Grain oriented electromagnetic steel sheet and manufacturing thereof

Method of making a grain oriented electromagnetic steel sheet having excellent magnetic properties, by a series of steps ranging from hot rolling to final finishing annealing for a silicon steel slab containing from about 0.001 to 0.07 wt % bismuth, wherein the average cooling rate for about five seconds measured immediately after the end of hot rolling is controlled within a range of from about 30 to 120.degree. C./second; the value of the ratio P.sub.H2O /P.sub.H2 of the atmosphere for the soaking step in decarburization annealing is adjusted within a range of from about 0.45 to 0.70; and a treatment is provided for inhibiting decomposition of the surface inhibitor during final finishing annealing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a grain oriented electromagnetic steel
 sheet adapted to be used for an iron core of a transformer or other
 electrical appliances.
 2. Description of the Related Art
 A grain oriented electromagnetic steel sheet as an iron core material for a
 transformer, a generator or a motor is required to have a high magnetic
 flux density and a low-iron loss as the most important properties.
 Various measures have so far been taken to achieve a low iron loss of the
 grain oriented electromagnetic steel sheet. Among others, importance has
 been attached to high integration of the grain orientations of the steel
 sheet in the {110} &lt;001&gt; orientation known also as Goss orientation. When
 grain orientations of the steel sheet are highly integrated in Goss
 orientation, &lt;001&gt; axes which are axes of easy magnetization of iron
 crystal would highly be integrated in the rolling direction. That is,
 force required for magnetization in the rolling direction becomes smaller,
 resulting in a smaller coercive force. As a result, hysteresis loss
 becomes smaller, thus permitting achievement of a low iron loss.
 Aligning grain orientations in Goss orientation greatly contributes to
 reduction of noise upon magnetization which is an important required
 property of a grain oriented electromagnetic material. Magnetostriction
 vibration and electromagnetic vibration of the iron core material are
 known to be causes of noise produced from a transformer. An improved
 degree of integration of grain orientations in Goss orientation inhibits
 generation of 90.degree. magnetic domain forming a cause of
 magnetostriction. Simultaneously with this, decreased excited current
 inhibits electromagnetic vibration, thus resulting in reduction of noise.
 For a grain oriented electromagnetic steel sheet, as described above,
 integration of &lt;001&gt; axes of crystal grains in the rolling direction is
 the most important subject. As an indicator of the degree of integration,
 the magnetic flux density, B.sub.8 (T) at a magnetization force of 800 A/m
 is often employed. That is, development efforts of a grain oriented
 electromagnetic steel sheet are promoted with improvement of magnetic flux
 density B.sub.8 as an important target. The iron loss is typically
 represented by an energy loss, W.sub.17/50 (W/kg) under conditions
 including an excited magnetic flux density of 1.7 T and an excited
 frequency of 50 Hz.
 The secondary recrystallization grains of the grain oriented
 electromagnetic steel sheet are formed through a phenomenon known as
 secondary recrystallization during the final finishing annealing. Enormous
 growth of crystal grains in Goss orientation is selectively caused by
 secondary recrystallization to increase the degree of integration in Goss
 orientation, thus obtaining a product having a desired magnetic property.
 In order to effectively accelerate integration of secondary
 recrystallization grains in Goss orientation, it is important to form a
 precipitation dispersion called an inhibitor which inhibits normal growth
 of primary recrystallization grains, uniformly throughout the steel and in
 an appropriate size. Presence of the inhibitor makes it possible to
 inhibit normal grain growth of primary recrystallization grains, and
 maintain a fine state of primary recrystallization grains even at high
 temperatures during final finishing annealing. At the same time, there is
 provided a higher selectivity for the growth of crystal grains in a
 preferred orientation, thus resulting in a higher degree of integration of
 crystal grains in Goss orientation and permitting achievement of a high
 magnetic flux density. In general, it is believed that a higher degree of
 integration in Goss orientation is available when the inhibitor is
 stronger and the normal growth inhibiting ability is great.
 A material having a small solubility in steel such as MnS, MnSe, Cu.sub.2-x
 S, Cu.sub.2-x Se or AlN is applicable as an inhibitor. For example,
 Japanese Patent Publication No. 33-4710 and Japanese Patent Publication
 No. 40-15644 disclose adding aluminum to a material, using a high
 reduction within a range of from 81 to 95% for the final cold rolling, and
 applying annealing before the final cold rolling, thereby causing
 precipitation of AlN, a strong inhibitor.
 Further, it is known that, in addition to the inhibitor constituents
 mentioned above, addition of Sn, As, Bi, Sb, B, Pb, Mo, Te, V, or Ge is
 effective for improvement of the degree of orientation integration of
 secondary recrystallization grains.
 From among these additional inhibitor constituents, P, As, Sb and Bi
 falling under the category of 5B family elements in the Periodic Table are
 known to intensify the normal grain growth inhibiting ability and improve
 magnetic property is cooperation with the main inhibitor such as MnS,
 MnSe, Cu.sub.2-x S, Cu.sub.2-x Se or AlN through segregation on grain
 boundaries. Among others, bismuth is considered helpful as a component
 intensifying the normal grain growth inhibiting ability through a grain
 boundary segregation effect because of a particularly low solubility in
 iron.
 A technique to improve magnetic property by adding bismuth is disclosed in
 Japanese Examined Patent Publication No. 51-29496 and Japanese Patent
 Examined Publication No. 54-32412. Japanese Patent Publication No.
 62-56924, Japanese Unexamined Patent Publication No. 2-813673 and Japanese
 Examined Patent Publication No. 7-62176 disclose methods of compositely
 adding AlN, MnSe or MnS together with bismuth into steel. These
 techniques, while utilizing the inhibiting power intensifying effect by
 bismuth, have not as yet been established manufacturing conditions
 appropriate for a material added with bismuth, and are therefore
 insufficient to obtain stably a grain oriented electromagnetic steel sheet
 having satisfactory magnetic property.
 Japanese Unexamined Patent Publications Nos. 6-88171, 6-88172, 6-88173 and
 6-88174 disclose the possibility of largely improving magnetic flux
 density by adding bismuth to an aluminum-based inhibitor. The effect
 itself of addition of bismuth has however been known, but the magnetic
 property improving effect has not as yet been stably derived.
 A method of stabilizing magnetic property of an electromagnetic steel sheet
 containing added bismuth is disclosed in Japanese Unexamined Patent
 Publication No. 6-158169. This publication, while mainly disclosing a
 technique of heating a steel slab having a low sulfur or selenium content
 to a low temperature and performing nitriding during heating, discloses
 also a manufacturing method comprising the steps of adding bismuth to
 steel and carrying out the latter half of decarburization annealing in a
 reducing atmosphere. However, the decarburization annealing conditions in
 this techniques mainly aims at stabilizing formation of a film. That is,
 optimum conditions for stabilizing the magnetic property improving effect
 for a material added with bismuth have not as yet been established.
 Regarding a separator for final finishing annealing, Japanese Unexamined
 Patent Publication No. 8-253819 discloses a technique of forming a film
 having an amount of coating of at least 5 g/m.sup.2 per side of the steel
 sheet. This technique has an object to improve the film through
 improvement of gas ventilation between coil layers, not providing a
 function of stabilizing magnetic property. Further, according to the
 result of research conducted by the present inventors, a simple increase
 in the amount of coated separator would result in a reverse effect for the
 stabilization of the magnetic property.
 As to the technique of using a low-activity material as an annealing
 separator for the silicon steel with added bismuth, Japanese Unexamined
 Patent Publication No. 6-256849 discloses a method of coating a material
 low in reactivity with SiO.sub.2 after application of a nitriding
 treatment. However, the function of bismuth in this technique is only to
 prevent decomposition of the inhibitor during a final finishing annealing
 unique to a mirror-finishing material including a nitriding step. Japanese
 Unexamined Patent Publication No. 7-173544 discloses a manufacturing
 method of a mirror-finished grain oriented electromagnetic steel sheet by
 coating an annealing separator added with a metal chloride onto a silicon
 steel with added bismuth. This technique has as well a main object to
 obtain a mirror surface by the addition of bismuth into the steel, and
 consequently, a satisfactory magnetic property cannot stably be obtained
 unless decarburization annealing conditions are controlled.
 Japanese Unexamined Patent Publication No. 9-202924 discloses a method of
 coating alumina as an annealing separator after carrying out
 decarburization annealing in an atmosphere not generating iron oxides, or
 removing oxides from the surface of the decarburization-annealed sheet. In
 this technique, alumina is used as an annealing separator for the purpose
 of obtaining a satisfactory magnetic property without being affected by
 the gas ventilation between coil layers during final finishing annealing.
 Application of this technique permits achievement of reduction of the
 amount of oxygen on the surface of the final-finishing-annealed sheet
 under the effect of the alumina separator, and stabilizes the magnetic
 property to some extent. However, since the decarburization annealing
 conditions are favorable only for mirror surface finishing, secondary
 recrystallization grains cannot be completely stabilized. When using
 alumina as an annealing separator, it becomes difficult to remove
 impurities from the steel, and brings about a problem of deterioration of
 hysteresis loss.
 In other words, addition of bismuth, being very helpful for the improvement
 of the magnetic property of a grain oriented electromagnetic steel sheet,
 tends to cause defective secondary recrystallization under the effect of
 various factors, and leaves a difficulty in stably obtaining a
 satisfactory magnetic property.
 The present invention has, as an object, to stabilize secondary
 recrystallization of a grain oriented electromagnetic steel sheet with
 added bismuth, and permit manufacture of a grain oriented electromagnetic
 steel sheet having excellent magnetic flux density and iron loss.
 SUMMARY OF THE INVENTION
 As a result of extensive studies, the present inventors reached the
 conclusion that, in order to stably obtain a satisfactory magnetic
 property from a silicon steel with added bismuth, it was important to
 create particular manufacturing conditions for the upstream processes such
 as hot rolling, as well as to optimize decarburization annealing
 conditions (particularly the atmosphere), and the final finishing
 annealing conditions. It was found also that, in formation of excessive
 forsterite film during final finishing annealing, a silicon steel
 containing added bismuth tended to cause odeterioration of the magnetic
 property. As a result of further studies carried out to solve this
 problem, we discovered the possibility of stably obtaining a grain
 oriented electromagnetic steel sheet having a high magnetic flux density
 by limiting formation of the forsterite film during finishing annealing
 using the silicon steel having added bismuth.
 More specifically, the present invention provides a manufacturing method of
 a grain oriented electromagnetic steel sheet having excellent magnetic
 properties, comprising the steps of: heating a silicon steel slab
 containing from about 0.03 to 0.10 wt % carbon, from about 2.0 to 5.0 wt %
 silicon, from about 0.04 to 0.15 wt % manganese, from about 0.01 to 0.03
 wt % one or more selected from sulfur and selenium, from about 0.015 to
 0.035 wt % soluble aluminum and from about 0.0050 to 0.0100 wt % nitrogen
 to a temperature of at least about 1,300.degree. C., hot-rolling the
 heated steel slab, then achieving a final thickness sheet through a
 combination of annealing and cold rolling, decarburization-annealing the
 annealed and cold-rolled steel sheet, and conducting a final finishing
 annealing; wherein the slab contains from about 0.001 to 0.070 wt %
 bismuth; the average cooling rate is controlled to about 30 to 120.degree.
 C./sec for a period of five seconds from immediately after the completion
 of hot rolling; the ratio P.sub.H2O /P.sub.H2 in the atmosphere in the
 soaking step of the decarburization annealing procedure is adjusted to a
 value within a range of from about 0.45 to 0.70, and treatment for
 inhibiting decomposition of the surface layer inhibitor is incorporated in
 the final finishing annealing. Another feature of the invention is that
 the amount of oxygen on the surface of the finally finishing-annealing
 sheet, which is an indicator of the effect of inhibiting decomposition of
 the surface layer inhibitor during final finishing annealing, is
 controlled.
 Still another aspect of the invention provides a method of manufacturing a
 grain oriented electromagnetic steel sheet having excellent magnetic
 properties, wherein the amount of MgO hydration of the annealing separator
 for the final finishing annealing, the amount of coating separator on the
 sheet surface, the amounts of added TiO.sub.2 in the separator, and values
 of the ratio P.sub.H2O /P.sub.H2 in the heating and the soaking steps of
 decarburization annealing are optimized for inhibiting decomposition of
 the surface layer inhibitor during final finishing annealing. Improvement
 of the film and magnetic property is accomplished by optimizing the
 soaking temperature in the decarburization annealing procedure and adding
 an inhibitor-intensifying element such as Sn, Ni, Cr or Ge.
 The invention provides also a grain oriented electromagnetic steel sheet
 having excellent magnetic properties, comprising a base metal portion of
 the final product containing up to about 0.0040 wt % carbon, from about
 2.0 to 5.0 wt % silicon, from about 0.02 to 0.15 wt % manganese, up to
 about 0.0025 wt % of one or two elements selected from sulfur and
 selenium, up to about 0.0015 wt % aluminum, up to about 25 wtppm nitrogen,
 from about 0.0002 to 0.0600 wt % bismuth, and the balance substantially
 iron, wherein the average value of the shift angle .theta. between the
 [001] axis of crystal grains and the rolling direction, measured 200 mm or
 more from both ends of the product coil, equal to or less than about
 5.0.degree..

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The development of the present invention will now be preliminarily
 described sequentially along with several illustrative experiments.
 EXPERIMENTS
 (Experiment 1)
 A steel ingot mainly containing 0.06 wt % carbon, 3.2 wt % silicon, 0.07 wt
 % manganese, 0.02 wt % selenium, 0.005 wt % sulfur, 0.022 wt % aluminum,
 0.0085 wt % nitrogen and 0.035 wt % bismuth was heated to 1,400.degree.
 C., held for 30 minutes and then hot rolled into a hot-rolled steel sheet
 having a thickness of 2.5 mm. The average cooling rate of the hot-rolled
 steel sheet during five seconds immediately after hot rolling was
 20.degree. C./sec or 40.degree. C./sec. Then, the hot-rolled steel sheet
 was subjected to a hot-rolled sheet annealing at 1,000.degree. C. for 30
 seconds, a pickling and then a primary cold rolling into a steel sheet
 having a thickness of 1.6 mm. Then, an intermediate annealing was applied
 to the cold-rolled steel sheet, and after pickling, the sheet was brought
 into a final thickness of 0.23 mm through a secondary cold rolling. Then,
 the resultant cold-rolled steel sheet was subjected to a decarburization
 annealing at a soaking temperature of 850.degree. C. for 100 seconds. The
 ratio of the water vapor partial pressure to the hydrogen partial pressure
 in the atmosphere of the soaking step of decarburization annealing
 (oxidation potential): P.sub.H2O /P.sub.H2 was altered to various levels
 within a range of from 0.30 to 0.80. The same value as in the soaking step
 was set for P.sub.H2O /P.sub.H2 of the heating step of decarburization
 annealing. After coating an annealing separator mainly comprising MgO onto
 the decarburization-annealed sheet, a final finishing annealing was
 applied at a maximum temperature of 1,200.degree. C. for five hours. Eight
 Epstein test pieces (30 mm wide and 280 mm long) were sampled in the
 rolling direction from the final finishing-annealed steel sheet and
 magnetic flux density B.sub.8 was measured on these test pieces by the
 Epstein test method.
 FIG. 1 illustrates the effects of P.sub.H2O /P.sub.H2 in the heating step
 and the soaking step of decarburization annealing on magnetic flux density
 B.sub.8. As in clear from FIG. 1 that a high magnetic flux density B.sub.8
 of at least 1.965 T was obtained by using a higher cooling rate
 immediately after the end of hot rolling and controlling P.sub.H2O
 /P.sub.H2 of the decarburization annealing atmosphere within a range of
 from 0.45 to 0.7. Even with a value of P.sub.H2O /P.sub.H2 within the
 range of from 0.45 to 0.7, on the other hand, a low cooling rate
 immediately after the end of hot rolling resulted in a low and unstable
 magnetic flux density B.sub.8, with a product containing 0.0122 wt %
 bismuth. For the portion of the product coil having a high magnetic flux
 density B.sub.8 excluding the both width ends for 200 mm each, crystal
 grains had an average value .theta. of the shift angle between the [001]
 axis of each grain and the rolling direction within a range of 2.5 to
 4.5.degree.. The average value .theta. of the shift angle of grain
 orientation is defined as follows, and the measuring method was as
 described below.
 1) The crystal grain orientation was measured at a pitch of 10 mm in the
 longitudinal direction and at a pitch of 10 mm in the width direction by
 the use of X-ray diffraction or the like for a portion of the entire width
 except for 200 mm on the both sides of the coil and about 100 mm in the
 longitudinal direction of the coil.
 2) The Angle (absolute value) between the grain [001] axis and the rolling
 direction was determined for each portion to be measured.
 3) Values of the grain orientation shift angle thus determined for the
 individual portions were averaged as .theta..
 (Experiment 2)
 The relationship between the cooling rate immediately after the end of hot
 rolling and the magnetic property of the product was investigated. The
 experiment was carried out under the same conditions as in Experiment 1
 except that the cooling rate immediately after the end of hot rolling was
 altered within a range of from 10 to 130.degree. C./second, with a
 P.sub.H2O /P.sub.H2 of 0.40 for the heating step and a P.sub.H2O /P.sub.H2
 of 0.60 for the soaking step of decarburization annealing. FIG. 2
 illustrates the effect of the cooling rate during those five seconds
 measured immediately after the end of hot rolling on magnetic flux density
 B.sub.8. FIG. 2 indicates that a high and stable magnetic flux density was
 available by controlling the cooling rate immediately after the end of hot
 rolling, within a range of from 30 to 120.degree. C./second. With a
 cooling rate immediately after hot rolling of over 120.degree. C./second,
 the hot-rolled steel sheet suffered from a seriously defective shape. The
 product contained bismuth within a range of 0.0140 wt %. The average value
 .theta. of shift angles between the [001] grain axis and the rolling
 direction of grains in the portion of the product coil (excluding 200 mm
 from both width ends) was within a range of from 2.4 to 3.5.degree..
 (Experiment 3)
 The relationship between the amount of added bismuth and the magnetic
 property of the product was investigated. The experiment was carried out
 under the same conditions as in Experiment 1 except that the amount of
 added bismuth was varied within a range of from 0 to 0.068 wt %, with a
 P.sub.H2O /P.sub.H2 ratio of 0.35 for the heating step and a P.sub.H2O
 /P.sub.H2 ratio of 0.55 for the soaking step of decarburization annealing.
 FIG. 3 illustrates the effect of the amount of added bismuth on magnetic
 flux density B.sub.8. It is revealed from FIG. 3 that the improvement of
 magnetic flux density was remarkable when the amount of added bismuth was
 from 0.001 to 0.07 wt %. The product contained from 0.0002 to 0.0505 wt %
 bismuth. The average value .theta. of the shift angle between the [001]
 grain axis and the rolling direction of the grains (in the portion of the
 product coil excluding 200 mm from both width ends) was within a range of
 from 1.5 to 3.9.degree..
 (Experiment 4)
 Steel ingots mainly comprising 0.06 wt % carbon, 3.2 wt % silicon, 0.07 wt
 % manganese, 0.02 wt % selenium, 0.005 wt % sulfur, 0.022 wt % aluminum
 and 0.0085 wt % nitrogen and containing 0 wt % or 0.035 wt % bismuth,
 respectively, were heated to 1,400.degree. C., held for 30 minutes, and
 then hot-rolled into hot rolled sheets having a thickness of 2.4 mm. The
 average cooling rate of the hot-rolled sheets, during the five seconds
 immediately following the end of hot rolling, was 70.degree. C./sec. Then,
 hot-rolled sheet annealing was applied to the resultant hot-rolled steel
 sheets at 1,000.degree. C. for 30 seconds, and after pickling, the sheets
 were subjected to primary cold rolling into cold-rolled steel sheets
 having a thickness of 1.8 mm. Then, an intermediate annealing was applied
 to the cold-rolled steel sheets at 1,1000.degree. C. for one minute, and
 after pickling, the sheets were rolled to a final thickness of 0.23 mm
 through secondary cold rolling. Then, the cold-rolled steel sheets were
 decarburization-annealed under conditions including a soaking temperature
 of 850.degree. C., a soaking period of 100 seconds and a P.sub.H2O
 /P.sub.H2 of 0.60.
 Subsequently, after coating an annealing separator mainly comprising MgO in
 a slurry form in various amounts of coating, finishing annealing was
 applied at a maximum temperature of 1,2000.degree. C. for five hours. For
 the annealing separator, the amount of MgO hydration was altered within a
 range of from 0.5 to 5.0 wt %, and TiO.sub.2 was added in an amount of 10
 weight parts relative to 100 weight parts of MgO (excluding the weight of
 hydration water). The amount of coating was altered within a range of from
 2 to 12 g/m.sup.2 per single side of the steel sheet. The amount of MgO
 hydration was determined by causing hydration by mixing in suspension MgO
 in pure water at 20.degree. C. for an hour, measuring the weight after
 drying at 300.degree. C. for a minute (W1) and the weight after drying at
 1,000.degree. C. for 60 minutes (W2), and performing calculation with use
 of the following formula:
EQU Amount of hydration=(W1-W2)/W1.times.100(%)
 Eight Epstein test pieces (30 mm width and 280 mm length) were sampled in
 parallel with the rolling direction from the final finishing-annealed
 steel sheet to measure magnetic flux density B.sub.8 by the Epstein test
 method.
 The amount of oxygen .sigma. (g/m.sup.2) per single side of the surface of
 the final finishing-annealed steel sheet was also measured. The value of
 .sigma. was determined by subtracting the amount of oxygen derived from a
 chemical analysis of the substrate alone after removal of a surface film
 from the amount of oxygen derived from a chemical analysis of the final
 finishing-annealed sheet with the surface film adhering thereto, and
 connecting the resultant value into an amount of deposited oxygen per
 single side of the steel sheet.
 FIG. 4 illustrates the effects of the amount of MgO hydration and the
 amount of coated separator on magnetic flux density B.sub.8. FIG. 4
 indicates that a magnetic flux density B.sub.8 of at least 1.96 T is
 achievable by appropriately controlling the amount of coated annealing
 separator and the amount of MgO hydration. The hatched portion in FIG. 4
 represents a range of stable availability of magnetic flux density
 B.sub.8. On the assumption that X represents the amount of MgO hydration
 (wt %) and Y represents the amount of coated separator per single side of
 the steel sheet after coating and drying (g/m.sup.2), the upper limit was
 expressed by the following formula (1):
EQU Y.ltoreq.-3X+15 . . . (1)
 FIG. 5 illustrates the effects of the amount of oxygen on the surface of
 the final finishing-annealed steel sheet and the addition of bismuth on
 magnetic flux density B.sub.8. FIG. 5 reveals that magnetic flux density
 B.sub.8 is regulated by .sigma. in a steel ingot containing added bismuth,
 wherein controlling .sigma. to equal to or less than 1.5 g/m.sup.2 is
 important for obtaining stably a high magnetic flux density B.sub.8. In a
 steel ingot without added bismuth, on the other hand, magnetic flux
 density B.sub.8 was high within a range of .sigma. from 1.5 to 2.5
 g/m.sup.2, and deterioration of B.sub.8 magnetivity outside this range was
 slow.
 Therefore, in order to stably obtain a satisfactory magnetic property in a
 steel containing added bismuth, it is important to control the amount of
 coated annealing separator and the amount of MgO hydration within the
 ranges shown in FIG. 4, or to limit the amount of oxygen .sigma. on the
 surface of the final finishing-annealed steel sheet to up to 1.5
 g/m.sup.2, as indicated in FIG. 5.
 (Experiment 5)
 The effects of the ratio P.sub.H2O /P.sub.H2 in decarburization annealing,
 the average cooling rate of the hot-rolled steel sheet during the five
 seconds measured immediately after the end of hot rolling, and the amount
 of oxygen a on the surface of the final finishing-annealed steel sheet on
 the magnetic property were investigated. The experiment was carried out
 under the same conditions as in Experiment 4 except that bismuth was added
 in an amount of 0.035 wt %; the value of P.sub.H2O /P.sub.H2 in
 decarburization annealing was varied; the average cooling rate of the
 hot-rolled steel sheet during five seconds immediately after the end of
 hot rolling was controlled at two levels of 20.degree. C./sec and
 50.degree. C./sec; TiO.sub.2 was added in an amount of 10 weight parts
 relative to 100 weight parts of MgO in the separator; and the amount of
 oxygen a on the surface of the final finishing-annealed steel sheet was
 adjusted to two levels of 1.0 g/m.sup.2 or 1.8 g/m.sup.2. FIG. 6
 illustrates the effects of the ratio P.sub.H2O /P.sub.H2 in the soaking
 step of decarburization annealing, the amount of oxygen on the surface of
 the finishing-annealed steel sheet, and the cooling rate immediately after
 hot rolling on magnetic flux density B.sub.8. According to FIG. 6, with
 .sigma.=1.0 g/m.sup.2 and an average cooling rate immediately after hot
 rolling of 50.degree. C./second, a very high magnetic flux density B.sub.8
 was stably achieved within a range of P.sub.H2O /P.sub.H2 of from 0.45 to
 0.70. With .sigma.=1.8 g/m.sup.2 or an average cooling rate immediately
 after hot rolling of 20.degree. C./second, in contrast, a sufficient
 property was unavailable even within a range of P.sub.H2O /P.sub.H2 of
 from 0.45 to 0.70. It is therefore possible to stably obtain a product
 having a high magnetic flux density by controlling the average cooling
 rate immediately after hot rolling, the atmosphere for decarburization
 annealing, and the amount of oxygen on the surface of the final
 finishing-annealed steel sheet satisfying prescribed conditions.
 (Experiment 6)
 An experiment was carried out to study constituents of the annealing
 separator. The experiment was conducted under the same conditions as in
 Experiment 4 except that bismuth was added in an amount of 0.035 wt %,
 with an amount of coated annealing separator of 6.5 g/m.sup.2 per single
 side, and an amount of hydration of 2.5 wt %. FIG. 7 illustrates the
 effect of the amount of added TiO.sub.2 in the annealing separator on
 magnetic flux density B.sub.8. As is clear from FIG. 7, a high magnetic
 flux density B.sub.8 is stably achieved by limiting the amount of added
 TiO.sub.2 to be added to the annealing separator to up to 10 weight parts
 relative to 100 weight parts of MgO. The increase in TiO.sub.2 causes an
 increase in oxygen source in the annealing separator, while limitation of
 the amount of added TiO.sub.2 causes a decrease in .sigma., thus
 permitting improvement of the degree of integration of secondary
 recrystallization grain orientations.
 (Experiment 7)
 Trace additive elements effective for stably obtaining an excellent
 magnetic property were studied. The experiment was carried out under the
 same conditions as in Experiment 4 except that 0.1 wt % tin, 0.1 wt %
 nickel, 0.1 wt % chromium and 0.1 wt % germanium were individually added
 to a steel ingot containing 0.06 wt % carbon, 3.3 wt % silicon, 0.07 wt %
 manganese, 0.02 wt % selenium, 0.03 wt % soluble aluminum, 0.0090 wt %
 nitrogen and 0.030 wt % bismuth. FIG. 8 illustrates the relationship
 between .sigma. and magnetic flux density B.sub.8 when adding tin, nickel,
 chromium and germanium. FIG. 8 reveals stable creation of a product having
 a higher magnetic flux density by adding tin, nickel, chromium and
 germanium in addition to the basic constituents. According to FIG. 8, as
 in FIG. 5, an increase in .sigma. causes a rapid deterioration of magnetic
 flux density B.sub.8. When tin, nickel, chromium and germanium are added
 as constituents of steel, a satisfactory magnetic property was typically
 represented by a magnetic flux density B.sub.8 of over 1.95 T even when
 .sigma. was over 1.5 g/m.sup.2. With .sigma..ltoreq.1.5 g/m.sup.2, there
 is created an excellent magnetic property of magnetic flux density
 B.sub.8.gtoreq.1.97 T.
 Achieving a higher magnetic flux density stably obtained by the addition of
 tin, nickel, chromium and germanium is considered to be due to the fact
 that these elements display an inhibitor effect in a solid-solution state
 in steel and have a function of intensifying the effect of inhibiting
 grain growth of bismuth concentrated on grain boundaries. Another
 probability is that concentration on the steel sheet surface layer
 inhibits dissipation of bismuth from the surface. Under these effects, a
 higher magnetic flux density can be achieved in a bismuth-containing
 material, and a satisfactory magnetic property can be reached even when
 .sigma. is over 1.5 g/m.sup.2.
 (Experiment 8)
 The effect of the atmospheres for the soaking step and the heating step of
 decarburization annealing was investigated. An experiment was carried out
 under the same conditions as in Experiment 1 except that the steel sheet
 was cooled at a cooling rate of 60.degree. C./sec during a period (five
 seconds) immediately after the end of hot rolling; the value of P.sub.H2O
 /P.sub.H2 in the soaking step of decarburization annealing was altered
 within a range of from 0.35 to 0.80; the atmosphere for the heating step
 of decarburization annealing was controlled separately from the soaking
 step; and the value of P.sub.H2O /P.sub.H2 was varied within a range of
 from 0.20 to 0.75. The heating step of decarburization annealing was
 measured in an in-furnace area corresponding to a range of sheet
 temperature of from 255 to 765.degree. C., and an average P.sub.H2O
 /P.sub.H2 value in this area was used as the value of P.sub.H2O /P.sub.H2
 for the heating step.
 FIG. 9 illustrates the relationship between P.sub.H2O /P.sub.H2 and
 magnetic flux density B.sub.8 for the heating step for cases with a
 P.sub.H2O /P.sub.H2 of 0.40, 0.50 and 0.60 for the soaking step. As in
 Experiment 1, a high magnetic flux density is obtained in cases with a
 P.sub.H2O /P.sub.H2 for the soaking step of 0.5 and 0.6. The value of
 B.sub.8 was further improved by using a lower P.sub.H2O /P.sub.H2 in the
 heating step than in the soaking step.
 FIG. 10 illustrates the effects of P.sub.H2O /P.sub.H2 in the heating and
 soaking steps on magnetic flux density B.sub.8 after finishing annealing.
 FIG. 10 reveals that a satisfactory magnetic flux density B.sub.8 is
 available by using a value of P.sub.H2O /P.sub.H2 for the heating step of
 decarburization annealing lower by 0.05 to 0.25 than that for the soaking
 step. The hatched portion in FIG. 10 represents a range within which a
 very high magnetic flux density of a magnetic flux density B.sub.8 of over
 1.97 T is available, and is expressed by the following formula (2) on the
 definition of X1 representing the ratio P.sub.H2O /P.sub.H2 in the
 atmosphere in the heating step and X2 representing the ratio P.sub.H2O
 /P.sub.H2 in the atmosphere in the soaking step:
EQU X2-0.25.ltoreq.X1.ltoreq.X2-0.05 (2)
 It is clear from this experiment that a more excellent magnetic flux
 density can be created by controlling the value of the ratio P.sub.H2O
 /P.sub.H2 for the heating step of decarburization annealing within a
 certain range lower than P.sub.H2O /P.sub.H2 for the soaking step.
 (Experiment 9)
 The relationship between the soaking temperature of decarburization
 annealing and the magnetic property of the product was investigated. An
 experiment was carried out under the same conditions as in Experiment 1
 except that the soaking temperature of decarburization annealing was
 varied within a range of from 750 to 950.degree. C., and cooling was
 performed at an average cooling rate of 60.degree. C./sec immediately
 after the end of hot rolling (five seconds), with a P.sub.H2O /P.sub.H2 of
 0.40 for the heating step and a P.sub.H2O /P.sub.H2 of 0.60 for the
 soaking step of decarburization annealing. The result is shown in FIG. 11.
 A high and stable magnetic flux density was obtained by controlling the
 soaking temperature of decarburization annealing within a range of from
 800 to 900.degree. C.
 (Experiment 10)
 The effects of temperature and atmosphere in the latter half of the soaking
 step of decarburization annealing were investigated. An experiment was
 carried out under the same conditions as in Experiment 1 except that, with
 a cooling rate immediately after hot rolling of 60.degree. C./sec, a
 soaking temperature of decarburization annealing of 850.degree. C., a
 P.sub.H2O /P.sub.H2 for the soaking step of 0.60 or 0.30, and a P.sub.H2O
 /P.sub.H2 for the latter half (corresponding to 20 seconds of soaking step
 immediately before temperature decrease) of 0.05 or the same value as for
 the soaking step, the latter half temperature was varied within a range of
 from 770 to 970.degree. C. FIG. 12 illustrates the relationship between
 the latter half temperature of the soaking step of decarburization
 annealing and the value of B.sub.8. Improvement of magnetic flux density
 B.sub.8 was achieved by controlling the latter half temperature of the
 soaking step of decarburization annealing within a range of from 820 to
 920.degree. C. and the value of P.sub.H2O /P.sub.H2 of 0.05, as compared
 with the case with no change in the latter half of the soaking step of
 decarburization annealing. With a P.sub.H2O /P.sub.H2 for the soaking step
 of decarburization annealing of about 0.30, however, the magnetic flux
 density B.sub.8 is at a low level irrespective of a change in the latter
 half of the soaking step of decarburization annealing. More specifically,
 an improvement of magnetic flux density can be achieved with control of
 the heating step atmosphere on the low oxidizing side, by using a
 P.sub.H2O /P.sub.H2 ratio for the soaking step of decarburization
 annealing within a range of from 0.45 to 0.70 and providing a reducing
 atmosphere zone in the latter half of the soaking step of decarburization
 annealing.
 It was concluded from the results as described above that a very excellent
 magnetic property could be achieved by controlling, in a bismuth-added
 steel, 1) the cooling rate immediately after the end of hot rolling, 2)
 atmosphere and temperature of decarburization annealing, and 3) the
 amount-of coated annealing separator, the amount of MgO hydration and the
 amount of added TiO.sub.2.
 The reasons of limiting the chemical compositions of the materials within
 the aforementioned ranges in the present invention will now be described.
 (C: about 0.03 to 0.10 wt %)
 Carbon is a constituent useful for improving the hot-rolled texture by
 phase transformation of iron. It is useful also for generating grains
 having Goss orientation. In order to cause carbon to effectively display
 these functions, it is necessary for the material to contain carbon in an
 amount of at least about 0.03 wt %. With a carbon content of over about
 0.10 wt %, however, defective decarburization is caused even by
 decarburization annealing, and normal secondary recrystallization is
 prevented. The carbon content should therefore be limited within a range
 of from about 0.03 to about 0.10 wt %.
 (Si: about 2.0 to 5.0 wt %)
 Silicon causes an increase in electric resistance and reduces the iron
 loss. This is a constituent necessary for making it possible to stabilize
 the body-centered cubic lattice structure of the iron and to apply a
 high-temperature heat treatment. In order to obtain these effects, it is
 necessary for a material to contain silicon in an amount of at least about
 2.0 wt %. However, a content of over about 5.0 wt % makes it difficult to
 perform cold rolling. The silicon content should therefore be limited
 within a range of from about 2.0 to 5.0 wt %.
 (Mn: about 0.04 to 0.15 wt %)
 Manganese effectively contributes to improvement of hot brittleness of
 steel. Further, when sulfur or selenium is mixed, manganese forms
 precipitates such as MnS or MnSe. These precipitates serve as inhibitors.
 A manganese content of under about 0.04 wt % has insufficient function as
 inhibitor. With a manganese content of over about 0.15 wt %, on the other
 hand, precipitates such as MnSe become coarse and lose their effect as
 inhibitors. The manganese content should therefore be limited within a
 range of from about 0.04 to 0.15 wt %.
 (S and/or Se: about 0.01 to 0.03 wt %)
 Sulfur and selenium are useful constituents serving as inhibitors as a
 second dispersed phase in steel through formation of MnSe, MnS, Cu.sub.2-x
 Se or Cu.sub.2-x S in combination with manganese or copper. A total
 content of sulfur and selenium of under about 0.01 wt % gives only a
 limited effect of addition. With a total content of over about 0.04 wt %,
 on the other hand, a solid solution is incomplete by slab heating, and
 also causes a defective product surface. The content of sulfur and/or
 selenium should therefore be limited within a range of from about 0.01 to
 0.03 wt %.
 (soluble Al: about 0.015 to 0.035 wt %)
 Aluminum is a useful constituent functioning as an inhibitor through
 formation of AlN acting as a second dispersed phase. An amount of added
 aluminum of under about 0.015 wt % cannot ensure a sufficient amount of
 precipitation. When the amount of addition is over about 0.035 wt %, on
 the other hand, AlN is precipitated in a coarse form and loses its
 function as an inhibitor. The soluble aluminum content should therefore be
 limited within a range of from about 0.015 to 0.035 wt %.
 (N: about 0.0050 to 0.010 wt %)
 Nitrogen is also a constituent necessary for forming AlN just as aluminum.
 With an amount of added nitrogen of under about 0.0050 wt %, precipitation
 of AlN is insufficient. Addition of nitrogen in an amount of over about
 0.010 wt % causes swelling on the surface during slab heating. The
 nitrogen content should therefore be limited within a range of from about
 0.0050 to 0.010 wt %.
 (Bi: about 0.001 to 0.070 wt %)
 Bismuth is found to be preferentially concentrated on grain boundaries of
 primary recrystallization grains. It reduces mobility of grain boundaries
 during annealing. As a result, addition of bismuth causes an increase in
 secondary recrystallization temperature, thus providing secondary
 recrystallization grains integrated in the Goss orientation and improving
 the magnetic flux density. These functions are similar to those of
 antimony and arsenic. Bismuth is advantageous in that its solubility in
 iron is particularly low, and its melting point is as low as about
 271.degree. C. This is considered to result in a superior function of
 segregating on grain boundaries, as compared with antimony and arsenic.
 This is considered to lead to a remarkable effect of imparting a normal
 grain growth inhibiting ability, and to effectively act for improvement of
 orientational integration.
 Bismuth, having a grain boundary segregating type inhibiting function
 intensifying constituent as antimony and the like, is considered to have a
 function of uniformly improving the magnetic property of a grain oriented
 electromagnetic steel sheet using inhibitors such as MnSe, MnS or
 AlN+(MnSe, MnS).
 With a bismuth content of under about 0.001 wt %, the aforementioned normal
 grain growth inhibiting effect based on grain boundary segregation cannot
 fully be realized. Because of a very low solubility in iron, it is
 difficult successfully to add bismuth in an amount of over about 0.07 wt
 %. The amount of added bismuth should therefore be limited within a range
 of from about 0.001 to 0.07 wt %. (Sn: about 0.02 to 0.5 wt %, Ni: about
 0.05 to 0.5 wt %, Cr: about 0.05 to 0.5 wt %, Ge: about 0.001 to 0.1 wt %)
 In addition to the above-mentioned basic constituents, a high magnetic flux
 density B.sub.8 can be stably obtained by adding one or more materials
 selected from the group consisting of from about 0.02 to 0.5 wt % tin,
 from about 0.05 to 0.5 wt % nickel, from about 0.05 to 0.5 wt % chromium
 and from about 0.001 to 0.1 wt % germanium to steel. Presence of these
 solid-solution type inhibitor elements is considered to intensify the
 normal grain growth inhibiting effect of bismuth. This effect is fully
 displayed only when deterioration of the inhibitor effect of bismuth is
 prevented by satisfying all the requirements set forth in the invention
 including the amount of coated annealing separator, the amount of MgO
 hydration, the decarburization annealing atmosphere and the hot rolling
 conditions. When the amounts of addition of these elements are under the
 above-mentioned ranges, the effect of intensifying the inhibiting function
 of bismuth is not realized. When the amounts of addition are above these
 ranges, on the other hand, the effect is saturated, and disadvantages are
 encountered such as a decrease in the saturated magnetic flux density and
 deterioration of surface quality. These elements should therefore
 preferably be added in amounts within the aforementioned ranges.
 In addition, individual or composite addition of antimony, arsenic,
 molybdenum, copper, phosphorus, boron, tellurium, vanadium or niobium for
 reinforcing the inhibiting power is effective for further improving the
 magnetic property.
 Antimony and arsenic have a function of improving the inhibiting power by
 segregating on grain boundaries as in the case of bismuth. These elements
 should preferably be added in an amount within a range of from about 0.001
 to 0.10 wt %.
 Molybdenum has a function of making acute the nuclei of secondary
 recrystallization grains in Goss orientation. The effect is particularly
 remarkable within a range of from about 0.001 to 0.20 wt %.
 Copper is, as manganese, an element forming precipitates in combination
 with selenium or sulfur and thus improving the inhibiting power. The
 effect is remarkable within a range of from about 0.01 to 0.30 wt %.
 Phosphorus is, as antimony, a constituent improving the inhibiting power by
 segregating on grain boundaries. A content of under about 0.010 wt % gives
 only an insufficient effect. A content of over about 0.030 wt % leads to
 instable magnetic property and surface quality. The phosphorus content
 should therefore be within a range of from about 0.010 to 0.030 wt %.
 Boron, tellurium, vanadium and niobium have a function of further
 increasing the normal grain growth inhibiting power by forming
 precipitates such as BN, MnTe, Vn, NbN and NbC in steel. Boron should
 preferably be added within a range of from about 0.0010 to 0.010 wt %, and
 vanadium, niobium and tellurium, within a range of from about 0.005 to
 0.10 wt %, respectively.
 The main manufacturing steps of the present invention will now be
 described.
 First, regarding the hot rolling conditions, the cooling rate after hot
 rolling is an important factor. An insufficient cooling rate after hot
 rolling makes it impossible for bismuth and AlN in the hot-rolled sheet to
 be uniformly dispersed, and this results in deterioration of the
 inhibiting power of the material which becomes non-uniform at different
 portions. This is considered to cause an insufficient and non-uniform
 secondary recrystallization, thus causing an unstable magnetic property.
 According to the results of experiments, the average cooling rate
 immediately after the end of hot rolling (for five seconds) should be at
 least about 30.degree. C./sec. On the other hand, a cooling rate of over
 about 120.degree. C./sec tends to cause a defective shape of the strip.
 The upper limit should therefore be about 120.degree. C./sec.
 For the decarburization annealing conditions, various factors are
 important. In the case of bismuth enhanced silicon steel, the result of
 our studies reveals that deterioration of the inhibitor in the surface
 region of the sheet during the final finishing annealing tends to cause
 deterioration of the magnetic property. As shown in FIG. 6, magnetic flux
 density B.sub.8 becomes stable at a high level by keeping a high P.sub.H2O
 /P.sub.H2 in the soaking step of decarburization annealing to some extent.
 This is attributable to sufficient formation of an oxide film (SiO.sub.2,
 Fe.sub.2 SiO.sub.4) on the surface of the decarburization-annealed steel
 sheet which inhibits oxidation of the inhibitor (AN, bismuth) on the
 surface layer, thereby permitting stable secondary recrystallization. A
 P.sub.H2O /P.sub.H2 becoming too high leads again to a decrease in
 magnetic flux density. This is considered to be due to the fact that
 excessive surface oxidation of the decarburization-annealed sheet causes a
 decrease in uniformity of the surface oxide layer, leading to a decrease
 in protectivity for the atmosphere. From the point of view of preventing
 deterioration of the inhibitor during the final finishing annealing and
 ensuring uniformity of the surface oxide layer of the decarburization
 annealed sheet, therefore, the value of P.sub.H2O /P.sub.H2 for the
 soaking step of decarburization annealing should be limited within a range
 of from 0.45 to 0.70 (FIG. 6).
 In order to stably obtain a satisfactory magnetic property with a
 bismuth-added material, however, the two aforementioned manufacturing
 conditions alone would be insufficient, and it is necessary to incorporate
 a treatment for inhibiting decomposition of the surface layer inhibitor
 during the final finishing annealing.
 The amount of oxygen on the surface of the final finishing-annealed sheet
 is one of the indicators showing the extent of decomposition of the
 surface layer inhibitor during the final finishing annealing. The
 appropriate range of the amount of oxygen on the surface of the final
 finishing-annealed sheet will therefore be described.
 The magnetic property of a bismuth-added material is considered susceptible
 to the effect of decomposition of the inhibitor during the final finishing
 annealing. In order to prevent this, only ensuring oxidizing property of
 the decarburization annealing atmosphere is not sufficient for a
 bismuth-added material, although it is effective for materials to which
 bismuth was not added. In the case of bismuth-added material, formation of
 the forsterite film during final finishing annealing exerts a remarkable
 effect on secondary recrystallization. For the purpose of inhibiting
 decomposition of the surface layer inhibitor, the amount of surface oxygen
 a per single side of the final finishing-annealed sheet should preferably
 be up to about 1.5 g/m.sup.2.
 When the inhibitor effect of bismuth is reinforced by adding tin, nickel,
 chromium or germanium into steel, a satisfactory magnetic property is
 achievable even with an amount of surface oxygen a of the final
 finishing-annealed sheet of over about 1.5 g/m.sup.2.
 In order to reduce the amount of surface oxidation a of the final
 finishing-annealed sheet, it is also effective to use an annealing
 separator comprising Al.sub.2 O.sub.3, SiO.sub.2, CaO, Sb.sub.2 O.sub.3 or
 a metal chloride individually or compositely mixed with MgO for
 stabilization of the magnetic property.
 For inhibiting decomposition of the surface layer inhibitor during the
 final finishing annealing, there are available methods of controlling the
 decarburization annealing atmosphere or the annealing separator.
 First, the method of controlling the decarburization annealing atmosphere
 will be described.
 The magnetic flux density is improved by applying a lower ratio P.sub.H2O
 /P.sub.H2 for the heating step than that for the soaking step in
 decarburization annealing, and further, applying a value lower by a
 certain value than the P.sub.H2O /P.sub.H2 ratio for the soaking step.
 This is attributable to the improved uniformity of subscale on the
 decarburization-annealed sheet and to the promoted effect of inhibiting
 bismuth oxidation in the surface layer as described above. With a view to
 obtaining this effect, the value of P.sub.H2O /P.sub.H2 for the heating
 step should preferably be lower than that for the soaking step. More
 preferably, assuming that P.sub.H2O /P.sub.H2 in the atmosphere for the
 heating step is represented by X1, and that in the atmosphere for the
 soaking step, by X2, it is desirable to perform control with a range
 satisfying X2-0.25.ltoreq.X1.ltoreq.X2-0.05. The value of P.sub.H2O
 /P.sub.H2 in the atmosphere for the heating step can be evaluated, for
 example, by averaging values of P.sub.H2O /P.sub.H2 within a region
 corresponding to a temperature region of about 30 to 90% of the soaking
 temperature (unit:centigrade). Improvement of magnetic flux density
 B.sub.8 is available by using a temperature for the latter half of the
 soaking step of decarburization annealing within a range of from about 820
 to 920.degree. C. and a reducing atmosphere having a P.sub.H2O /P.sub.H2
 ratio of up to about 0.15. This is considered to be due to the improvement
 of subscale density of the decarburization-annealed sheet brought about by
 the reduction of the oxide layer of the surface of the
 decarburization-annealed sheet. It is therefore desirable to use a
 temperature for the latter half of the soaking step of decarburization
 annealing within a range of from about 820 to 920.degree. C. and the
 P.sub.H2O /P.sub.H2 ratio of the atmosphere of up to about 0.15. A period
 of time shorter than five seconds for this treatment leads to insufficient
 reduction of the surface of the decarburization-annealed sheet. With a
 period of over about 200 seconds, it is difficult to ensure a sufficient
 period of time for the treatment in an oxidizing atmosphere. The treatment
 time should therefore preferably be within a range of from about 5 to 200
 seconds.
 It is also desirable to employ a reducing atmosphere for the latter half of
 the soaking step of decarburization annealing, and a lower P.sub.H2O
 /P.sub.H2 in the atmosphere for the heating step than that in the soaking
 step except for the latter half, most preferably lower by about 0.05 to
 0.25. A synergistic effect of the subscale uniformity and the reducing
 treatment of the subscale surface brought about by the optimization of the
 heating step further densities the subscale and have a function of
 bringing secondary recrystallization closer to the ideal state.
 The method of controlling the annealing separator will now be described.
 In order to improve the magnetic property by reducing the amount of surface
 oxygen of the final finishing-annealed sheet of a bismuth-added material,
 it is effective to reduce the amount of water introduced between layers of
 the final finishing-annealed coil through adjustment of the amount of
 coated annealing separator and the amount of MgO hydration. That is, by
 assuming that the amount of MgO hydration is represented by X (wt %), and
 the amount of coated separator per single side of steel sheet after
 coating and drying, by Y (g/m.sup.2), the formula: Y.ltoreq.-3X+15 should
 preferably be satisfied.
 It is known that addition of an appropriate amount of TiO.sub.2 into the
 annealing separator accelerates film formation during final finishing
 annealing, thereby permitting achievement of a satisfactory appearance of
 product. Usually, TiO.sub.2 is added in an amount within a range of from
 about 10 to 15 wt % relative to 100 weight parts of MgO. While TiO.sub.2
 contributes to film formation as an oxygen source in the annealing
 separator, and excessive film formation with the bismuth-added material
 tends to cause decomposition of the surface layer inhibitor and
 deterioration of the magnetic property. It is therefore desirable, as
 shown in FIG. 7, to limit the amount of TiO.sub.2 added into the annealing
 separator to up to about 10 weight parts relative to about 100 weight
 parts of MgO. Adding a compound of strontium, antimony, boron, zirconium,
 niobium or chromium which are known assistants to the annealing separator
 is effective for improving the film properties.
 The soaking temperature of decarburization annealing is considered to exert
 an effect of decarburization property and primary recrystallized grain
 size of the decarburization-annealed sheet. Applying a soaking temperature
 of decarburization annealing within a range of from 800 to 900.degree. C.
 is considered to lead to sufficient removal of carbon in steel, enabling
 the primary recrystallized grain size of the decarburization-annealed
 sheet to take a value appropriate for secondary recrystallization. As a
 result, it is relatively easy to obtain a high and stable magnetic flux
 density. With a soaking temperature of decarburization annealing of
 outside the aforementioned range, more carbon remains in the steel, and
 the primary grain size becomes too small or too large: an ideal secondary
 recrystallization texture is unavailable and the magnetic property of the
 product tends to deteriorate. For these reasons, the soaking temperature
 during decarburization annealing should preferably be limited within a
 range of from about 800 to 900.degree. C.
 Even when hot-rolled sheet annealing or intermediate annealing is omitted,
 the effects of the aforementioned manufacturing conditions sufficiently
 serve to improve the magnetic property. There is therefore imposed no
 particular limitation on the presence of hot-rolled sheet annealing or
 intermediate annealing. The present invention is therefore applicable to
 any process of hot-rolled sheet annealing and then achieving a final
 thickness through two or more runs of cold rolling including an
 intermediate annealing, a process of achieving a final thickness through
 two or more runs of cold rolling including an intermediate annealing
 without applying hot-rolled sheet annealing, and a process conducting
 hot-rolled annealing and then achieving a final thickness through a single
 run of cold rolling.
 Applying magnetic domain refining to a grain oriented electromagnetic steel
 sheet based on the above-mentioned manufacturing conditions is very
 important for reducing the iron loss, and magnetic domain refining is
 effectively applicable in the invention. Applicable methods for magnetic
 domain refining include a method of introducing linear strain by means of
 a laser beam, as disclosed in Japanese Examined Patent Publication No.
 57-2252, or by means of a plasma flame as disclosed in Japanese Unexamined
 Patent Publication No. 62-96617, and the introduction of a linear notch in
 a direction substantially perpendicular to the rolling direction prior to
 final finishing annealing as disclosed in Japanese Examined Patent
 Publication No. 3-69968. It is also possible to obtain a material having a
 very low iron loss by mirror-surface-treating the surface of a final
 finishing-annealed sheet obtained by the method of the present invention
 and then artificially forming a tensile coating, or by combining a
 magnetic domain refining.
 In the final product, the contents of carbon, sulfur, selenium, nitrogen
 and aluminum are considerably reduced from the contents thereof in the
 slab under the effect of decarburization annealing and the purifying
 treatment in final finishing annealing. The minimum C content in the
 product is about 2 ppm in the usual industrial process. The manganese and
 bismuth contents also decrease during finishing annealing, but remain to
 some degree in the product. The silicon content shows almost no change
 from that in the slab. The product therefore comprises up to about 0.0040
 wt % carbon, from about 2.0 to 5.0 wt % silicon, from about 0.02 to 0.15
 wt % manganese, up to about 0.0025 wt % sulfur and/or selenium, up to
 about 0.0015 wt % aluminum, up to about 25 wtppm nitrogen, and from about
 0.0002 to 0.0600 wt % bismuth. Further, According to the manufacturing
 method of the invention, the average value .theta. of the shift angle
 between the [001] grain axis and the rolling direction in the portion of
 the product coil except for 200 mm from both width ends of the product
 coil, is about 5.degree. or less.
 EXAMPLES
 Example 1
 A silicon steel slab comprising 0.060 wt % carbon, 3.30 wt % silicon, 0.070
 wt % manganese, 0.020 wt % aluminum, 0.0075 wt % nitrogen, 0.0040 wt %
 antimony, 0.020 wt % selenium, 0.020 wt % molybdenum and 0.001 wt %
 sulfur, and containing bismuth in an amount of 0 wt %, 0.001 wt %, 0.030
 wt %, or 0.060 wt %, and the balance substantially iron was heated by
 induction heating to 1,400.degree. C. for 60 minutes, and then hot rolled
 to a hot-rolled thickness of 2.5 mm. Cooling was applied at cooling rate
 of 50.degree. C./sec during five seconds immediately after the end of the
 final pass of hot rolling. Then, the hot-rolled sheet was subjected to
 hot-rolled sheet annealing at 950.degree. C. for one minute, pickling, and
 primary cold rolling into a cold-rolled sheet having a thickness of 1.6
 mm. Subsequently, the cold-rolled sheet was subjected to intermediate
 annealing at 1,050.degree. C. for one minute, pickling, and then secondary
 cold rolling into a cold-rolled sheet having a final thickness of 0.23 mm.
 The cold-rolled sheet was then subjected to decarburization annealing at
 850.degree. C. for 100 seconds with two levels of P.sub.H2O /P.sub.H2 in
 the soaking step of 0.40 and 0.55. Then, an annealing separator prepared
 by adding 10 wt % TiO.sub.2 to MgO of which the amount of hydration was
 adjusted to 3.0 wt % was coated onto the surface of the
 decarburization-annealed sheet in amounts of two levels including 4.0
 g/m.sup.2 and 8.0 g/m.sup.2. Subsequently, final finishing annealing was
 applied to the decarburization-annealed sheet at a maximum temperature of
 1,200.degree. C. for five hours. The amount of surface oxygen .sigma. of
 the resultant finishing-annealed sheet was measured. Then, an insulating
 tensile coating mainly comprising magnesium phosphate containing colloidal
 silica was applied to the final finishing-annealed sheet into a product
 sheet. Linear strain areas were introduced into the product sheet at
 intervals of 7 mm relative to the rolling direction at an angle of 900 to
 the rolling direction by means of a plasma flame.
 Epstein test pieces (280L.times.30W) corresponding to 500 g were cut in
 parallel with the rolling direction from the product obtained as described
 above to measure the magnetic flux density B.sub.8 and the iron loss
 W.sub.17/50 by the Epstein test method. The resultant magnetic property of
 the product is shown in Table 1. In the grain oriented electromagnetic
 steel sheet manufactured under conditions meeting the present invention, a
 product having a very high magnetic flux density magnetic flux density
 B.sub.8 was obtained. The final product of this example contained up to
 0.0035 wt % carbon, 3.24 wt % silicon, 0.055 wt % manganese, 0.0001 wt %
 sulfur, 0.0007 wt % selenium, 0.0010 wt % aluminum and 7 wtppm nitrogen in
 the substrate. The bismuth contents were 0.0004 wt %, 0.0182 wt % and
 0.0394 wt %, respectively, for the amounts of added bismuth of 0.0001 wt
 %, 0.030 wt % and 0.060 wt %. The final product of this example had an
 average value .theta. of shift angle between the [001] grain axis and the
 rolling direction in the portion of the product coil excluding 200 mm from
 the both ends of the product coil within a range of from 2.0 to
 3.1.degree..
 TABLE 1
 Amount of
 Amount surface oxygen
 of P.sub.H20 /P.sub.H2 in Amount of of final
 added decarburization coated finishing-
 Bi annealing separator annealed sheet B.sub.8 W.sub.17/50
 Symbol (wt %) atmosphere (g/m.sup.2) (g/m.sup.2 per side) (T) (W/kg)
 Remarks
 1A 0 0.040 4 1.08 1.905 0.871
 Comparative
 example
 1B 0 0.040 8 2.15 1.940 0.762
 Comparative
 example
 1C 0 0.055 4 1.12 1.910 0.865
 Comparative
 example
 1D 0 0.055 8 2.26 1.935 0.776
 Comparative
 example
 1E 0.001 0.040 4 1.10 1.925 0.789
 Comparative
 example
 1F 0.001 0.040 8 2.18 1.911 0.866
 Comparative
 example
 1G 0.001 0.055 4 1.15 1.970 0.662
 Example of the

Invention
 1P 0.060 0.055 8 2.29 1.952 0.722
 Comparative
 example
 Example 2
 A silicon steel slab comprising 0.065 wt % carbon, 3.40 wt % silicon, 0.065
 wt % manganese, 0.05 wt % copper, 0.022 wt % aluminum, 0.0082 wt %
 nitrogen, 0.02 wt % molybdenum, 0.016 wt % selenium, 0.009 wt % sulfur,
 0.045 wt % bismuth and the balance iron was heated by induction heating to
 1,400.degree. C. for 60 minutes, and then, hot-rolled to a hot-rolled
 sheet having a thickness of 2.5 mm. Four levels of cooling rate of
 20.degree. C./sec, 30.degree. C./sec, 60.degree. C./sec and 100.degree.
 C./sec were provided for five seconds immediately after the end of the
 final pass of hot rolling. Subsequently, hot-rolled sheet annealing was
 applied to the hot-rolled sheet at 950.degree. C. for a minute, and after
 pickling, the sheet was subjected to primary cold rolling into a
 cold-rolled sheet having a thickness of 1.6 mm. Subsequently, the
 cold-rolled sheet was subjected to intermediate annealing at 1,050.degree.
 C. for one minute, pickling, and then secondary cold rolling into a
 cold-rolled sheet having a final thickness of 0.23 mm. The cold-rolled
 sheet was then subjected to decarburization annealing at 850.degree. C.
 for 100 seconds with two levels of P.sub.H2O /P.sub.H2 in the soaking step
 of 0.40 and 0.55. Then, an annealing separator comprising MgO having an
 amount of hydration of 0.8 wt % was coated onto the surface of the
 decarburization-annealed sheet in an amount of 4.0 g/m.sup.2.
 Subsequently, final finishing annealing was applied to the
 decarburization-annealed sheet at a maximum temperature of 1,200.degree.
 C. for five hours. The amount of surface oxygen of the resultant final
 finishing-annealed sheet was measured. Then, after hydrochloric acid
 pickling, the surface of the final finishing-annealed sheet was
 mirror-surface treated through electrolytic polishing in an NaCl bath, and
 then, a tension was imparted to the steel sheet surface by
 vapor-depositing TiN onto the steel sheet surface. Then, an insulating
 coating mainly comprising magnesium phosphate containing colloidal silica
 was applied. Further, linear strain areas were introduced into the product
 sheet at intervals of 5 mm relative to the rolling direction at an angle
 of 85.degree. to the rolling direction by means of a plasma flame. Epstein
 test pieces corresponding to 500 g were cut from the product obtained, to
 measure the magnetic flux density B.sub.8 and the iron loss W.sub.17/50 by
 the Epstein test method. The resultant magnetic property of the product is
 shown in Table 2. In the grain oriented electromagnetic steel sheet
 manufactured under conditions meeting the present invention, a product
 having a very excellent magnetic property was stably obtained. The final
 product of this example contained up to 0.0030 wt % carbon, 3.33 wt %
 silicon, 0.058 wt % manganese, 0.0003 wt % sulfur, 0.0010 wt % selenium,
 0.007 wt % aluminum, 5 wtppm nitrogen and 0.0222 wt % bismuth in the
 substrate. The final product of this example had an average shift angle
 value .theta. within a range of from 1.9 to 2.9.degree..
 TABLE 2
 Amount of
 Average cooling surface oxygen
 rate (.degree. C./s) of final
 immediately after P.sub.H20 /P.sub.H2 during finishing-
 hot rolling (for 5 decarburization annealed sheet B.sub.8
 W.sub.17/50
 Symbol sec) annealing (g/m.sup.2 per side) (T) (W/kg)
 Remarks
 2A 20 0.040 0.61 1.935 0.652
 Comparative
 example
 2B 30 0.040 0.65 1.942 0.642
 Comparative
 example
 2C 60 0.040 0.68 1.945 0.644
 Comparative
 example
 2D 100 0.040 0.64 1.939 0.638
 Comparative
 example
 2E 20 0.050 0.59 1.928 0.667
 Comparative
 example
 2F 30 0.050 0.57 1.975 0.501 Example
 of the

Invention
 Example 3
 A silicon steel slab comprising 0.065 wt % carbon, 3.30 wt % silicon, 0.065
 wt % manganese, 0.05 wt % copper, 0.025 wt % aluminum, 0.0075 wt %
 nitrogen, 0.02 wt % molybdenum, 0.015 wt % selenium, 0.010 wt % sulfur, 0
 wt % or 0.020 wt % bismuth, and the balance iron was heated by induction
 heating at 1,400.degree. C. for 60 minutes, and then hot-rolled into a
 hot-rolled sheet having a thickness of 2.5 mm. The hot-rolled sheet was
 cooled at a cooling rate of 60.degree. C./sec for five seconds immediately
 after the end of the final pass of hot rolling. Then the hot-rolled sheet
 was pickled without hot-rolled sheet annealing, and subjected to primary
 cold rolling into a cold-rolled sheet having a thickness of 1.6 mm.
 Subsequently, the cold-rolled sheet was subjected to intermediate
 annealing at 1,050.degree. C. for one minute, pickled, and cold-rolled by
 secondary cold rolling into a cold-rolled sheet having a final thickness
 of 0.27 mm. Then, grooves each having an angle with the rolling direction
 of 85.degree., a width of 100 .mu.m, and a width of 25 .mu.m at intervals
 of 3.0 mm in the rolling direction were formed on the cold-rolled sheet by
 resist etching, and then, decarburization annealing was applied at
 850.degree. C. for 100 seconds. P.sub.H2O /P.sub.H2 in the soaking step of
 decarburization annealing was 0.43 or 0.65. Then, an annealing separator
 mainly comprising MgO of an amount of hydration of 3.0 wt % and added with
 7 weight parts or 12 weight parts TiO.sub.2 relative to 100 weight parts
 MgO was coated onto the surface of the decarburization-annealed sheet in
 an amount of coating of 4.0 g/m.sup.2 per single side. Then, final
 finishing annealing was applied at a maximum temperature of 1,200.degree.
 C. for five hours, and an insulating coating mainly comprising magnesium
 phospate containing colloidal silica was applied to obtain a product.
 Epstein test pieces corresponding to 500 g were cut from the thus obtained
 product to measure the magnetic flux density B.sub.8 and the iron loss
 W.sub.17/50 by the Epstein test method.
 The magnetic property of the result product is shown in Table 3. In the
 grain oriented electromagnetic steel sheet manufactured under the
 conditions of the present invention, there is stably created a product
 having a very excellent magnetic property.
 The final product of this example of the invention contained up to 0.0020
 wt % carbon, 3.24 wt % silicon, 0.060 wt % manganese, 0.0008 wt % sulfur,
 0.0009 wt % selenium, 0.0010 wt % aluminum, 5 wtppm nitrogen, and 0.0012
 wt % bismuth in the substrate thereof. The final product of this example
 had an average value .theta. of shift angle of 2.2.degree..
 TABLE 3
 Amount of
 surface
 oxygen of
 final
 Amount finishing-
 of Amount of annealed
 added P.sub.H20 /P.sub.H2 during added TiO.sub.2 sheet
 Bi decarburization (relative to (g/m.sup.2 per B.sub.8
 W.sub.17/50
 Symbol (wt %) annealing 100 g MgO in g) side) (T) (W/kg)
 Remarks
 3A 0 0.43 7 0.95 1.884 0.785
 Comparative

Within or

out of