent Publication No. 35-2657 by Satoru Taguchi et, al. According to the cold-rolling method, a relatively higher magnetization property (B 8 value) can be obtained, but the resulting product does not have a magnetization property which offsets the cost, and thus cannot replace the conventional grain oriented electrical steel sheet. The magnetization property "B 8 value" of the grain oriented electrical steel sheet has been significantly improved since the technique disclosed in Japanese Examined Patent Publication No. 51-13469 was invented. The B 8 value of equal to or more than 1.88 Tesla is standardized by JIS (Japanese Industrial Standard), and products having a B 8 value of about 1.92 Tesla haven been commercially available. Under the above-mentioned situations, the product of double oriented electrical steel sheet is required to have a magnetization property (B 8 value) corresponding to the above-mentioned grain oriented electrical steel sheet. As processes for improving the magnetic flux density of double oriented electrical steel sheet, a process wherein a hot-rolled material is annealed and then a cold-rolled in the mutually rectangular direction is disclosed in Japanese Examined Patent Publication 38-8213, a process wherein a material is nitrided in the course from post-primary recrystallization to the start of secondary recrystallization in Japanese Examined Patent Publication 1-43818, and a process wherein after the cross cold-rolling, the material is further cold-rolled in the initial cold-rolled direction at a reduction rate of 5-33% in Japanese Unexamined Patent Publication 1-272718. EP-A-0318051 discloses a process for manufacturing a double oriented electrical steel sheet which comprises hot-rolling a silicon steel slab into a hot-rolled sheet, subjecting the hot-rolled sheet to cold-rolling at a reduction rate of 40-80%, subsequently subjecting the sheet to another cold-rolling at a reduction rate of 30-70% in a direction crossing the said cold-rolled direction, annealing the cold-rolled sheet in a wet hydrogen atmosphere for decarburization, and carrying out final finishing annealing which comprises a stage for completing secondary recrystallization followed by a stage for purification. It never mentions the temperature to which the silicon steel slab is heated before the hot rolling, and is primarily unconcerned with the S content of the steel, but mentions S-contents of 0.012 and 0.018%. SUMMARY OF THE INVENTION The present invention is aimed at the establishment of a process capable of stably manufacturing a double oriented electrical steel sheet having a high magnetic flux density. The present invention provides a process for manufacturing a double oriented electrical steel sheet which comprises hot-rolling a silicon steel slab into a hot-rolled sheet, subjecting the hot-rolled sheet to cold-rolling at a reduction rate of 40-80%, subsequently subjecting the sheet to another cold-rolling at a reduction rate of 30-70% in a direction crossing the said cold-rolled direction, annealing the cold-rolled sheet in a wet hydrogen atmosphere for decarburization, and carrying out final finishing annealing which comprises a stage for completing secondary recrystallization followed by a stage for purification, the silicon steel slab being heated to a temperature of not more than 1270°C before hot-rolling the heated slab into said hot-rolled sheet, the decarburization annealing being at 750 to 950°C, the secondary recrystallization being completed at 920 to 1100°C, and the silicon steel slab containing 1.8-4.8% by weight of Si, 0.008-0.048% by weight of acid soluble Aℓ, totally 0.0028-0.0100% by weight of N, not more than 0.007% by weight of S and the balance being Fe and unavoidable impurities, whereby the double oriented electrical steel sheet obtained has a B 8 value of at least 1.91 Tesla in said two cold-rolling directions. Preferably, the nitriding of the sheet is such that the N content in the raw material is totally 0.002-0.060% by weight at any time during the former annealing stage for decarburization, during an additional annealing stage thereafter, or during the heating stage in the final finishing annealing stage by the time of the start of the secondary recrystallization. Attached Figs.1 and 2 show the steel and the magnetic flux density (B 8 value) of the product with a varied amount of S in the steel and a varied slab heating temperature. A basic metallurgical principal applied in the process of manufacturing a double oriented electrical steel sheet is the phenomenon of secondary recrystallization. The following factors have been known for regulating the secondary crystallization: (1) a primarily recrystallized texture that facilitates growth of crystalline grain having an objective crystal orientation; (2) fine precipates or substitutional elements that have the effect of suppressing the growth of crystalline grains having orientations deviating from the object, i.e. the existence of an inhibitor; (3) a grain size distribution of primarily recrystallized grains which is as uniform as possible and of suitable average size; and (4) a secondary recrystallization annealing cycle that selectively grows the objective grains with a steel sheet possessing the requirements of (1), (2), and (3). All of these factors are known in the production of grain oriented electrical steel sheets, but have not yet been known in the production of double oriented electrical steel sheets. According to our investigation, the techniques disclosed in Japanese Examined Patent Publication No. 35-2657, and Japanese Unexamined Patent Publication No. 1-272718 as mentioned above deal with the factor (1), and the technique described in Japanese Examined Patent Publication No. 1-43818 deals with the factor (2). Concerning factor (4), the subject of EP-A-0318051, we have obtained a novel finding. Specifically, a {110} <UVW> orientation exists together with the desired "{100} <001>" orientation, in the secondarily recrystallized grains obtained in the cross cold-rolling method. The larger the orientation, {110} <UVW>, the more B 8 deteriorates. Furthermore, the secondary recrystallization temperature of {110} <UVW> oriented grains has been found to be higher than that of {100} <001> oriented grains. By completing the secondary recrystallization at the relatively lower temperature ranging from 920 to 1100°C, the rate of the {100} <001> oriented grains is enhanced before the growth of {110} <UVW> oriented grains, and this makes it possible to improve B 8 . This invention provides conditions which can completely realize this technical idea, and make it possible to obtain a high magnetic flux density stably. In addition, the present invention, which can shorten the annealing cycle for the secondary recrystallization, has the advantageous effect of lowering the production cost. The contents of the present invention will now be explained specifically. In the secondary recrystallization for the purpose of producing a double oriented electrical steel sheet, an S type (MnS) inhibitor has hitherto been used as is available. However, we have discovered that this MnS inhibitor is rather harmful in the production of a double oriented electrical steel sheet in the secondary recrystallization. Its existence is the cause for deterioration of the magnetic flux density. The inventors conducted the following experiments concerning the S type inhibitor: A molten steel containing 0.049% by weight of, C, 3.25% by weight of Si, 0.14% by weight of Mn, 0.27% by weight of acid soluble Aℓ, and totally 0.0073% by weight of N was divided into five portions, and slabs wherein the content of S (by weight) was adjusted to 0.0010%, 0.0070%, 0.016%, 0.023%, and 0.035%, respectively were cast. After coarsely rolling the slabs, they were divided into five portions. The coarsely rolled materials were heated at 1100°C, 1150°C, 1270°C, 1320°C, and 1380°C, respectively, to produce hot-rolled sheets having a 1.5 mm thickness. The sheets were annealed at 1000°C for 2 minutes, and then cold-rolled to a thickness of 0.55 mm in the same direction as the hot-rolled direction. Subsequently, the sheet was cold-rolled to a 0.23 mm thickness in the direction vertical to that of the first cold-rolled direction (cross cold-rolling). Thereafter, the rolled sheet was subjected to decarburization and annealing in a wet hydrogen atmosphere at 820°C for 120 seconds, MgO containing 3% ferromanganese being applied thereto. The sheet was then heated up to 1200°C at a heating rate of 30°C/hr in an atmosphere of 75% H 2 + 25% N 2 , and then annealed at 1200°C for 20 minutes in a 100% H 2 atmosphere. The magnetic flux densities of the resulting products are shown in Fig.1. It can be understood from Fig.1 that the lower the content of S in the steel, and the lower the slab-heating temperature, the higher the B 8 value is. From the measurement of the orientation of crystalline grains of this product, the product having a higher S content in the steel and obtained at the higher slab-heating temperature contained more {110} <UVW> oriented grains. From repeated observations of the steel sheet during the heating in the annealing for the secondary re-crystallization, it was found that the higher the content of S in the steel and the higher the slab-heating temperature, the more the tendency to delay the progress of the secondary recrystallization is. It can thus be assumed that when the content of S is larger and the slab heating temperature increased, MnS is dissolved in a larger amount and MnS precipitates become more and finer. Therefore, the effect of suppressing the grain growth as inhibitor is enhanced, thereby delaying the progress of the secondary recrystallization. Such delaying of the progress of the secondary recrystallization makes the phenomenon that "{100} <001> orientation appears at a low temperature and {100} <UVW> orientation appears at a high temperature" more significant, thereby lowering the B 8 value. As described above, in spite of the common sense of availability of MnS in the secondary recrystallization for the production of a grain oriented electrical steel sheet, excess MnS was instead found to have an adverse effect upon the secondary crystallization of {100} <001> orientation, which is a double oriented electrical steel sheet. The content of Si as the steel component will now be restricted. If the content of Si exceeds 4.8% by weight, the material tends to crack on cold-rolling and it is difficult to carry out rolling. Conversely, the magnetic flux density becomes higher as the Si content gets smaller, but the crystal orientation is destroyed if the transformation of α into γ occurs at the annealing stage for the secondary crystallization. Consequently, the lower limit of the Si content is defined to be 1.8% by weight, the translation of α into γ then notoccurring. In the present invention, when forming the inhibitor from the inital stage of the process, it is necessary that 0.008-0.48% by weight of acid soluble Aℓ, and 0.0028-0.0100% by weight of total N are contained. If the content of acid soluble Aℓ is less than 0.008% by weight or the total content of N is less than 0.0028% by weight, no secondary re-crystallization occurs due to shortage of the amount of the inhibitor. Conversely, if the content of acid soluble Aℓ exceeds 0.048% by weight, no secondary recrystallization occurs because of the inhomogeneous distribution of AℓN. Further, if the total content of N exceeds 0.010% by weight, there arises a surface blister blemish called a "blister" which occurs during the stage of hot-rolling and spreads during the stage of cold-rolling. In the case of forming the inhibitor in an intermediate stage of the process, 0.008-0.048% by weight of acid soluble Aℓ is incorporated, and a nitriding is applied at any time during a short time decarburization stage after the final cold-rolling, during an additional annealing stage carried out thereafter, or during the heating up stage in the finishing annealing stage by the time of the start of the secondary recrystallization so that a nitride "AℓN" or "(Aℓ,Si)N" is formed to be 0.002-0.060% by weight of the total N content to act as the inhibitor. Because a high content of S changes the B 8 value for the worse, the content of S is defined to be not more than 0.007% by weight. A silicon steel slab containing the components mentioned above is hot-rolled into a hot-rolled sheet. As the gist of the present invention is that solid solution of MnS by heating of the slab is suppressed in order to decrease the inhibition effect of MnS upon the grain growth, the slab-heating temperature is low. The upper limit of this temperature is 1270°C, so that no slag occurs. The lower limit may be a temperature capable of hot-rolling, for example, 1000°C. Immediately thereafter, or after further heating at a temperature of 750-1200°C for 30 seconds to 30 minutes for short time annealing, the cold-rolling is applied in the lengthwise direction of the hot-rolled sheet and in the cross direction thereof. The short time annealing is preferable in terms of enhancing the magnetic flux density of the product, but the production cost is increased. Consequently, whether or not the short time annealing is applied may be decided after taking into consideration the desired level of the magnetic flux density of the product. The cold-rolling wherein the direction of the first cold-rolling is in the hot-rolled direction of the stock can produce a product having a higher magnetic density than that obtained by the cold-rolling wherein the direction of the first cold-rolling is crossed perpendicular to the hot-rolled direction of the stock. However, whether the first rolling is in or perpendicular to the hot-rolled direction, the resulting product is always a double oriented electrical steel sheet having {100} <001> orientation or an orientation in the same vicinity. In order to remove a small amount of C contained in the steel, the material after cold-rolling is subjected to decarburization at 750-950°C for a short time in a wet hydrogen atmosphere. The means for the formation of an inhibitor by the nitriding treatment in the course from post-final cold-rolling to the start of the secondary recrystallization in the finishing annealing stage, which is one embodiment of the present invention, will now be explained. The means for penetrating nitrogen into the steel sheet which can be used should not be specifically restricted, but include, for example, a method wherein the cold steel sheet is nitrided during decarburization annealing in an atmosphere having nitriding capability, e.g. in an atmosphere containing ammonia gas; a method wherein the decarburized steel sheet is additionally annealed and at this time the steel sheet is nitrided; or a method wherein the decarburized steel sheet is nitrided in the early stage of finishing annealing before the start of secondary recrystallization in the atmosphere having a nitriding capability. Where the subject of the finishing annealing mentioned above is a strip coil, especially of a large size, it is difficult to penetrate nitrogen into the space between the strip layers and thus there is a fear of insufficient and inhomogeneous nitriding of the steel sheet. Consequently, it is desirable to secure the gaps between sheets to a level more than a specific value, or to take means for adding a metal nitride which discharges nitrogen in the course of the finishing annealing, into an annealing separator such as an ammono compound, prior to the finishing annealing. Further, the decarburized sheet or the nitriding-treated sheet is finally annealed after the application of an annealing separator such as MgO. As a finishing annealing condition, it is essential that the secondary crystallization is completed at a temperature of 920-1100°C. Concrete means for expressing the secondary crystallization include maintaining a temperature of 920-1100°C for a period of 5 hours or more, which is the period for the secondary crystallization to be completed, or to heat up at a rate not more than 30°C/hr in the temperature range mentioned above. Since it is an essential condition of the present invention for the inhibition effect of MnS on the grain growth to be low, the secondary recrystallization can be completed at a lower temperature and for shorter time, and thus the heating rate can be higher in comparison with a prior filed patent (EP-A-0318051 and Japanese Unexamined Patent Publication No. 2-141531). The present invention, therefore, can lower the production cost due to higher annealing efficiency. The sheet in which the secondary crystallization has been thus completed can be annealed at a temperature of 1150-1200°C for 5-20 hours in a hydrogen atmosphere for the purpose of purification such as for removing N and S. Working examples will now be explained. Example 1: The same hot-rolled sheet, having a 1.5 mm thickness, as the hot-rolled sheet used for obtaining the results of Fig.1 was annealed at 1000°C for 2 minutes, and then cold-rolled in the same direction as the hot-rolled direction, to a 0.55 mm thickness. Subsequently, the sheet was cold-rolled in a direction vertical to the first cold-rolled direction, to a 0.23 mm thickness (cold cross-rolling). The sheet was then annealed in a wet hydrogen atmosphere at 820°C for 120 seconds for decarburization, and then MgO containing 3% ferromanganese nitride was applied thereto. The sheet was heated up to 1020°C at a heating rate of 50°C/hr, maintained in a 75% H 2 + 25% N 2 atmosphere for 20 hours to secondarily recrystallize out. Thereafter, it was heated up to 1200°C at a heating rate of 25°C/hr, and maintained in a 100% H 2 atmosphere to be purified. The magnetic properties of the resulting product are shown in Fig.2. Example 2: Two kinds of cast strips containing 0.048% by weight of C, 3.30% by weight of Si, 0.070% by weight of acid soluble Aℓ, totally 0.0072% by weight of N, and the balance being Fe and unavoidable impurities, which further contained either 0.0060% by weight or 0.021% by weight of S,were heated to 1150°C or 1320°C, and then hot-rolled into hot-rolled sheets having a 1.8 mm thickness. The sheet was annealed at 1000°C for 2 minutes, and then cold-rolled in the same direction as the hot-rolled direction to a 0.75 mm thickness. Subsequently, the sheet was cold-rolled in a direction vertical to the first cold-rolled direction to a 0.30 mm thickness. The sheet was then annealed in a wet hydrogen atmosphere at 820°C for 150 seconds for decarbonization, and then MgO containing 3% ferromanganese nitride was applied thereto. The sheet was heated up to 1020°C at a heating rate of 50°C/hr in a 75% H 2 + 25% N 2 atmosphere, and maintained for either 5 hours, 10 hours, or 20 hours to secondarily recrystallize out. In each case, the sheet was then heated up to 1200°C at a heating rate of 25°C /hr, and maintained in a 100% H 2 atmosphere for 20 hours to be purified. The magnetic properties of the resulting products are shown in Table 1. The product having a small content of S in the steel had a higher B 8 . However, in the case where a higher slab heating temperature, i.e. 1320°C was applied, longer soaking time was required for the secondary recrystallization to obtain high B 8 values. The product having a content of S as large as 0.021% by weight in the steel did not have a high B 8 . Especially, in the case of a lower slab heating temperature and a longer soaking time for the secondary crystallization, poor secondary crystallization (the portion where secondarily recrystallize is not completed) was observed (marked by * in the table). Example 3: Cast strips containing 0.048% by weight of C, 3.27% by weight of Si, 0.13% by weight of Mn, 0.0060% by weight of S, and the balance being Fe and unavoidable impurities, which further contained an amount of acid soluble Aℓ and a total amount of N listed in Table 2,were heated to 1230°C, and then hot-rolled into hot-rolled sheets having a 1.8 mm thickness. The sheets were annealed at 1000°C for 2 minutes, and then cold-rolled in the same direction as the hot-rolled direction to a 0.75 mm thickness. Subsequently, the sheets were cold-rolled in a direction vertical to the first cold-rolled direction to a 0.30 mm thickness. The sheets were then annealed in a wet hydrogen atmosphere at 800°C for 150 seconds for decarbonization, and then MgO was applied thereto. The sheets were heated up to 1000°C at a heating rate of 50°C/hr in a 75% H 2 + 25% N 2 atmosphere, and maintained for 10 hours. Subsequently, the sheets were then heated up to 1200°C at a heating rate of 25°C /hr, and maintained for 20 hours in a 100% H 2 atmosphere to be purified. The magnetic properties of the resulting products are shown in Table 2. Sample No. Acid soluble Al content Steel (%) N content in Steel (%) B 8 (T) in 1st cold-rolled direction B 8 (T) in 1st cold-rolled direction 1 0.028 0.0023 1.85 1.80 2 0.028 0.0078 1.90 1.91 3 0.007 0.0079 1.68 1.58 4 0.027 0.0076 1.91 1.91 5 0.052 0.0077 1.57 1.56 In the samples deviating from the present invention, i.e., that which had a small N content in the steel (sample No.1), and those which had too low an acid soluble Aℓ content (sample No.3) or too high a soluble Aℓ content (sample No.5), most portions did not secondarily crystallize out and had a low B 8 . Example 4: Cast strips containing 0.048% by weight of C, 3.27% by weight of Si, 0.13% by weight of Mn, 0.0060% by weight of S, and the balance being Fe and unavoidable impurities, which further contained an amount of acid soluble Aℓ and a total amount of N listed in Table 3,were heated to 1230°C, and then hot-rolled into hot-rolled sheets having a 1.8 mm thickness. The sheets were annealed at 1000°C for 2 minutes, and then cold-rolled in the same direction as the hot-rolled direction to a 0.75 mm thickness. Subsequently, the sheets were cold-rolled in a direction vertical to the first cold-rolled direction to a 0.30 mm thickness. The sheets were then annealed in a wet hydrogen atmosphere at 800°C for 150 seconds for decarbonization. After about 0.120% of N was added in an ammonia atmosphere, MgO was applied to the sheets. The sheets were heated up to 1000°C at a heating rate of 50°C/hr in a 75% H 2 + 25% N 2 atmosphere, and maintained for 10 hours. Subsequently, the sheets were then heated up to 1200°C at a heating rate of 25°C /hr, and maintained for 20 hours in a 100% H 2 atmosphere to be purified. The magnetic properties of the resulting products are shown in Table 3. Sample No. Acid soluble Al content Steel (%) N content in Steel (%) B 8 (T) in 1st cold-rolled direction B 8 (T) in 1st cold-rolled direction 1 0.028 0.0023 1.92 1.90 2 0.028 0.0078 1.94 1.93 3 0.007 0.0079 1.72 1.67 4 0.027 0.0076 1.93 1.94 5 0.052 0.0077 1.63 1.57 In the samples deviating from the present invention, i.e. that which had too little a soluble Aℓ content (sample No.3) or a too high a soluble Aℓ content (sample No.5), most portions did not secondarily crystallize out and had a low B 8 . Example 5: Cast strips containing 0.048% by weight of C, 3.27% by weight of Si, 0.13% by weight of Mn, 0.0060% by weight of S, 0.028% by weight of acid soluble Aℓ, totally 0.028% by weight of N, and the rest of Fe and unavoidable impurities, were heated to 1230°C, and then hot-rolled into hot-rolled sheets having a 1.8 mm thickness. One of the sheets was directly cold-rolled and the other was cold-rolled after annealing at 1000°C for 2 minutes, in the same direction as the hot-rolled direction, to a 0.75 mm thickness. Subsequently, the sheet was cold-rolled in the direction vertical to the first cold-rolled direction to a 0.30 mm thickness. The sheets were then annealed in a wet hydrogen atmosphere at 820°C for 150 seconds for decarburization, and then MgO containing 3% ferromanganese was applied thereto. The sheets were heated up to 1100°C at a heating rate of 20°C/hr in a 75% H 2 + 25% N 2 atmosphere, and maintained for 10 hours. Subsequently, the sheets were then heated up to 1200°C at a heating rate of 50°C /hr, and maintained for 20 hours in a 100% H 2 atmosphere to be purified. The magnetic characteristics of the resulting products are shown in Table 4. Annealing of Hot-rolled Sheet B 8 (T) in 1st cold-rolled direction B 8 (T) in 1st cold-rolled direction None 1.83 1.80 1000°C for 2 min. 1.93 1.93 By annealing the hot-rolled sheet, a product having a high B 8 could be obtained. As described above, the present invention can efficiently and stably produce a double oriented electrical steel sheet having a B 8 value similar to or better than that of the best current level of grain oriented electrical steel sheet.

Claims: A process for manufacturing a double oriented electrical steel sheet which comprises hot-rolling a silicon steel slab into a hot-rolled sheet, subjecting the hot-rolled sheet to cold-rolling at a reduction rate of 40-80%, subsequently subjecting the sheet to another cold-rolling at a reduction rate of 30-70% in a direction crossing the said cold-rolled direction, annealing the cold-rolled sheet in a wet hydrogen atmosphere for decarburization, and carrying out final finishing annealing which comprises a stage for completing secondary recrystallization followed by a stage for purification, the silicon steel slab being heated to a temperature of not more than 1270°C before hot-rolling the heated slab into said hot-rolled sheet, the decarburization annealing being at 750 to 950°C, the secondary recrystallization being completed at 920 to 1100°C, and the silicon steel slab containing 1.8-4.8% by weight of Si, 0.008-0.048% by weight of acid soluble Aℓ, totally 0.0028-0.0100% by weight of N, not more than 0.007% by weight of S and the balance being Fe and unavoidable impurities, whereby the double oriented electrical steel sheet obtained has a B 8 value of at least 1.91 Tesla in said two cold-rolling directions. A process according to claim 1 wherein the decarburized sheet is nitrided so that the N content in the raw material is totally 0.002-0.060% by weight at any time during the former annealing stage for decarburization, during an additional annealing stage thereafter, or during the heating stage in the final finishing annealing stage by the time of the start of the secondary recrystallization. A process according to claim 1 or 2 wherein the hot-rolled sheet is annealed at a temperature of 750 to 1200° for 30 seconds to 30 minutes before cold rolling.

Inventors: SUGA YOZO, USHIGAMI YOSHIYKI, SUGA, YOZO, USHIGAMI, YOSHIYUKI, USHIGAMI, YOSHIYKI
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1991
Main classification: C21D8
IPC codes: H01F1, C21D8, C22C38