(1) Field of the Invention
A grain oriented electrical steel sheet is mainly used as an iron core material of an electrical equipment such as a transformer or the like, and the steel sheet is required to have superior magnetic properties such as good exciting and watt loss characteristics. A magnetic flux density B.sub.8 at a magnetic field intensity of 800 A/m is usually used as the numerical value showing the exciting characteristic, and the watt loss W.sub.17/50 per kg observed when the sample is magnetized at a frequency of 50 Hz to 1.7 tesla (T) is used as the numerical value showing the watt loss characteristic. The magnetic flux density is the most dominant factor for the watt loss characteristic, and in general, the higher the magnetic flux density, the larger the secondary recrystallized grain diameter and the more unsatisfactory the watt loss characteristic. Nevertheless, by control of the magnetic domain, the watt loss characteristic can be improved regardless of the secondary recrystallized grain diameter.
This grain oriented electrical steel sheet is prepared by developing a Goss structure having a (110) plane on the surface of the steel sheet and a &lt;001&gt; axis in the rolling direction by causing the secondary recrystallization at the final finish annealing step. To obtain good magnetic properties, the &lt;001&gt; axis, which is the easy magnetization axis, must agree precisely with the rolling direction. The directionality of the secondary recrystallized grains can be greatly improved by the method in which MnS, AlN or the like is utilized as the inhibitor and final rolling is carried out under a high reduction ratio, and as a result, the watt loss characteristic is greatly improved.
In the production of a grain oriented electrical steel sheet, annealing of a hot-rolled sheet is generally carried out after hot rolling for a uniformation of the structure and precipitation. For example, in the process using AlN as the main inhibitor, at the step of annealing a hot-rolled sheet, a treatment for the precipitation of AlN is carried out to control the inhibitor, as disclosed in Japanese Examined Patent Publication No. 46-23820.
In general, a grain oriented electrical steel sheet is prepared through main steps such as casting, hot rolling, annealing, cold rolling, decarburization annealing, and finish annealing, the production consumes a large quantity of energy, and therefore, the manufacturing costs are higher than in the usual steel production process.
Recently, improvements have been made in this production process consuming a large quantity of energy, and demands for a simplification of the steps and reduction of the energy consumption are now increasing. As the means for satisfying this desire, there has been proposed a process in which in the production method using AlN as the main inhibitor, the precipitation of AlN at the step of annealing a hot-rolled sheet is replaced by the high-temperature winding after hot rolling (Japanese Examined Patent Publication No. 59-45730). Indeed, in this process, the magnetic properties can be maintained to some extent even if the step of annealing a hot-rolled sheet is omitted, but in the usual process where the sheet is wound in the form of a coil having 5 to 20 tons, a positional difference of the heat history is brought about in the coil during the cooling step, and thus the precipitation of AlN is inevitably uneven and the final magnetic properties differ according to parts in the coil, resulting in lowering of the yield.
Under this background, the inventors noted the recrystallization phenomenon after the final pass of finish hot rolling, which was little taken into account in the conventional technique, and examined a process of omitting the step of annealing a hot-rolled sheet by utilizing this phenomenon in the method of carrying out cold rolling once at a reduction ratio higher than 80%.
In connection with hot rolling of a grain oriented magnetic steel sheet, as the means for preventing an insufficient secondary recrystallization (formation of linear micrograins continuous in the rolling direction) caused by coarsening and growth of crystal grains of the slab at the step of heating the slab at a high temperature (for example, at a temperature not lower than 1300.degree. C.), there has been proposed a process in which, at the hot rolling step, the high reduction rolling for promoting crystallization is carried out at a temperature of 960.degree. to 1190.degree. C. at a reduction of at least 30% per pass to divide coarse crystal grains (Japanese Examined Patent Publication No. 60-37172). According to this proposal, the formation of linear micrograins can be controlled, but a production process comprising the carrying out of the annealing of a hot-rolled sheet is the premise thereof.
In the production process using MnS, MnSe or Sb as the inhibitor, there has been proposed a method in which the magnetic properties are improved by continuously carrying out hot rolling at a temperature of 950.degree. to 1200.degree. C. and a reduction ratio of at least 10% and then cooling the sheet at a cooling rate not lower than 3.degree. C./sec to precipitate MnS, MnSe or the like uniformly and finely (Japanese Unexamined Patent Publication No. 51-20716). Furthermore, there has been proposed a method in which hot rolling is carried out at a low temperature to control the advance of recrystallization and the magnetic properties are improved by preventing the (110)&lt;001&gt; oriented grains formed by shear deformation from being reduced by the subsequent recrystallization (Japanese Examined Patent Publication No. 59-32526 and Japanese Examined Patent Publication No. 59-35415). In these conventional techniques, the production by single cold rolling without annealing of a hot-rolled sheet is not even examined. In connection with the hot rolling of a silicon steel slab having an ultra-low carbon content, there has been proposed a method in which hot rolling under high reduction at a low temperature, which results in an accumulation of strain in the hot-rolled sheet, is carried out, and by the recrystallization at the subsequent annealing of the hot-rolled sheet, coarse crystal grains, characteristic of an ultra-low carbon content material, are divided (Japanese Examined Patent Publication No. 59-34212). But the production comprising an one stage cold rolling without the annealing of the hot-rolled sheet is not examined in this method.