Production method for grain-oriented electrical steel sheet

Abstract

Grain-oriented electrical steel sheets with good magnetic properties are industrially stably produced, by using as the material, a steel slab having a predetermined composition, wherein after cold rolling and before the start of secondary recrystallization annealing, the cold rolled sheet is subjected to nitriding treatment with nitrogen content of 50 mass ppm or more and 1000 mass ppm or less, and a total content of 0.2 mass % to 15 mass % of a sulfide and/or sulfate is contained in an annealing separator, and a staying time in the temperature range of 300 C. to 800 C. in the heating stage of secondary recrystallization annealing of 5 hours or more is secured to precipitate silicon nitride (Si.sub.3N.sub.4) and MnS, and using the silicon nitride in combination with MnS as inhibiting force for normal grain growth to significantly reduce variation of magnetic properties.

Claims

1. A production method for a grain-oriented electrical steel sheet, the method comprising: subjecting a steel slab to hot rolling, without re-heating or after re-heating, to obtain a hot rolled sheet, the steel slab having a composition containing, by mass % or mass ppm, C: 0.08% or less, Si: 2.0% to 4.5% and Mn: 0.5% or less, S: less than 50 ppm, Se: less than 50 ppm, O: less than 50 ppm, sol.Al: 80 ppm or less, N in a range satisfying [sol.Al](14/27) ppmN80 ppm, and the balance being Fe and incidental impurities; then subjecting the hot rolled sheet to annealing and cold rolling to obtain a cold rolled sheet of final sheet thickness; then subjecting the cold rolled sheet to primary recrystallization annealing; then applying an annealing separator thereon; and then subjecting the cold rolled sheet to secondary recrystallization annealing, wherein after cold rolling and before the start of secondary recrystallization annealing, the cold rolled sheet is subjected to nitriding treatment to obtain a nitrogen content of 50 mass ppm or more and 1000 mass ppm or less, the annealing separator contains 50 mass % or more of MgO and a total content of 0.2 mass % to 15 mass % of a sulfide and/or sulfate, and a staying time in the temperature range of 300 C. to 800 C. in the heating stage of the secondary recrystallization annealing of 5 hours or more is secured.

2. The production method for a grain-oriented electrical steel sheet according to claim 1, wherein the sulfide and/or sulfate is a sulfide and/or sulfate of one or more of Ag, Al, La, Ca, Co, Cr, Cu, Fe, In, K, Li, Mg, Mn, Na, Ni, Sn, Sb, Sr, Zn and Zr.

3. The production method for a grain-oriented electrical steel sheet according to claim 1, wherein the composition of the steel slab further contains, by mass %, one or more of Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Cr: 0.01% to 1.50%, P: 0.0050% to 0.50%, Mo: 0.01% to 0.50% and Nb: 0.0005% to 0.0100%.

4. The production method for a grain-oriented electrical steel sheet according to claim 2, wherein the composition of the steel slab further contains, by mass %, one or more of Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Cr: 0.01% to 1.50%, P: 0.0050% to 0.50%, Mo: 0.01% to 0.50% and Nb: 0.0005% to 0.0100%.

5. The production method for a grain-oriented electrical steel sheet according to claim 1, wherein the total content of the sulfide and/or sulfate contained in the annealing separator is 2 mass % to 15 mass %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be further described below with reference to the accompanying drawings, wherein:

(2) FIG. 1 shows electron microscope photographs of a microstructure subjected to decarburization annealing, followed by nitriding treatment such that the nitrogen content is 100 mass ppm (FIG. 1A) or 500 mass ppm (FIG. 1B), subsequently heated to 800 C. at a predetermined heating rate, and then immediately subjected to water-cooling, as well as a graph (FIG. 1C) showing the identification results of precipitates in the above microstructure obtained by EDX (energy-dispersive X-ray spectrometry).

DETAILED DESCRIPTION

(3) Details of the present invention are described below.

(4) First, reasons for limiting the chemical compositions of the steel slab to the aforementioned range in the present invention will be explained. Here, unless otherwise specified, indications of % and ppm regarding components shall each stand for mass % and mass ppm.

(5) C: 0.08% or less

(6) C is a useful element in terms of improving primary recrystallized textures. However, if the content thereof exceeds 0.08%, primary recrystallized textures deteriorate. Therefore, C content is limited to 0.08% or less. From the viewpoint of magnetic properties, the preferable C content is in the range of 0.01% to 0.06%. If the required level of magnetic properties is not very high, C content may be set to 0.01% or less for the purpose of omitting or simplifying decarburization during primary recrystallization annealing.

(7) Si; 2.0% to 4.5%

(8) Si is a useful element which improves iron loss properties by increasing electrical resistance. However, if the content thereof exceeds 4.5%, it causes significant deterioration of cold rolling manufacturability, and therefore Si content is limited to 4.5% or less. On the other hand, for enabling Si to function as a nitride-forming element, Si content needs to be 2.0% or more. Further, from the viewpoint of iron loss properties, the preferable Si content is in the range of 2.0% to 4.5%,

(9) Mn: 0.5% or less

(10) Since Mn provides an effect of improving hot workability during manufacture, it is preferably contained in the amount of 0.03% or more. However, if the content thereof exceeds 0.5%, primary recrystallized textures worsen and magnetic properties deteriorate. Therefore, Mn content is limited to 0.5% or less.

(11) S, Se and O: less than 50 ppm (individually)

(12) If the content of each of S, Se and O is 50 ppm or more, it becomes difficult to develop secondary recrystallization. This is because primary recrystallized textures are made non-uniform by coarse oxides or MnS and MnSe coarsened by slab heating. Therefore, S, Se and O are all suppressed to less than 50 ppm. The contents of these elements may also be 0 ppm.

(13) sol,Al: less than 100 ppm

(14) Al forms a dense oxide film on a surface of the steel sheet, and could make it difficult to control the degree of nitridation at the time of nitriding treatment or obstruct deearburization. Therefore, Al content is suppressed to less than 100 ppm in terms of sol.Al. However, Al, which has high affinity with oxygen, is expected to bring about such effects as to reduce the amount of dissolved oxygen in steel and to reduce oxide inclusions which would lead to deterioration of magnetic properties, when added in minute quantities during steelmaking. Therefore, in order to curb deterioration of magnetic properties, it is advantageous to add Al in an amount of 20 ppm or more. The content thereof may also be 0 ppm.
[sol.Al](14/27)ppmN80 ppm

(15) The present invention has a feature that silicon nitride is precipitated after performing nitridation. Therefore, it is important that N is contained beforehand in steel in an amount equal to or more than the N content required to precipitate as AlN with respect to the amount of Al contained in steel. In particular, since Al and N are bonded at a ratio of 1:1, by containing N in an amount satisfying (sol.Al (mass ppm))[atomic weight of N (14)/atomic weight of Al (27)] or more, it is possible to completely precipitate a minute amount of Al contained in steel before nitriding treatment. On the other hand, since N could become the cause of defects such as blisters at the time of slab heating, N content needs to be suppressed to 80 ppm or less. The content thereof is preferably 60 ppm or less.

(16) The basic components are as described above. In the present invention, the following elements may be contained according to necessity as components for improving magnetic properties in an even more industrially reliable manner.

(17) Ni: 0.005% to 1.50%

(18) Ni provides an effect of improving magnetic properties by enhancing the uniformity of texture of the hot rolled sheet, and, to obtain this effect, it is preferably contained in an amount of 0.005% or more. On the other hand, if Ni content exceeds 1.50%, it becomes difficult to develop secondary recrystallization, and magnetic properties deteriorate. Therefore, Ni is preferably contained in a range of 0.005% to 1.50%,

(19) Sn: 0.01% to 0.50%

(20) Sn is a useful element which improves magnetic properties by suppressing nitridation and oxidization of the steel sheet during secondary recrystallization annealing and facilitating secondary recrystallization of crystal grains having good crystal orientation, and to obtain this effect, it is preferably contained in an amount of 0.01% or more. On the other hand, if Sn is contained in an amount exceeding 0.50%, cold rolling manufacturability deteriorates. Therefore, Sn is preferably contained in the range of 0.01% to 0.50%.

(21) Sb: 0.005% to 0.50%

(22) Sb is a useful element which effectively improves magnetic properties by suppressing nitridation and oxidization of the steel sheet during secondary recrystallization annealing and facilitating secondary recrystallization of crystal grains having good crystal orientation, and to obtain this effect, it is preferably contained in an amount of 0.005% or more. On the other hand, if Sb is contained in an amount exceeding 0.50%, cold rolling manufacturability deteriorates. Therefore, Sb is preferably contained in the range of 0.005% to 0.50%.

(23) Cu: 0.01% to 0.50%

(24) Cu provides an effect of effectively improving magnetic properties by suppressing oxidization of the steel sheet during secondary recrystallization annealing and facilitating secondary recrystallization of crystal grains having good crystal orientation, and to obtain this effect, it is preferably contained in an amount of 0.01% or more. On the other hand, if Cu is contained in an amount exceeding 0.50%, hot rolling manufacturability deteriorates. Therefore, Cu is preferably contained in the range of 0.01% to 0.50%,

(25) Cr: 0.01% to 1.50%

(26) Cr provides an effect of stabilizing formation of forsterite films, and, to obtain this effect, it is preferably contained in an amount of 0.01% or more. On the other hand, if Cr content exceeds 1.50%, it becomes difficult to develop secondary recrystallization, and magnetic properties deteriorate. Therefore, Cr is preferably contained in the range of 0.01% to 1.50%.

(27) P: 0.0050% to 0.50%

(28) P provides an effect of stabilizing formation of forsterite films, and, to obtain this effect, it is preferably contained in an amount of 0.0050% or more. On the other hand, if P content exceeds 0.50%, cold rolling manufacturability deteriorates. Therefore, P is preferably contained in a range of 0.0050% to 0.50%.

(29) Mo: 0.01% to 0.50%, Nb: 0.0005% to 0.0100%

(30) Mo and Nb both have an effect of suppressing generation of scabs after hot rolling by for example, suppressing cracks caused by temperature change during slab heating. These elements become less effective for suppressing scabs, however, unless Mo content is 0.01% or more and Nb content is 0.0005% or more. On the other hand, if Mo content exceeds 0.50% and Nb content exceeds 0.0100%, they cause deterioration of iron loss properties if they remain in the finished product as, for example, carbide or nitride. Therefore, it is preferable for each of Mo content and Nb content to be within the above mentioned ranges.

(31) Next, the production method for the present invention will be explained.

(32) A steel slab adjusted to the above preferable chemical composition range is subjected to hot rolling without being re-heated or after being re-heated. When re-heating the slab, the re-heating temperature is preferably approximately in the range of 1000 C. to 1300 C. This is because slab heating at a temperature exceeding 1300 C. is not effective in the present invention where little inhibitor element is contained in steel in the form of a slab, and only causes an increase in costs, while slab heating at a temperature of lower than 1000 C. increases the rolling load, which makes rolling difficult.

(33) Then, the hot rolled sheet is subjected to hot band annealing as necessary, and subsequent cold rolling once, or twice or more with intermediate annealing performed therebetween to obtain a final cold rolled sheet.

(34) The cold rolling may be performed at room temperature. Alternatively, warm rolling where rolling is performed with the steel sheet temperature raised to a temperature higher than room temperature for example, around 250 C. is also applicable.

(35) Then, the final cold rolled sheet is subjected to primary recrystallization annealing.

(36) The purpose of primary recrystallization annealing is to anneal the cold rolled sheet with a rolled microstructure for primary recrystallization to adjust the grain size of the primary recrystallized grains so that they are of optimum grain size for secondary recrystallization. In order to do so, it is preferable to set the annealing temperature of primary recrystallization annealing approximately in the range of 800 C. to below 950 C. Further, by setting the annealing atmosphere during primary recrystallization annealing to an atmosphere of wet hydrogen-nitrogen or wet hydrogen-argon, primary recrystallization annealing may be combined with decarburization annealing.

(37) Further, in the present invention, nitriding treatment is performed after the above cold rolling and before the start of secondary recrystallization annealing. As long as the degree of nitridation is controlled, any means of nitridation can be used and there is no particular limitation. For example, as performed in the past, gas nitriding may be performed directly in the form of a coil using NH.sub.3 atmosphere gas, or continuous nitriding may be performed on a running strip. Here, preferable treatment conditions are a treatment temperature of 600 C. to 800 C. and a treatment time of 10 seconds to 300 seconds. Further, it is also possible to utilize salt bath nitriding treatment with higher nitriding ability than gas nitriding. Here, a preferred salt bath is a salt bath of an NaCNNa.sub.2CO.sub.3NaCl system. Here, the preferable treatment conditions are a salt bath temperature of 400 C. to 700 C. and a treatment time of 10 seconds to 300 seconds.

(38) The important point of the above nitriding treatment is the formation of a nitride layer on the surface layer. In order to suppress diffusion into steel, it is preferable to perform nitriding treatment at a temperature of 800 C. or lower, yet, by shortening the duration of the treatment (e.g. to around 30 seconds), it is possible to form a nitride layer only on the surface even if the treatment is performed at a higher temperature.

(39) Here, it is necessary for the nitrogen content after performing nitridation to be 50 mass ppm or more and 1000 mass ppm or less. If the nitrogen content is less than 50 mass ppm, a sufficient effect cannot be obtained, whereas if it exceeds 1000 mass ppm, an excessive amount of silicon nitride precipitates and secondary recrystallization hardly occurs. Preferably, the nitrogen content is in a range of 200 mass ppm to less than 1000 mass ppm.

(40) After subjecting the steel sheet to the above primary recrystallization annealing and nitriding treatment, an annealing separator is applied on a surface of the steel sheet. In order to form a forsterite film on the surface of the steel sheet after secondary recrystallization annealing, it is necessary to use an annealing separator mainly composed of magnesia (MgO). However, if there is no need to form a forsterite any suitable oxide with a melting point higher than the secondary recrystallization annealing temperature, such as alumina (Al.sub.2O.sub.3) or calcia (CaO), can be used as the main component of the annealing separator.

(41) An annealing separator mainly composed of magnesia (MgO) refers to an annealing separator containing magnesia (MgO) of 50 mass % or more, preferably 80 mass % or more.

(42) Here, it is important to contain a sulfide and/or sulfate in an annealing separator in an amount of 0.2 mass % to 15 mass %, in order to form MnS during secondary recrystallization annealing to obtain a grain growth inhibiting effect, thereby increasing the intensity of the Goss orientation which is an ideal orientation of secondary recrystallization.

(43) This is because if the content of a sulfide and/or sulfate in an annealing separator is less than 0.2 mass %, the above effect is not obtained, whereas if the content thereof exceeds 15 mass %, base film formation becomes difficult.

(44) Therefore, the content of a sulfide and/or sulfate in an annealing separator is in the range of 0.2 mass % to 15 mass %. The range is preferably 2 mass % to 10 mass %.

(45) Further, if Cu is contained as a steel component, CuS precipitates as a sulfide in addition to MnS and, as is the case with MnS, contributes to improving the grain growth inhibiting effect.

(46) Further, as a sulfide and/or sulfate to add to an annealing separator, a sulfide and/or sulfate of one or more of Ag, Al, La, Ca, Co, Cr, Cu, Fe, in, K, Li, Mg, Mn, Na, Ni, Sn, Sb, Sr, Zn and Zr is/are preferable.

(47) Subsequently, secondary recrystallization annealing is performed. During this secondary recrystallization annealing, it is necessary to secure a staying time in the temperature range of 300 C. to 800 C. in the heating stage of 5 hours or more. During the staying time, the nitride layer mainly composed of Fe.sub.2N, Fe.sub.4N in the surface layer formed by nitriding treatment is decomposed and N diffuses into the steel. As for the chemical composition of the present invention, Al which is capable of forming MN does not remain, and therefore N as a grain boundary segregation element diffuses into steel using grain boundaries as diffusion paths.

(48) Silicon nitride has poor compatibility with steel (i.e. the misfit ratio is high), and therefore the precipitation rate is very low. Nevertheless, since the purpose of precipitation of silicon nitride is to inhibit normal grain growth, it is necessary to have a sufficient amount of silicon nitride selectively precipitated at grain boundaries at the stage of 800 C. at which normal grain growth proceeds. Regarding this point, silicon nitride cannot precipitate in grains, yet by setting the staying time in the temperature range of 300 C. to 800 C. to 5 hours or more, it is possible to selectively precipitate silicon nitride at grain boundaries by allowing silicon to be bound to N and Si diffusing along the grain boundaries. Although an upper limit of the staying time is not necessarily required, performing annealing for more than 150 hours is unlikely to increase the effect. Therefore, the upper limit is preferably set to 150 hours. A more preferable staying time is in a range of 10 hours to 100 hours. Further, as the annealing atmosphere, either of N.sub.2, Ar, H.sub.2 or a mixed gas thereof is applicable.

(49) After the start of decomposition of a sulfide and/or sulfate during secondary recrystallization annealing, since the diffusion rate of S is lower than N, diffusion proceeds while forming MnS (and further CuS) from the surface layer, and the concentration of S in the surface layer becomes significantly higher than that of nitride. As a result, grain growth in the surface layer is strongly inhibited, and secondary recrystallization starts from the inner parts in the sheet thickness direction. In the surface layer of the steel sheet, a large texture variation is caused due to the frictional force between the surface layer and rolls during hot rolling or cold rolling, and as a result, there is a higher probability of secondary recrystallized grains with displaced orientations being generated. Therefore, by enhancing the grain growth inhibiting effect in the surface layer part, the intensity of the Goss orientation which is an ideal orientation of secondary recrystallization grains is significantly increased compared to nitriding treatment alone,

(50) As described above, with a grain-oriented electrical steel sheet obtained by applying the above process to a slab that contains a limited amount of Al in steel, with an excessive amount of N with respect to AlN precipitation added thereto, and contains little inhibitor components such as MnS or MnSe, it is possible to selectively form coarse silicon nitride (with a precipitate size of 100 nm or more), as compared to conventional inhibitors, at grain boundaries at the stage during the heating stage of secondary recrystallization annealing before secondary recrystallization starts, and with the sulfide or sulfate contained in the annealing separator being decomposed and diffused (luring the secondary recrystallization annealing, it is possible to allow MnS (and CuS) to precipitate densely at the surface layer. Although there is no particular limit on the upper limit of the precipitate size of silicon nitride, it is preferably 10 m or less.

(51) FIG. 1 shows electron microscope photographs for observation and identification of a microstructure subjected to decarburization annealing, followed by nitriding treatment such that the nitrogen content is 100 mass ppm ((a) of FIG. 1) or 500 mass ppm ((b) of FIG. 1), subsequently heated to 800 C. at a heating rate such that the staying time in the temperature range of 300 C. to 800 C. is 8 hours, and then immediately subjected to water-cooling, which were observed and identified using an electron microscope. Further, graph (c) in FIG. 1 shows the results of identification of precipitates in the aforementioned microstructure by EDX (energy-dispersive X-ray spectrometry).

(52) It can be seen from FIG. 1 that unlike fine precipitates conventionally used (with a precipitate size of smaller than 100 nm), even the smallest one of the coarse silicon nitride precipitates at the grain boundary has a precipitate size greater than 100 urn.

(53) The use of pure silicon nitride which is not precipitated compositely with Al which is a feature of the present invention, has significantly high stability from the viewpoint of effectively utilizing Si which exists in steel in order of several % and provides an effect of improving iron loss properties. That is, components such as Al or Ti, which have been used in conventional techniques, have high affinity with nitrogen and provide precipitates which still remain stable at high temperature. Therefore, these components tend to remain in steel insistently, and the remaining components could become the cause of deteriorating magnetic properties.

(54) However, when using silicon nitride, it is possible to achieve purification of precipitates which are harmful to magnetic properties simply by purifying nitrogen and sulfur, which diffuse relatively quickly. Further, when using Al or Ti, control in ppm order is necessary from the viewpoint that purification is eventually required and that an inhibitor effect must surely be obtained. However, when using Si and S, such control is unnecessary during steelmaking, and this is also an important feature of the present invention.

(55) In production, it is clear that utilizing the heating stage of secondary recrystallization is most effective for precipitation of silicon nitride in terms of energy efficiency, yet it is also possible to selectively precipitate silicon nitride at grain boundaries by utilizing a similar heat cycle. Therefore, in production, it is also possible to perform silicon nitride dispersing annealing before time consuming secondary recrystallization.

(56) After the above secondary recrystallization annealing, it is possible to further apply and bake an insulating coating on the surface of the steel sheet. Such an insulating coating is not limited to a particular type, and any conventionally known insulating coating is applicable. For example, preferred methods are described in JPS5079442A and JPS4839338A where a coating liquid containing phosphate-chromate-colloidal silica is applied on a steel sheet and then baked at a temperature of around 800 C.

(57) It is possible to correct the shape of the steel sheet by flattening annealing, and further to combine the flattening annealing with baking treatment of the insulating coating.

EXAMPLES

Example 1

(58) A steel slab having a composition containing C: 0.04%, Si: 3.4%, Mn: 0.08%, S: 0.002%, Se: 0.001%, O: 0.001%, Al: 0.006%, N: 0.0035%, Cu: 0.10%, and Sb: 0.06%, with the balance including Fe and incidental impurities, was heated at 1200 C. for 30 minutes, and then subjected to hot rolling to obtain a hot rolled sheet with a thickness of 2.2 mm. Then, the steel sheet was subjected to annealing at 1065 C. for 1 minute, and subsequent cold rolling to obtain a final sheet thickness of 0.23 mm. Then, samples of the size of 100 mm400 mm were collected from the center part of the obtained cold rolled coil, and primary recrystallization annealing combined with decarburization was performed in a lab. Then, the samples were subjected to gas treatment or nitriding treatment by salt bath treatment under the conditions shown in Table 1 to increase the nitrogen content in steel.

(59) As the nitriding condition for gas treatment, a mixed atmosphere of NH.sub.3: 30 vol % and N.sub.2: 70 vol % was used. Further, as the nitriding condition for salt bath treatment, a ternary system salt of NaCNNa.sub.2CO.sub.3NaCl was used.

(60) The N content of the steel sheet after the above nitriding treatment was measured.

(61) Then, magnesium sulfate was added under the conditions shown in Table 1 to an annealing separator mainly composed of MgO and containing 5% of TiO.sub.2 and made into a water slurry state and then applied, dried and baked on the samples, and subsequently, the samples were subjected to final annealing under the conditions shown in Table 1, and then a phosphate-based insulating tension coating was applied and baked thereon to obtain products.

(62) For the obtained products, the magnetic flux density B.sub.8 (T) at a magnetizing force of 800 A/m was evaluated.

(63) TABLE-US-00001 TABLE 1 Annealing Final Annealing Separator Condition Additive Amount Staying Time in Nitriding Treatment N Content of Magnesium Temperature Range Magnetic Means of Content of Temperature Time after Treatment Sulfate of 300 C. to Properties Treatment Treatment ( C.) (s) (mass ppm) (mass %) 800 C. (h) B.sub.8 (T) Remarks Condition 1 None 35 0 20 1.852 Comparative Example Condition 2 Salt Bath Nitridation 550 120 350 0 20 1.913 Comparative Example Condition 3 Salt Bath Nitridation 550 120 350 5 20 1.949 Inventive Example Condition 4 Salt Bath Nitridation 550 120 350 5 4 1.906 Comparative Example Condition 5 Salt Bath Nitridation 550 600 700 10 20 1.940 Inventive Example Condition 6 Gas Nitridation 750 20 120 0 20 1.909 Comparative Example Condition 7 Gas Nitridation 750 20 120 5 20 1.955 Inventive Example Condition 8 Gas Nitridation 750 80 520 5 20 1.958 Inventive Example Condition 9 Gas Nitridation 750 80 520 10 20 1.963 Inventive Example Condition 10 Gas Nitridation 750 5 40 10 20 1.903 Comparative Example Condition 11 Gas Nitridation 750 3600 2900 10 20 1.777 Comparative Example

(64) As can be seen in Table 1, it is clear that magnetic properties are improved in the inventive examples compared to those produced in the conventional inhibitor-less manufacturing process.

Example 2

(65) A steel slab containing components shown in Table 2 (the contents of S, Se, and O each being less than 50 ppm) was heated at 1200 C. for 20 minutes, subjected to hot rolling to obtain a hot rolled sheet with a thickness of 2.5 mm, and then the hot rolled sheet was subjected to annealing at 1050 C. for 1 minute, and then cold rolling to obtain a final sheet thickness of 0.27 mm, and then decarburization annealing where the cold rolled sheet is retained at an annealing temperature of 840 C. for 2 minutes, in an atmosphere of P(H.sub.2O)/P(H.sub.2)=0.4. Then, some of the coils were subjected to gas nitriding treatment (in an atmosphere of NH.sub.3: 30 vol %+N.sub.2: 70 vol %) at 750 C. for 20 seconds, and the N content of the steel sheets was measured.

(66) Then, annealing separators, each mainly composed of MgO with 10% of TiO.sub.2 and 10% of aluminum sulfate added thereto, were mixed with water, made into slurry state and applied on the steel sheets, respectively, which in turn were wound into coils and then subjected to Final annealing at a heating rate where the staying time in the temperature range of 300 C. to 800 C. was 30 hours. Then, a phosphate-based insulating tension coating was applied and baked thereon, and flattening annealing was performed for the purpose of flattening the resulting steel strips to obtain products.

(67) Epstein test pieces were collected from the product coils thus obtained and the magnetic flux density B.sub.8 thereof was measured. The measurement results are shown in Table 2.

(68) TABLE-US-00002 TABLE 2 Annealing Separator Chemical Composition Additive Amount Si C Mn sol. Al N Others Gas N Content of Magnesium Magnetic (mass (mass (mass (mass (mass (mass Nitriding after Treatment Sulfate Properties No. %) ppm) %) ppm) ppm) %) Treatment (mass ppm) (mass %) B.sub.8 (T) Remarks 1 3.35 400 0.03 180 70 Not 70 0 1.821 Comparative Performed Example 2 3.35 400 0.03 180 70 Performed 190 0 1.858 Comparative Example 3 3.35 400 0.03 80 20 Not 20 0 1.852 Comparative Performed Example 4 3.35 400 0.03 80 20 Performed 140 0 1.895 Comparative Example 5 3.35 400 0.03 80 50 Not 50 0 1.885 Comparative Performed Example 6 3.35 400 0.03 80 50 Performed 130 8 1.951 Inventive Example 7 1.85 400 0.03 80 50 Not 50 0 1.875 Comparative Performed Example 8 1.85 400 0.03 80 50 Performed 90 0 1.903 Comparative Example 9 3.35 200 0.1 50 20 Not 20 8 1.888 Comparative Performed Example 10 3.35 200 0.1 50 40 Performed 150 8 1.945 Inventive Example 11 3.35 600 0.08 60 40 Not 40 0 1.878 Comparative Performed Example 12 3.35 600 0.08 60 40 Performed 140 8 1.948 Inventive Example 13 3.35 600 0.08 60 40 N: 0.01, Performed 150 8 1.955 Inventive Sb: 0.02 Example 14 3.35 600 0.08 60 40 Sn: 0.03 Performed 150 8 1.954 Inventive Example 15 3.35 600 0.08 60 40 Cr: 0.03, Performed 140 8 1.952 Inventive Mo: 0.05 Example 16 3.35 600 0.08 60 40 Cu: 0.05 Performed 130 8 1.950 Inventive Example 17 3.35 600 0.08 60 40 P: 0.01, Performed 140 8 1.953 Inventive Nb: 0.001 Example

(69) It can be seen from Table 2 that all of the inventive examples obtained in accordance with the present invention exhibited high magnetic flux density.

Example 3

(70) A steel slab having a composition containing C: 0.03%, Si: 3.3%, Mn: 0.09%, S: 0.003%, Se: 0.001%, O: 0.001%, Al: 0.005%, N: 0.003%, Cu: 0.09% and Sb: 0.05%, with the balance including Fe and incidental impurities, was heated at 1220 C. for 20 minutes, subjected to hot rolling to obtain a hot rolled sheet with a thickness of 2.5 mm. Then, the hot rolled sheet was subjected to annealing at 1050 C. for 1 minute, then cold rolling to obtain a final sheet thickness of 0.27 mm, and then decarburization annealing where the cold rolled sheet was retained at an annealing temperature of 840 C. for 2 minutes, in an atmosphere of P(H.sub.2O)/P(H.sub.2)=0.4. Then, after performing salt bath nitriding treatment at 550 C. for 240 seconds (using a ternary system salt of NaCNNa.sub.2CO.sub.3NaCl), the N content of the steel sheet was measured. The N content was 240 mass ppm.

(71) Then, annealing separators, each mainly composed of MgO with 10% of TiO.sub.2 and a sulfide or sulfate added thereto, as shown in Table 3, were mixed with water and made into slurry state and applied on the steel sheets, respectively, which in turn were wound into coils and then subjected to final annealing at a heating rate where the staying time in the temperature range of 300 C. to 800 C. was 30 hours. Then, a phosphate-based insulating tension coating was applied and baked thereon, and flattening annealing was performed for the purpose of flattening the steel strips to obtain products.

(72) Epstein test pieces were collected from the product coils thus obtained and the magnetic flux density B.sub.8 thereof was measured. The measurement results are shown in Table 3.

(73) TABLE-US-00003 TABLE 3 Annealing Separator Additive Magnetic Type of Amount of Properties Sulfide, Sulfide, Sulfate B.sub.8 No. Sulfate (mass %) (T) Remarks 1 None 0 1.872 Comparative Example 2 Ag.sub.2SO.sub.4 10 1.965 Inventive Example 3 Al.sub.2(SO.sub.4).sub.3 10 1.963 Inventive Example 4 LaSO.sub.4 10 1.955 Inventive Example 5 CaSO.sub.4 10 1.955 Inventive Example 6 CoSO.sub.4 10 1.952 Inventive Example 7 Cr.sub.2(SO.sub.4).sub.2 10 1.954 Inventive Example 8 CuSO.sub.4 10 1.956 Inventive Example 9 FeSO.sub.4 10 1.953 Inventive Example 10 In.sub.2(SO.sub.4).sub.3 10 1.965 Inventive Example 11 K.sub.2SO.sub.4 10 1.952 Inventive Example 12 Li.sub.2SO.sub.4 10 1.955 Inventive Example 13 MgSO.sub.4 10 1.962 Inventive Example 14 MnSO.sub.4 10 1.961 Inventive Example 15 Na.sub.2SO.sub.4 10 1.957 Inventive Example 16 NiSO.sub.4 10 1.965 Inventive Example 17 SnSO.sub.4 10 1.957 Inventive Example 18 Sb.sub.2(SO.sub.4).sub.3 10 1.958 Inventive Example 19 SrSO.sub.4 10 1.955 Inventive Example 20 ZnSO.sub.4 10 1.952 Inventive Example 21 Zr(SO.sub.4).sub.2 10 1.950 Inventive Example 22 MgS 10 1.963 Inventive Example 23 MnS 10 1.955 Inventive Example 24 Na.sub.2S.sub.2O.sub.3 10 1.956 Inventive Example

(74) It can be seen from Table 3 that all of the inventive examples obtained in accordance with the present invention exhibited high magnetic flux density.