PRODUCTION METHOD OF UNIDIRECTIONAL ELECTROMAGNETIC STEEL SHEET HAVING EXCELLENT IRON LOSS AND HIGH FLUX DENSITY

Abstract: This invention provides a unidirectional electromagnetic steel sheet having a high flux density and an extremely low iron loss by winding a steel sheet in a coil form, applying a high temperature finish annealing, adjusting a mean grain size of secondary recrystallized grains inside a roll surface to 11 ∼ 50 m/m and applying tension coating and artificial magnetic domain control treatment to a unidirectional electromagnetic steel sheet having a flux density B a of at least 1.88 to provide it with a tension of at least 0.7 kg/mm².

Description: The present invention relates to a method of producing a grain oriented electrical steel sheet having a high magnetic flux density and a very low iron loss, wherein a magnetic domain control is carried out at the surface of the steel sheet. A method of reducing iron loss by carrying out an artificial magnetic domain control on the surface of a grain oriented electrical steel sheet having a high magnetic flux density in the direction substantially at a right angle relative to the rolling direction is known. Specifically, the foregoing method is disclosed in, e.g., official gazettes of Japanese Unexamined Patent Publication No. 18566/1980 and Japanese Unexamined Patent Publication No. 73724/1983, entitled "A Method of Irradiating a Laser Light Beam with a Distance Kept Between the Adjacent Laser Light Beams," an official gazette of Japanese Unexamined Patent Publication No. 96036/1986 entitled "A Method of Forming an Invading Substance with a Distance Kept Between the Adjacent Invading Substances," an official gazette of Japanese Unexamined Patent Publication No. 117218/1986 entitled "A Method of Forming a Groove With a Distance Kept Between the Adjacent Grooves," an official gazette of Japanese Laid-Open Patent No. 117284/1986 entitled "A Method of Removing Part of a Steel Matrix and Then Coating the Removed Part of the Steel Matrix With a Film of Phosphoric Acid Based Tension Adding Agent With a Distance Kept Between the Adjacent Removed Parts" and an official gazette of Japanese Unexamined Patent Publication No. 151511/1987 entitled "A Method of Irradiating a Plasma Flame With a Distance Kept Between the Adjacent Plasma Flames." In practice, employment of the artificial magnetic domain control technology as mentioned above has made it possible to considerably reduce the iron loss of the grain oriented electrical steel sheet. Nevertheless, in spite of the aforementioned reduction of iron loss, strong demands for a ferrous material having a greater reduction of iron loss and smaller fluctuation of iron loss have been raised by many users. To satisfactorily meet these requirements, it has become even more necessary to develop a ferrous material having a greater reduction of iron loss. It is an object of the present invention to provide a method of introducing a grain oriented electrical steel sheet meeting the above requirements. This object is achieved by the method of claim 1. In view of the aforementioned requirements, the inventor conducted research and development work into the development of a grain oriented electrical steel sheet having the required properties, and as a result of this research and development work, found that a remarkably good reduction of iron loss can be obtained with respect to a grain oriented electrical steel sheet having a high magnetic flux density, by carrying out a control to allow an average grain size of each secondarily recrystallized grain to remain within a predetermined range wherein the grain oriented electrical steel sheet is treated such that it is covered with a layer of tension coating and a magnetic domain control is carried out therefor in the direction substantially at a right angle relative to the rolling direction, after completion of the secondary recrystallization, whereby, the present invention was created. Specifically, the present invention provides a method of producing a grain oriented electrical steel sheet having a very good reduction of iron loss while a magnetic flux density is kept higher than 1.88T in the presence of a magnetizing force of 800 A/m, wherein the method is practiced such that a finally cold rolled steel sheet is subjected to annealing for decarburization, wound in the form of a coil while coated with a separating agent for an annealing operation, subjected to finish annealing at a high temperature, then subjected to flattening annealing with the separating agent removed therefrom, thereafter, before or after completion of the flattening annealing operation, covered with a film of tension coating to allow an intensity of tension per an unit sectional area of the steel sheet to be kept higher than 0.7 kg/mm², and moreover, before or after completion of the tension coating operation or the flattening annealing operation, an artificial magnetic domain control is carried out for the surface of the steel sheet, wherein the method is characterized in that an average grain size of each secondarily recrystallized grain in the rolling surface is regulated to remain within 10 to 50 mm while components in a ferrous material for the steel sheet and conditions for treating the steel sheet are properly controlled. The invention is described in more detail in connection with the drawings, in which: Fig. 1 illustrates a relationship among a magnetic flux density B₈, an average grain size of each secondarily recrystallized grain and an iron loss W 15/50 after steel sheets were subjected to finish annealing at a high temperature while held in the flat state, a laser light beam then irradiated to the surface of each grain oriented electrical steel sheet which was covered with a layer of tension coating, and thereafter, a magnetic domain control carried out for the grain oriented electrical steel sheet; Fig. 2 illustrates a relationship between a magnetic flux density B₈ and an average grain size of each secondarily recrystallized grain with respect to grain oriented electrical steel sheets produced such that the steel sheets were subjected to finish annealing at a high temperature while held in the bent state, subjected to flattening annealing after completion of the finish annealing operation, were then covered with a film of tension coating, respectively, and moreover, a magnetic domain control carried out therefor by irradiating a laser light beam to the surface of each of the steel sheets; and Fig. 3 illustrates a relationship among a reduction ratio for a final cold rolling, a magnetic flux B₈ and an average grain size of each secondarily recrystallized grain with respect to the respective grain oriented electrical steel sheets after the steel sheets were subjected to finish annealing at a high temperature while held in the flat state. The best mode of carrying out the present invention has been practically recognized based on results derived from the following Experiments (I) to (III). (EXPERIMENT I) Steel sheets each containing Si of 3.2% and having one or two or more selected frcm a group of materials, i.e., MnS, MnSe, CuxS, Sn and Sb utilized therefor as an inhibitor in addition to AlN were finally cold rolled to a thickness of 0.17 mm, and subsequently, were subjected to annealing for decarburization and then coated with a separator utilized for an annealing. Then, the sheets were subjected to finish annealing at a high temperature while held in the flat state, and after completion of the finish annealing operation, the separator was removed from each of the steel sheets, whereby various kinds of grain oriented electrical steel sheets were produced. Thereafter, each of the resultant grain oriented electrical steel sheets was covered with a film of tensile coating, to allow an intensity of tension per an unit sectional area of each grain oriented electrical steel sheet to be maintained at a level of 1.0 kg/mm². Subsequently, a pulsed laser light beam was irradiated to the surface of each grain oriented electrical steel sheet in the direction at a right angle relative to the rolling direction under conditions of an energy density of 2.0 J/cm², an irradiation width of 0.25 mm and an irradiation distance of 5 mm. After completion of the irradiating operation, a magnetic flux density B₈ (magnetic flux density in the presence of a magnetizing force of 800 A/m) and an iron loss W 15/50 were measured with respect to the respective grain oriented electrical steel sheets. In addition, while the surface film was removed therefrom, a grain size of each secondarily recrystallized grain in the rolling surface was measured by employing a line segment method with respect to three directions, i.e., the rolling direction, the direction of 45 degrees relative to the rolling direction and the direction at a right angle relative to the rolling direction, whereby an average grain size was determined with respect to the respective grain oriented electrical steel sheets (it should be noted that an average grain size determined for carrying out the present invention with respect to all the grain oriented electrical steel sheets was measured by employing the aforementioned method). A relationship among the average grain size, the magnetic flux density B₈ and the iron loss W 15/50 is illustrated in Fig. 1. Referring to Fig. 1, the abscissa designates an average grain size and the ordinate designates a magnetic flux density B₈. In the drawing, four marks (i.e., a double circle mark, a single circle mark, a triangular mark and a X-shaped mark) designate an iron loss W 15/50, respectively. As apparent from Fig. 1, it has been found that an excellent reduction of iron loss can be obtained when the average grain size is larger than 11 mm and the magnetic flux density B₈ is kept higher than 1.88T. ( EXPERIMENT II) Steel sheets were treated by the same method as that employed for Experiment I, until the step of coating a separating agent for an annealing operation. Thereafter, these sheets were subjected to finish annealing at a high temperature while bent in the rolling direction with a radius of curvature of 400 mm, and the separating agent was then removed. Then, the sheets subjected to flattening annealing, and after completion of the flattening annealing operation, were covered with a film of tensile coating and a laser light beam was then irradiated to the surface of each of the steel sheets by employing the same method as that employed for Experiment I. Subsequently, a magnetic flux density B₈ and an average grain size of each secondarily recrystallized grain were measured with respect to the respective steel sheets. A relationship between the magnetic flux density B₈ and the average grain size is illustrated in Fig. 2. Referring to Fig. 2; the abscissa designates an average grain size and the ordinate designate a magnetic flux density B₈. As apparent from Fig. 2, where the steel sheets were subjected to finish annealing at a high temperature while held in the bent state, it is clearly recognized that there is a tendency for the magnetic flux density B₈ to be deteriorated when the average grain size is excessively enlarged. In addition, it is found that the magnetic flux density B₈ is remarkable deteriorated when the average grain size exceeds 50 mm. In this connection, it is presumable from Fig. 1 that, where the average grain size exceeds 50 mm, the magnetic flux density B₈ is deteriorated, resulting in a corresponding deterioration of the iron loss. It should be noted that the respective steel sheets were annealed while the opposite surfaces thereof were oriented in the vertical direction and they were normally held in the wound state in the form of a coil, because the finish annealing operation at a high temperature took a long time while they were kept at a high temperature. A radius of curvature as measured along the inner periphery of each coil was smaller than about 400 mm during the finish annealing operation at a high temperature. This is because if a radius of curvature is larger, the installation is unavoidably enlarged, resulting in grain oriented electrical steel sheets disadvantageously produced at a high cost. It has been found from the results derived from Experiment I and Experiment II that a very good reduction of iron loss can be obtained by controlling an average grain size of each secondarily recrystallized grain to within 11 to 50 mm with respect to the respective grain oriented electrical steel sheets each having a high magnetic flux density which were produced by allowing them to be subjected to finish annealing at a high temperature by employing the conventional method while they were held in the wound state in the form of a coil, covering them with a layer of tension coating and then treating them under a magnetic domain control in the direction at a substantially right angle relative to the rolling direction after completion of the secondary recrystallization. (EXPERIMENT III) Silicon steel slabs each containing C of 0.065%, Si of 3.0%, Mn of 0.075%, S of 0.025%, an acid soluble aluminum of 0.0260%, N of 0.0085% and a residual of unavoidably contained elements were heated at a temperature of 1,350°C for 120 minutes and then hot rolled to have a thickness of 1.1 to 5.0 mm. The hot rolled steel plates were annealed at a temperature of 1,120°C for 2 minutes and then cooled down to 300°C at a cooling rate of 30°C/sec. Thereafter, they were cold rolled to a thickness of 0.285 mm, subjected to annealing for decarburization at a temperature of 850°C for 3 minutes in a wet atmosphere comprising H₂ of 75% and N₂ of 25%, coated with a separator containing a magnesia as a main component for an annealing and then subjected to finish annealing at a high temperature while they were held in the flat state. The finish annealing at a high temperature was performed such that the atmosphere comprising H₂ of 75% and N₂ of 25% was maintained during elevation of the working temperature, the working temperature was elevated up to 1,200°C at an elevating rate of 15°C/hour and the cold rolled steel sheets were then annealed at a temperature of 1,200°C for 20 hours in a hydrogen atmosphere. Thereafter, a magnetic flux density B₈ and an average grain size of each secondarily recrystallized grain were measured with respect to the respective products of grain oriented electrical steel sheets. A relationship among a reduction ratio of the cold rolling, the magnetic flux density B₈ and the average grain size is illustrated in Fig. 3. Referring to Fig. 3, the abscissa designates a reduction ratio of the cold rolling and the ordinate designates a magnetic flux density and an average grain size. As is apparent from Fig. 3, the grain oriented electrical steel sheet each having excellent properties such that the reduction ratio of the cold rolling remains within 83 to 92%, the average grain size ranges from 11 to 50 mm and the magnetic flux density is kept higher than 1.88T can be obtained. As apparent from the above description, according to the present invention, the grain oriented electrical steel sheets each having-a high magnetic flux density and a very low value of iron loss can be obtained by allowing the respective grain oriented electrical steel sheets (derived from, e.g., Experiment III) having excellent properties such that the average grain size ranges from 11 to 50 mm and the magnetic flux density is kept higher than 1.88T to be covered with a layer of tension coating which assures an intensity of tension higher than 0.7 kg/mm² and then treating the surface of each of the grain oriented electrical steel sheets under an artificial magnetic domain control. Next, description will be given of a chemical composition of each grain oriented electrical steel sheet, and the reason why the foregoing chemical composition is determined therefor. Preferably, the carbon content is lower than 0.12%, as if it exceeds 0.12%, it becomes difficult to accomplish decarburizaiton during an annealing operation to be performed for the purpose of decarburization. Also, preferably the silicon content remains within 2.5 to 4.5%, as if it is lower than 2.5%, each grain oriented electrical steel sheet fails to show a very good reduction of iron loss. If it exceeds 4.5%, the workability is deteriorated. Preferably, the manganese content remains within 0.030 to 0.200%, as if lower than 0.030%, the workability is deteriorated. If it exceeds 0.200%, the grain oriented electrical steel sheets do not show a very good reduction of iron loss. Preferably, the total content of one or both of sulfur and selenium remains within 0.01 to 0.06%, as if it is lower than 0.01% or exceeds 0.06%, the grain oriented electrical steel sheets do not show a very good reduction of iron loss. Preferably, the acid soluble aluminum has a content which remains within 0.010 to 0.050%, as if lower than 0.010%, the grain oriented electrical steel sheets do not have an excellent property of magnetic flux density. If it exceeds 0.050%, the secondary recrystallization is accomplished incorrectly. Preferably, the nitrogen content remains within 0.0030 to 0.0100%, as if it is lower than 0.0030%, the secondary recrystallization is accomplished incorrectly. If it exceeds 0.0100%, a flaw in the form of a blister appears on the surfaces of the grain oriented electrical steel sheets. It has been found that the grain oriented electrical steel sheet does not have an excellent magnetic property unless it is at least once annealed at a temperature of from 1,050 to 1,200°C and then quickly cooled until a final cold rolling is performed after completion of the hot rolling. An intensity of tension per unit sectional area of each grain oriented electrical steel sheet appearing in the presence of a surface film (inclusive of forstelite) should be kept higher than 0.7 kg/mm², as if lower than 0.7 kg/mm², the grain oriented electrical steel sheets do not show a very good reduction of iron loss. When the magnetic flux density is made higher than 1.88T while a magnitude of magnetizing force is maintained at a level of 800 A/m, the grain oriented electrical steel sheets show a very good reduction of iron loss. If the magnetic flux density is made lower than 1.88T, the grain oriented electrical steel sheets do now show a very good reduction of iron loss. It has been found that each grain oriented electrical steel sheet exhibits a very good reduction of iron loss when produced under conditions such that the average grain size of each secondarily recrystallized grain remains within 11 to 50 mm, the surface of each grain oriented electrical steel sheet is coated with a film which ensures that the intensity of tension per an unit sectional area thereof is kept higher than 0.7 kg/mm², the magnetic flux density is kept higher than 1.88T while the magnitude of magnetizing force is maintained at the level of 800 A/m and artificial magnetic domain control is carried out for the surface of each grain oriented electrical steel sheet in the direction at a substantially right angle relative to the rolling direction. It is considered that, where magnetic domain control is carried out for the grain oriented electrical steel sheet of the present invention, the factor of a deterioration of the iron loss when the average grain size is smaller than 11 mm harmfully affects a magnetic domain forming pattern wherein the fine grain boundary serves to minimize the iron loss. In addition, it is considered that where steel sheets are subjected to finish annealing at a high temperature while held in the bent state (on the basis of industrial products), deviation of a Goss orientation from the rolling surface or the like malfunction due to the flattening annealing performed after completion of the annealing at a high temperature is concerned with undesirable reduction of the magnetic flux density at the time when the average grain size exceeds 50 mm. It has been found that grain oriented electrical steel sheets each having excellent properties such that the magnetic flux density B₈ is kept higher than 1.88T and the average grain size of each secondarily recrystallized grain remains within 11 to 50 mm while a substance of AlN is utilized therefor as a main inhibitor can be produced under conditions such that they are at least once annealed at a temperature of from 1,050 to 1,200°C until a final cold rolling is performed after completion of the hot rolling, are quickly cooled after completion of the annealing and are then subjected to final cold rolling at a reduction ratio of 83 to 92%. EMBODIMENT 1 Silicon steel slabs each containing C of 0.080%, Si of 3.2%, Mn of 0.075%, an acid soluble aluminum of 0.0250% and N of 0.0085% and moreover containing one or two or more selected from a group of elements, i.e., S of 0.025% or 0.015%, Se of 0.020%, Sn of 0.12%, Cu of 0.07% and Sb of 0.020% were heated at a temperature of 1,350°C for 120 minutes so that they were hot rolled to provide hot rolled plates each having a thickness of 0.9 to 4.4 mm. The hot rolled plates were annealed at various temperature within the range from 1,000 to 1,220°C and then cooled down to 300°C at a cooling rate of 35°C/sec. Thereafter, they were subjected to final cold rolling in accordance with a production process I or II to be described later. Specifically, in a case of the production process I, the hot rolled sheets were subjected to final cold rolling immediately after completion of the annealing operation therefor. On the other hand, in a case of the production process II, after the hot rolled plates were annealed, they were subjected to intermediate cold rolling to have a predetermined thickness, respectively, they were then annealed at a temperature of 1,000°C for 100 seconds and, thereafter, they were cooled down to 300°C at a cooling rate of 25°C/sec. Subsequently, they were subjected to final cold rolling. After completion of the final cold rolling, the hot rolled sheets were subjected to annealing for decarburizaiton at a temperature of 850°C for 3 minutes in a wet atmosphere comprising H₂ of 75% and N₂ of 25%, they were then coated with a separator containing a magnesia as a main component for the annealing operation and thereafter, they were wound in the form of a coil having a radius of curvature of about 400 mm, respectively, so that they were subjected to finish annealing at a high temperature. While they were subjected to finish annealing at a high temperature, the atmosphere comprising H₂ of 75% and N₂ of 25% was maintained during elevation of the working temperature and they were heated up to a temperature of 1,200°C at an elevating rate of 15°C/hour so that they were annealed at a temperature of 1,200°C for 20 hours in a hydrogen atmosphere. Thereafter, the separating agent utilized for the annealing operation was removed and a magnetic domain control treatment, a tension coating operation, an annealing operation and others were then performed in accordance with one of four kinds of methods, i.e., an A method, a B method, a C method and a D method each of which will be described later. When the A method was employed, the tension coating operation was performed for the respective steel sheets such that an intensity of tension per an unit sectional area of each steel sheet was set to 1.0 kg/mm². Then, they were subjected to flattening annealing at a temperature of 850°C for 30 seconds in additional consideration for baking the layer of tension coating. Subsequently, a pulsed laser light beam was irradiated to the surface of each steel sheet in the direction at a right angle relative to the rolling direction under conditions of an energy density of 2.0 J/cm², an irradiation width of 0.25 mm and irradiation distance of 5 mm. When the B method was employed, each steel sheet was coated with antimony powder after it was treated in accordance with the A method. Thereafter, it was annealed at a temperature of 800°C for 2 hours. When the C method was employed, a pulse laser light beam was irradiated to the surface of each steel sheet in the direction at a right angle relative to the rolling direction under conditions of an energy density of 3.0 J/cm², an irradiation width of 0.2 mm and an irradiation distance of 5 mm so as to partially remove the forstelite layer from the surface of each steel sheet. Then, the steel sheet was immersed in a nitric acid solution having a concentration of 61% for 20 seconds. Subsequently, a tension coating was performed for the steel sheet such that an intensity of tension per an unit sectional area of the steel sheet was maintained at a level of 1.0 kg/mm². Thereafter, the steel sheet was subjected to flattening annealing at a temperature of 850°C for 30 seconds in additional consideration for baking the tension coating layer. Finally, when the D method was employed, a strain was introduced into each steel sheet under a load of 180 kg/mm² with the aid of a gear-shaped roll which was designed such that a gear tooth pitch was dimensioned to 8 mm, a radius of curvature at the apex of each gear tooth was dimensioned to 100 microns and the gear tooth serving as a blade extended at an angle of 75 degrees relative to the rolling direction. Then, a tension coating operation was performed such that an intensity of tension per an unit sectional area of each steel sheet was maintained at a level of 1.0 kg/mm². Thereafter, the steel sheet was subjected to flattening annealing at a temperature of 850°C for 30 seconds. After the steel sheet was treated by employing one of the A method, the B method, the C method and the D method, the magnetic flux density B₈ and the iron loss were measured. Subsequently, the surface film was removed from the steel sheet which in turn was washed in an acid solution so that an average grain size of each secondarily recrystallized grain in the rolling surface was measured. Components in each steel sheet, a thickness of each hot rolled sheet, a production process (I or II), a soaking temperature during an annealing for hot rolled sheets, a thickness of each steel sheet after completion of an intermediate cold rolling, a thickness of each steel sheet after completion of a final cold rolling, a reduction ratio for the final rolling, an average grain size of each secondarily recrystallized grain, a magnetic domain controlling method (A, B, C or D), a magnetic flux density B₈ and an iron loss are shown in Table 1, respectively. As is apparent from Table 1, according to the embodiment of the present invention, an grain oriented electrical steel sheet having a high magnetic flux density and a very good reduction of iron loss can be obtained. The present invention makes it possible to provide a ferrous material having a remarkable low iron loss which is preferably employable for a core or the like in a transformer. Consequently, an energy loss in the transformer or the like electric equipment can be substantially reduced by using the ferrous material of the present invention.

Claims: A method of producing a grain oriented electrical steel sheet showing a very good reduction of iron loss while a magnetic flux density is kept higher than 1.88T in the presence of a magnetizing force of 800 A/m wherein a slab containing up to 0.12% of C, 2.5 to 4.5% of Si, 0.030 to 0.200% of Mn, 0.01 to 0.06% total amount of S and/or Se, 0.010 to 0.050% of acid soluble Al and 0.0030 to 0.0100% of N, with the balance being Fe and unavoidable impurities, is hot rolled, the obtained hot rolled sheet is annealed at 1050 to 1200°C, said sheet is cold rolled with a reduction ratio of 82 to 93%, the finally cold rolled steel sheet is subjected to annealing for decarburization, is wound in the form of a coil while coated with a separator for an annealing operation, is then subjected to finish annealing at a high temperature, is then subjected to flattening annealing after said separator is removed therefrom, thereafter, before or after completion of said flattening annealing, is covered with a film of tension coating to keep an intensity of tension per a unit sectional area of the steel sheet higher than 0.7 kg/mm², and moreover, before or after completion of said tension coating or said flattening annealing, an artificial magnetic domain control is carried out at the surface of the steel sheet, whereby an average grain size of each secondarily recrystallized grain in the rolling surface is regulated to remain within the range of from 11 to 50 mm.

Inventors: NAKASHIMA S, NAKASHIMA, S., NAKASHIMA, S. Nippon Steel Corp. R&amp;D Lab.-III
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1989
Main classification: C21D9
IPC codes: C21D9, C21D8, H01F1