Title of Invention	"A NON ORIENTED MAGNETIC STEEL SHEET AND METHOD OF PRODUCTION THEREOF"
Abstract	A non-oriented magnetic steel sheet characterized by containing, by mass%. Si: 1.5% or less, Mn:0.40% to 1.5%, Sol.A 1:0.01% to 0.04%, Ti:0.0015% or less, N;0.0030% or less, S:0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and optionally containing one or more of Sn , Cu, Ni, REM, Ca, Mg and where Sn , Cu and Ni in total amount of 0.01% to 0.50%, and REM, Ca and Mg in a total of 0.001%) to 0.05%, and the balance of Fe and unavoidable impurities, and 10% or more of the particles of sulfide containing Mn , complexly precipitating with the B precipitates, and a distribution density of a total of MnS, CU2S, and their complex sulfides of 3.0x105 /mm2 or less, and a distribution density of the Ti precipitates not reaching a diameter of 0.1 µm of 1..0xl03/mm2 or less.

Title of Invention

"A NON ORIENTED MAGNETIC STEEL SHEET AND METHOD OF PRODUCTION THEREOF"

Abstract

A non-oriented magnetic steel sheet characterized by containing, by mass%. Si: 1.5% or less, Mn:0.40% to 1.5%, Sol.A 1:0.01% to 0.04%, Ti:0.0015% or less, N;0.0030% or less, S:0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and optionally containing one or more of Sn , Cu, Ni, REM, Ca, Mg and where Sn , Cu and Ni in total amount of 0.01% to 0.50%, and REM, Ca and Mg in a total of 0.001%) to 0.05%, and the balance of Fe and unavoidable impurities, and 10% or more of the particles of sulfide containing Mn , complexly precipitating with the B precipitates, and a distribution density of a total of MnS, CU2S, and their complex sulfides of 3.0x105 /mm2 or less, and a distribution density of the Ti precipitates not reaching a diameter of 0.1 µm of 1..0xl03/mm2 or less.

Full Text	TECHNICAL FIELD The present invention relates to a nonoriented magnetic steel sheet used as a core material of electrical equipment and a method of production of the same, more particularly relates to a nonoriented magnetic steel sheet excellent in stampability and magnetic characteristics of stress relief annealing and a method of production of the same. BACKGROUND ART In recent years, the global rise in energy saving in electrical equipment has led to demands for higfier performance characteristics in nonoriented magnetic steel sheet used as core materials of rotating machines. The motors for "high efficiency model" electrical products are now using high grade materials increased in the content of Si and Al to raise the specific resistance and increased in crystal grain size. On the other hand, general use model motors are also being required to be improved in performance, but due to severe cost restrictions, it is difficult to switch to high grade materials such as with high efficiency models. The steel..sheet used for general use models has been strikingly improved in watt loss by giving it an Si content of 1.5% or less and promoting the crystal grain growth at the time of the stress relief annealing performed after stamping out the motor core. Further, recently, an increasing number of users are utilizing the scrap produced when stamping out cores as materials for castings. From the viewpoint of securing the castability of the scrap, it has become necessary to reduce the content of Al in the steel sheet to less than 0.05%. To improve the crystal grain growth at the time of stress relief annealing, it is important to reduce or render harmless the precipitates unavoidably mixed in the steel. The designs of materials of nonoriented magnetic steel sheet with Al: less than 0.05% may be roughly divided into two groups. One, as seen in Japanese Patent Publication (A) No. 54-163720, is the method of adding 0.002% or so of B to Al-killed steel (Sol. Al: 0.02% or so) to form a nitride constituted by BN and suppress the precipitation of A1N harmful to crystal grain growth. On the other hand, as seen in Japanese Patent Publication (A) No. 7-150248, the method of controlling the ratios of the oxides of Si02 and MnO included in the Si-killed steel (Sol. Al As a method for suitably controlling the nitrides or oxides produced along with the deoxidation and further reducing or rendering harmless the sulfides to improve the magnetic characteristics, for example Japanese Patent Publication (A) No. 58-117828 discloses a method of limiting the S to 0.005% or less to obtain an effect of improvement of the magnetic characteristics, Japanese Patent Publication (A) No. 58-164724 discloses a method of adding Ca or a rare earth element to fix the S when including S: 0.01 to 0.02%, and Japanese Patent Publication (A) No. 4-63228 discloses a method of limiting S to 0.0050% or less and limiting the heating temperature of the slab 1100°C or less so as to prevent fine precipitation of MnS during the hot rolling. DISCLOSURE OF THE INVENTION With further reduction of the watt loss required, it is becoming difficult to produce the desired sheet sufficiently and stably by the above techniques. The present invention was made in consideration of this problem and provides a steel sheet with a good crystal grain growth of stress relief annealing, low watt loss, and high magnetic flux density by means of improvement focusing on the B added for the purpose of suppressing precipitation of A1N and the S as an element unavoidably mixed in the steel. The present invention was made to solve the above problem and has as its gist the following: (1) A nonoriented magnetic steel sheet characterized by containing, by mass%, Si: 1.5% or less, Mn: 0.4% to 1.5%, Sol. Al: 0.01% to 0.04%, Ti: 0.0015% or less, N: 0.0030% or less, S: 0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and the balance of Fe and unavoidable impurities, 10% or more of the particles of sulfide containing Mn, in terms of ratio of number, complexly precipitating with the B precipitates. (2) A nonoriented magnetic steel sheet characterized by containing, by mass%, Si: 1.5% or less, Mn: 0.4% to 1.5%, Sol. Al: 0.01% to 0.04%, Ti: 0.0015% or less, N: 0.0030% or less, S: 0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and the balance of Fe and unavoidable impurities, a crystal grain size of the steel sheet being 30 urn or less and a crystal grain size of stress relief annealing of 750°Cx2 hours being 50 pm or more. (3) A nonoriented magnetic steel sheet as set forth in (1) or (2), characterized in that a distribution density of a total of MnS, CuaS, and their complex sulfides is 3.0xl05/mm2 or less. (4) A nonoriented magnetic steel sheet as set forth in any one of (1) to (3), characterized in that a distribution density of the Ti precipitates not reaching a diameter of 0.1 jjm is l.OxloVmm2 or less. (5) A nonoriented magnetic steel sheet as set forth in any one of (1) to (4), characterized by further containing one or more of Sn, Cu, and Ni in a total mass% of 0.01% to 0.50% and/or one or more of REM, Ca, and Mg in an amount of 0.001 to 0.5%. (6) A method of production of a nonoriented magnetic steel sheet as set forth in any one of (1) to (5), characterized, during finish annealing after steelmaking, hot rolling, pickling, and cold rolling, by heating a slab for hot rolling by keeping it in a range of 1150°C to 1250°C for 5 minutes or more, then keeping it in a range of 1050°C to less than 1150°C for 15 minutes or more, then immediately hot rolling it. (7) A method of production of a nonoriented magnetic steel sheet as set forth in any one of (1) to (6), characterized, during finish annealing after steelmaking, hot rolling, pickling, and cold rolling, l making an outlet temperature of finish rolling of the hot rolling 800°C or more. (8) A method of production of a nonoriented magnetic steel sheet as set forth in (6), characterized by making a finish outlet temperature T (°C) of the hot rolling, in steel sheet containing Sn, T>900- 1000xSn[mass%]. BEST MODE FOR WORKING THE INVENTION The inventors ran against the problem that in steel with an Si of 1.5% or less, even if reducing the unavoidably included element S to 0.0010% or so and reducing the slab heating temperature to 1100°C or less like in Japanese Patent Publication (A) No. 4-63228, the watt loss of the stress relief annealing fluctuates and will not stabilize. The inventors investigated the causes and learned that despite the amount of S and the slab heating temperature being low, the steel has a large amount of MnS, CU2S, and their complex sulfides finely dispersed in it and the crystal grain growth of the stress relief annealing is remarkably suppressed. Observed more closely, these sulfides form spheres of diameters of 0.1 to 0.3 (jm or so, but the centers contain Ti precipitates of diameter equivalents of 0.05 (am or so. The reason why the sulfides take this form of precipitation is that the TiN initially precipitating after the casting or by the heating of the hot rolled slab finely disperses and forms the nuclei for precipitation of the sulfides. To deal with this situation, the inventors took note of sulfides with a faster speed of growth and easier roughening compared with TiN. That is, as opposed to the current situation of using TiN as the nuclei for complex precipitation of sulfides, the inventors experimented with using sulfides as the nuclei for complex precipitation of TiN and as a result discovered that stable, low watt loss is obtained. Further, they discovered that a low watt loss is also obtained stably by complex precipitation of sulfides for the BN formed fixing the excess N explained later and thereby completed the present invention. Below, the results of experiments leading to the present invention will be explained. (Experiment 1) A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 0.6%, Mn: 0.1 to 0.8%, Sol. Al: 0.03%, Ti: 0.0012%, N: 0.0021%, S: 0.0005 to 0.0025%, B: 0.0020%, and Sn: 0.08%. These slabs were soaked at 1200°C for 20 minutes, raised in temperature over 10 minutes to 1100°C/ soaked for 30 minutes, then hot rolled to a sheet thickness of 2.5 mm, pickled, and cold rolled to a sheet thickness of 0.50 mm. The thus obtained cold rolled sheets were finish annealed at 800°C for 10 seconds, then were stress relief annealed at 750°C for 2 hours and measured for crystal grain size and watt loss. As a result, as shown in Table 1, in Samples 8, 9, 11, and 12 with Mn of 0.4% or more and with S of 0.0012 and 0.0025%, the crystal grain sizes of the stress relief annealing were 50 \|om or more and good watt losses were obtained. Next, the stress relief annealed samples were observed for precipitates, whereupon in Samples 8, 9, 11, and 12 with good watt loss, complex precipitation with the B precipitate was observed at a numerical ratio of 10% or more of the sulfides including the Mn. On the other hand, in the other samples with the poor watt loss, a large number of fine precipitates of less than a diameter of 0.1 um thought to be precipitates of B was observed. The effect of improvement of the crystal grain growth of the stress relief annealing and watt loss appearing at Mn: 0.4% or more and S: 0.0012 and 0.0025% is believed to appear as follows: First, by increasing the Mn, the precipitation start temperature of MnS rises. Due to this, MnS precipitates at the time of heating of the hot rolling before the BN and, further, the later precipitating BN precipitates complexly about MnS nuclei. Due to this, it is possible to suppress the formation of fine precipitates of B and obtain good crystal grain growth of the stress relief annealing and watt loss. On the other hand, with Mn of less than 0.4%, at the stage where BN precipitates, the precipitation of MnS is not sufficient. Further, with S is 0.0005%, the amount of precipitation of MnS itself is small, so probably in both cases the nuclei for precipitation of BN are insufficient and the characteristics of stress relief annealing are not improved by the sole, fine precipitation of B. 7 Table 1 Mn S Crystal Complex Code (* U grain size (Urn) W15/50 (W/kg) precipitation ratio (%) Remarks 1 0.0005 30 4.88 0 Comp . ex . 2 0.1 0.0012 25 5.02 0 Comp. ex. 3 0.0025 19 5.21 5 Comp . ex . 4 0.0005 34 4.78 5 Comp . ex . 5 0.2 0.0012 29 4.89 5 Comp . ex . 6 0.0025 21 5.05 5 Comp . ex . 7 0.0005 35 4.72 40 Comp . ex . 8 0.4 0.0012 62 3.65 30 Inv. ex. 9 0.0025 54 4.03 20 Inv. ex. 10 0.0005 43 4.55 50 Comp . ex . 11 0.8 0.0012 65 3.51 45 Inv. ex. 12 0.0025 58 3.88 40 Inv. ex. {Experiment 2) A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0034%, Si: 0.75%, Mn: 0.15 to 0.72%, Sol. Al: 0.019%, Ti: 0.0008 to 0.0017%, N: 0.0018%, S: 0.0023%, B: 0.0025%, Sn: 0.03%, Cu: 0.01%, and Ni: 0.02%. These slabs were soaked at 1200°C for 5 minutes, lowered in temperature to 1100°C, soaked for 30 minutes/ then hot rolled to a sheet thickness of 2.7 mm, pickled, then cold rolled to a sheet thickness of 0.50 mm. The thus obtained cold rolled sheets were finish annealed at 800°C for 10 seconds, then were stress relief annealed at 750°C for 2 hours then observed for the precipitates and crystal grain sizes of the steel sheets and measured for watt loss. As a result, as shown in Table 2, in Samples 4, 5, If and 8 with Mn of 0.4% or more and with Ti of 0.0015% or less, the sulfide densities of the stress relief annealing were 3.0xl05 or less, the average crystal grain sizes were 50 ^m or more, and the watt losses were good. In these samples, it was confirmed that there were many spherical sulfides with diameters of 0.2 to 0.3 jam and a large number had a plurality of fine Ti precipitates with diameter equivalents less than 0.1 pm precipitated at their outer circumferences. On the other hand, in Samples 1 to 3 with Mn of low 0.15%, the sulfide densities of the stress relief annealing were 4.5xl05/mm2 or more, the average crystal grain sizes were small 35 jjm or less, and the watt losses were poor. It was confirmed that the sulfides seen in these samples had diameters of small 0.1 pm or less and a large number of sulfides contained Ti precipitates at their centers. Further, in Samples 6 and 9 with Mn of 0.4% or more and with Ti of over 0.0015% as well, the sulfide densities of the stress relief annealing were high 3.8xl05/mm2 or more, the average crystal grain sizes were also 45 \|im or less, and the watt losses were relatively poor. In these samples, the sulfides varied in diameter in a broad range of 0.05 to 0.3 pm or less. The form of complexing with the Ti precipitates also varied including the outer circumferences or the centers. Note that the distribution density of Ti precipitates with diameter equivalents of less than 0.1 jam was a low less than 1.0xl03/mm2 in all samples. The effect of raising the Mn to 0.4% or more and limiting Ti to 0.0015% or less in this way is believed to be as follows: Further, by raising the Mn, the precipitation start temperature of the MnS rises, while by lowering the Ti, the precipitation start temperature of the TiN falls. Due to this, the order of precipitation inverts from the usual and MnS precipitates before the TiN. Next the MnS rising in precipitation start temperature precipitates and grows at the 1200°C of the first stage of the hot rolling heating. On the other hand, the TiN dropping in precipitation start temperature is believed to precipitate about the nuclei of the already roughened MnS at the 1100°C of the second stage of the hot rolling heating. Table 2 Code Mn Ti Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Mm) W15/50 (W/kg) Remarks (%) I 0.15 0.0008 4.5xl05 2 0.0013 4.8xl05 3 0.0017 5.4xl05 4 0.42 0.0008 2.8xl05 5 0.0013 2.7xl05 6 0.0017 3.9xl05 7 0.0008 l.SxlO5 8 0.72 0.0013 1.9xl05 9 0.0017 3.8xl05 (Experiment 3) Next, to investigate the effects of the heating conditions of the hot rolling, slabs of Experiment 1 with Mn: 0.4% and S: 0.0025% were hot rolled at heats of two levels of soaking at 1200°C for 60 minutes and soaking at 1100°C for 60 minutes, then were pickled and further processed by the same process as in Experiment 1 to fabricate and compare and evaluate samples. As a result, as shown in Table 3, compared with Sample 1 with the good results (Sample 9 in Experiment 1), in Sample 2 soaked at 1200°C, the ratio of complex precipitation was substantially zero, a large number of independently fine B precipitates were observed, and the crystal grain size of the stress relief annealing and watt loss were the worst. On the other hand, in Sample 3 soaked at 1100°C, while not to the extent of the 1200°C heated material (Sample 2), the ratio of complex precipitation was a low 5% and the crystal grain size and watt loss were not that good. This result is interpreted as follows. First, when soaking at 1200°C, since Mn is a high 0.4-2% and MnS precipitates, the B is hot rolled in the unprecipitated state, so during the hot rolling or stress relief annealing, independent fine B precipitates are formed and the crystal grain growth and watt loss remarkably deteriorate. Next, when soaked at 1100°C, the distribution of the MnS becomes rough and the number of nuclei for complex precipitation of BN becomes insufficient, so it is believed that part of the B ends up precipitating independently and finely and therefore a good crystal grain size and watt loss cannot be obtained. Table 3 Code Hot rolling heating temperature Crystal grain size (Urn) W15/50 (W/kg) Complex precipitation ratio (%) Remarks 1 1200°C-»1100°C 54 4.03 25 Inv. ex. 2 1200°C 19 5.68 0 Comp. ex. 3 1100°C 35 4.51 5 Comp . ex . (Experiment 4) Next, to investigate the effects of the heating cycle of the hot rolling, slabs of Experiment 2 with Mn: 0.42% and Ti: 0.0013% were hot rolled at heats of two levels of soaking at 1200°C for 60 minutes and soaking at 1100°C for 60 minutes, then were pickled and further processed by the same process as in Experiment 2 to fabricate and compare and evaluate samples. As a result, as shown in Table 4, compared with Sample 1 with the good results (Sample 5 in Experiment 2), in Sample 2 soaked at 1200°C, the sulfide density was low, but the distribution density of the Ti precipitates was high and the crystal grain size of the stress relief annealing and watt loss were the worst. On the other hand, in Sample 3 soaked at 1100°C, the Ti precipitate density was low, but the sulfide density was high and the crystal grain size and the watt loss were both excellent. This result is interpreted as follows. First, when soaking at 1200°C, the Mn was a high 0.42% and roughening of the MnS proceeded, but the slab was hot rolled with the TiN not yet precipitated, independent fine Ti precipitates were formed during the hot rolling or at the time of stress relief annealing, and the crystal grain growth and watt loss remarkably deteriorated. Next, when soaked at 1100°C, the growth of MnS was insufficient, so it is believed the number of sulfides increased and good crystal grain size and watt loss could not be obtained. Table 4 Code Hot rolling heating temperature Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Mm) W15/50 (W/kg) Remarks 1 1200°C-»1100°C 2.7xl05 2 1200°C l.SxlO5 5.6xl03 25 5.44 Comp. ex. 3 1100°C 4.8xl05 Summarizing the above, the present invention discovers that by optimizing the amount of Mn and the heating cycle of the hot rolling so as to cause MnS to preferentially and optimally precipitate and simultaneously cause fine BN to complexly precipitate, both MnS and BN are simultaneously made harmful and the crystal grain growth and watt loss are improved. To realize this, the amount of Mn has to be raised and simultaneously the amount of Al has to be lowered. The reason is that Al consumes N as A1N and the AlN itself inhibits crystal grain growth. On the other hand, with the method of production of not adding any Al at all, the amount of Al is kept extremely low, which is perfect for formation of BN, but the large number of SiC>2MnO inclusions remaining in the steel are flattened along with the increase in the amount of Mn and conversely the grain growth ends up being worsened. Therefore, the present invention discovered that the optimal range of Al for suppression of flattening due to modification of the inclusions and for preferential precipitation of BN is 0.01% to 0.04%. Further, the present invention discovers that by optimizing the precipitation temperatures of MnS and TiN and optimizing the heating cycle of the hot rolling so as to first cause the coarse precipitation of MnS and then cause the complex precipitation of fine TiN, both precipitates are rendered harmless and the crystal grain growth and watt loss are improved. To realize this, it is necessary to raise the amount of Mn, raise the precipitation temperature of MnS, further lower the amount of Al to suppress the formation of A1N, and promote the precipitation of TiN. As the method of keeping the amount of Al extremely low, the method of using Si for deoxidation in steelmaking may be mentioned, but in this case, if increasing the amount of Mn, it is learned that the inclusions are flattened and conversely the watt loss is made worse. Therefore, in the present invention, by controlling the amount of Al to the relatively small range of 0.01 to 0.04%, it is possible to maintain the precipitation of TiN while modifying the inclusions to thereby increase the amount of Mn. Further, in this range of amount of Al, the excess N after precipitation of TiN results in easy fine precipitation of A1N harmful to grain growth, so this is avoided by adding a small amount of B to obtain BN. This technical idea was first discovered by the present invention. For example, Japanese Patent Publication (A) No. 58-117828 defines a content of Si: 0.1 to 1.0%, Al: less than 0.1%, Mn: 0.75 to 1.5%, N/B: 0.7 to 1.2 of B, but does not define complex precipitation of MnS and BN and the amount of Sol. Al for realizing this or the heating temperature of the hot rolling, so completely differs from the present invention in technical idea and could not have enabled deduction of the present invention. Further, Japanese Patent Publication (A) No. 2000-248344 defines a content of Si: 1.8% or less, Sol. Al: 0.05 to 0.20%, and Mn: 0.05 to 1.5%, but if Sol. Al is over 0.04%, A1N ends up precipitating with priority over BN, so the technical idea of the present invention of precipitating BN around MnS does not stand. Next, the reasons for numerical limitation of the ingredients and products in the present invention will be explained. Si is an element effective for increasing the electrical resistance, but if added over 1.5%, the hardness rises, the magnetic flux density falls, and the cost rises, so the content was limited to 1.5%. Mn is an important element for realizing the present invention. The present invention has as its main gist the precipitation of BN and/or TiN around sulfide containing MnS. Therefore, it is necessary to make MnS sufficiently precipitate before the precipitation temperature of BN and/or TiN. In the present invention containing Ti: 0.0015% or less, N: 0.0030% or less, and B/N of 0.5 to 1.5 of B, the object is achieved by making the Mn 0.4% or more. Further, if added over 1.5%, the drop in the saturated magnetic flux density becomes remarkable. In addition, the y-Mx transformation temperature falls and control of the structure of the hot rolled sheet becomes difficult, so 1.5% was made the upper limit. Al is an element required for deoxidation of steel. If less than Sol. 0.01%, the unremoved oxygen remains in the steel and oxides of SiC^'MnO are formed. These are flattened along with the effects of the Mn added in an amount of 0.4% or less and thereby inhibit crystal grain growth, so the lower limit of Sol. Al was made 0.01%. Further, if Sol. Al exceeds 0.04%, A1N precipitates instead of BN and achievement of the present invention becomes difficult. Further, there is also the viewpoint of securing the precipitation of TiN and the utilization of scrap in the demand sector. From this, the upper limit of Sol. Al was made 0.04%. Ti forms TIN and remarkably degrades the grain growth. It is an unavoidably included element, so reducing it to zero is industrially difficult. In the present invention, the upper limit was set to 0.0015% as the allowable amount able to be rendered harmless by complex precipitation with MnS, Cu2S, and their complex sulfides etc. If Ti exceeds 0.0015%, the precipitation start temperature of TiN becomes higher and the preferential precipitation of MnS can no longer be controlled. N forms TiN and A1N in addition to BN. In the present invention containing Sol. Al: 0.01 to 0.04%, if A1N is formed, the crystal grain growth remarkably deteriorates, so it is necessary to add B to suppress the formation of A1N. Therefore, if the N becomes higher, the amount of B added has to be increased, so excess addition of B invites embrittlement of the steel sheet and reduces the productivity, so the upper limit of N was made 0.0030%. S is necessary for forming the sulfide forming the nuclei for precipitation of BN and/or TiN. By including 0.0010% or more, the object of the present invention is achieved. However, if exceeding 0.0040%, the amount of precipitation of sulfide itself increases and the crystal grain growth is inhibited, so 0.0040% was made the upper limit. B is an element which has to be added to suppress the formation of A1N harmful to crystal grain growth, but 0.5 or more in terms of B/N has to be added to satisfy this object. Even if added in excess to N, the effect is saturated, so the upper limit was made 1.5 in terms of B/N. Sn, Cu, and Ni have the effect of suppressing nitridation and oxidation of the surface of the steel sheet during annealing, particularly stress relief annealing. The steel of the present invention containing Sol. Al: 0.01 to 0.04% is particularly easily nitrided, so these are preferably added. If the amount of addition is less than 0.01%, there is no effect. Further, if over 0.50%, even if added, the effect becomes saturated and the cost increases, so the range of the amount of addition was made 0.01% to 0.50%. Note that Sn, Cu, and Ni are equal in the effects of suppressing nitridation and oxidation, so may be added alone or in combination to meet the above range of the amount of addition. In addition, one or more of REM, Ca, and Mg may be added in an amount of 0.001 to 0.5%. In particular, Sn is an element extremely effective for improvement of the magnetic flux density in the present invention. The reason is that in the present invention, Mn is increased, so inevitably, the y-»cx transformation temperature becomes lower and the grain growth of the hot rolled sheet cannot be sufficiently promoted, so this has to be made up for. If the amount of addition is less than 0.01%, there is no effect, while even if added over 0.50%, the effect becomes saturated and the cost increases, so the range of the amount of addition was made 0.01% to 0.50%. Further, Sn also has the effect of suppressing nitridation and oxidation of the surface of the steel sheet during stress relief annealing. Addition is preferable from this viewpoint. Regarding the complex precipitation characterizing the present invention, the ratio of the number of sulfides including Mn with which B precipitates complexly precipitate is limited to 10% or more. This is based on the results of observation in samples with almost no independent fine B precipitates. The crystal grain size is an important factor for achieving both stampability and magnetic characteristics. In steel sheet used for stamping, if the grain size exceeds 30 \|om, the stampability deteriorates, so the crystal grain size was made 30 jam or less. Further, in an electrical product, if the grain size is less than 50 nm, the required watt loss is not satisfied, so the crystal grain size at the generally performed stress relief annealing of 750°Cx2 hours was set to 50 (am or more. If the MnS, Cu2S, and their complex sulfides are too great, the crystal grain growth is inhibited. To obtain a crystal grain size of 50 ^m or more in the stress relief annealing, the distribution density must be made 3.0xl05/mm2 or less. The "distribution density" referred to here is the number of precipitates from which Mn, S or Cu, S, or Mn, Cu, and S are detected when observing a mirror polished, then chemically polished sample by a scan type or transmission type electron microscope, divided by the area of the observation field (the total area when observing a plurality of fields). Ti precipitates complexly precipitate about nuclei of sulfides so can be rendered harmless even if so fine so as to be less than 0.1 jjm converted to diameter. A crystal grain size of 50 JJITI or more is obtained in stress relief annealing and a good watt loss is obtained when the distribution density of the Ti precipitates with a diameter equivalent of less than 0.1 jam is l.OxloVmm2 or less, so this was made the upper limit. Next the reasons for limitation of the production conditions in the present invention will be explained. The slab heating in the hot rolling has to be made a two stage continuous cycle in order to make the BN and/or TiN precipitate around nuclei of sulfides including MnS, Cu2S, and their complex sulfides to render them harmless. In the steel of the present invention with Mn increased to 0.4% or more, the precipitation and growth of MnS becomes remarkable in the range of temperature of 1150°C or more, but even over 1250°C, this ends up entering into solid solution, so the first stage of the heating temperature was made 1150°C to 1250°C. Note that the speed of growth of the MnS is fast, so the residence time in this temperature range need only be 5 minutes or more. Next, TiN and BN complexly precipitate over the MnS at a temperature of less than 1150°C/ so the second stage of the heating temperature was made less than 1150°C. From the viewpoint of securing reliability etc., the lower limit temperature was made 1050°C. Note that with a second stage of the heating temperature of less than 1050°C, the TiN precipitates, but the combination with sulfides is insufficient, so independent, fine precipitates increase in the middle of the hot rolling. The second stage heating time was made 15 minutes or more considering the precipitation times of TiN and BN. More preferably, it is 30 minutes or more. The higher the outlet temperature of the finish rolling of the hot rolling above 800°C but not more than the y-MX transformation temperature, the higher the magnetic flux density, but that temperature is eased depending on the amount of addition of Sn, so considering the degree of this relaxation when Sn is added, T>900-lOOOxSn[mass%] was set. Example 1 A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 0.55%, Mn: 0.12 to 0.96%, Sol. Al: 0.033%, Ti: 0.0008%, N: 0.0025%, S: 0.0032%, B: 0.0017%, and Sn: 0.02 to 0.09%. These slabs were raised in temperature to 1230°C, then immediately lowered in temperature to 1120°C, held there for 30 minutes, then hot rolled to a sheet thickness of 2.5 mm. Note that the outlet temperature of the finish rolling was 855°C. The hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finish annealed at 825°C for 10 seconds, and stress relief annealed at 750°C for 2 hours. The thus obtained samples were measured for crystal grain size and watt loss and for magnetic flux density and were observed for precipitates by a transmission type electron microscope. As a result, as shown in Table 5, in Samples 7 to 12 with Mn of 0.4% or more, the average crystal grain sizes of the stress relief annealing were 50 jam or more, good watt losses were obtained, and the ratios of the number of complex precipitates were 10% or more. Further, in Samples 8, 9, 11, and 12 satisfying 855>900-1000xSn, high approximately 0.02T magnetic flux densities were obtained. Table 5 Code Mn Sn Crystal grain size (Urn) W15/50 (W/kg) B50 (T) Complex precipitation ratio (%) Remarks (%) 1 0.02 18 5.51 1.71 0 Comp . ex . 2 0.12 0.05 19 5.48 1.74 0 Comp . ex . 3 0.09 21 5.47 1.74 0 Comp . ex . 4 0.02 29 5.01 1.71 5 Comp . ex . 5 0.26 0.05 30 4.92 1.74 5 Comp . ex . 6 0.09 31 4.87 1.74 5 Comp . ex . 7 0.43 0.02 55 4.12 1.70 15 Inv. ex. 8 0.05 57 4.09 1.73 16 Inv. ex. 9 0.09 59 4.08 1.73 15 Inv. ex. 10 0.96 0.02 61 3.96 1.70 35 Inv. ex. 11 0.05 64 3.92 1.73 42 Inv. ex. 12 0.09 65 3.90 1.73 46 Inv. ex. Example 2 A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 1.3%, Mn: 0.29 to 1.08%, Al: 0.027%, Ti: 0.0013%, N: 0.0019%, S: 0.0026%, B: 0.0024%, and Sn: 0.07%. These slabs were raised in temperature to 1230°C, then immediately lowered in temperature to 1090°C, held there for 30 minutes, then hot rolled. In this heating, the time the slabs were kept at a temperature of 1150°C or more was 15 minutes, and the outlet temperature of the finish rolling was 840°C. In addition, the slabs were tested by being heated at a constant temperature of 1230°C or 1090°C for 60 minutes, then immediately hot rolled. The thus obtained hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 ram, finish annealed at 850°C for 10 seconds, stress relief annealed at 750°C for 2 hours, measured for crystal grain size and watt loss and for magnetic flux density, and observed for precipitates by a transmission type electron microscope. As a result, as shown in Table 6, in Samples 10 to 12 with Mn of 0.4% or more and with hot rolling temperatures of a two-stage cycle of 1230-»1090°C, the average crystal grain sizes of the stress relief annealing were 50 (jm or more, good watt losses were obtained, and the ratios of the number of complex precipitates were also 10% or more. Further, in all samples, 840>900-1000xSn(=0.07%) was satisfied and high magnetic flux densities were obtained. Table 6 Code Hot rolling heating temperature Mn (%) Crystal grain size (Jim) W15/50 (W/kg) B50 (T) Complex precipitation ratio (%) Remarks 1 1090°C 0.29 44 4.11 1.73 5 Comp. ex. 2 0.42 40 4.03 1.72 5 Comp. ex. 3 0.71 42 4.00 1.72 5 Comp. ex. 4 1.08 44 3.96 1.72 5 Comp. ex. 5 0.29 32 4.66 1.73 0 Comp. ex. 6 1230°C 0.42 30 4.53 1.73 0 Comp. ex. 7 0.71 32 4.38 1.73 5 Comp. ex. 8 1.08 33 4.34 1.73 5 Comp. ex. 9 0.29 44 4.01 1.72 5 Comp. ex. 10 i Q^nT1 ki nQfiT 0.42 57 3.39 1.72 20 Inv. ex. 11 0.71 59 3.35 1.72 30 Inv. ex. 12 1.08 64 3.21 1.72 50 Inv. ex. Example 3 A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0038%, Si: 0.51%, Mn: 0.12 to 0.84%, Sol. Al: 0.025%, Ti': 0.0008 to 0.0024%, N: 0.0025%, S: 0.0035%, and B: 0.0016%. These slabs were raised in temperature to 1240°C, then immediately lowered in temperature to 1120°C, held there for 30 minutes, then hot rolled to a sheet thickness of 2.7 mm. In this heating, the time during which the slabs were kept at 1150°C or more in temperature was 22 minutes. Further, the outlet temperature of the finish rolling was 820°C. This hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finish annealed at 825°C for 10 seconds, stress relief annealed at 750°C for 2 hours, then observed for the precipitate and crystal grain size of the steel sheets and measured for watt loss. As a result, as shown in Table 7, in Samples 7, 8, 10, and 11 with Mn of 0.4% or more and Ti of 0.0015% or less, the sulfide densities of the stress relief annealing were S.OxlO5 or less, the average crystal grain sizes were 50 (Jin or more, and good watt losses were obtained. In these samples, there were many spherical sulfides with a diameter of 0.2 to 0.3 pm and a large number of the sulfides were confirmed to have a plurality of Ti precipitates precipitated at their outer circumferences. On the other hand, in Samples 1 to 6 with low Mn of 0.12 and 0.25%, the sulfide densities of the stress relief annealing were high, the average crystal grain sizes were small, and the watt losses were poor. The sulfides seen in these samples had small diameters of 0.1 pm or less. A large number of the sulfides could be confirmed to contain fine Ti precipitates. Further, in Samples 9 and 12 with Mn of 0.4% or more, but Ti of over 0.0015% as well, the sulfide densities of the stress relief annealing were high, the average crystal grain sizes were also small, and the watt losses were relatively poor. In these samples, the sulfides varied in diameter over a broad range of 0.05 to 0.3 pm. The forms of combination with the Ti precipitates were also varied including the outer circumferences and centers of the sulfides. Note that the distribution densities of the Ti precipitates with diameters of less than O.lpm were low ones of less than 1.0xl03/mm2 in all samples. Table 7 Code Mn Ti Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Urn) W15/50 (W/kg) Remarks ( ) 1 0.0006 6.3xl05 2 0.12 0.0011 7.7xl05 3 0.0016 8.9xl05 4 0.0006 3.5xl05 5 0.25 0.0011 4.3xl05 6 0.0016 4.9xl05 7 0.0006 1.3xl05 8 0.48 0.0011 l.SxlO5 9 0.0016 3.6xl05 10 0.0006 1.2xl05 11 0.84 0.0011 l.SxlO5 12 0.0016 3.3xl05 Example 4 A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0022%, Si: 1.2%, Mn: 0.31 to 1.44%, Sol. Al: 0.03%, Ti: 0.0013%, N: 0.0016%, S: 0.0031%, B: 0.0021%, and Sn: 0.02%. The slabs were raised in temperature to 1220°C, then immediately lowered in temperature to 1070°C, held there for 20 minutes, then hot rolled. In this heating, the time during which the slabs were kept at 1150°C or more in temperature was 15 minutes. In addition to this, the slabs were tested by heating at a constant temperature of 1220°C or 1070°C for 45 minutes then immediately hot rolled. The thus obtained hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finished annealed at 850°C for 5 seconds, stress relief annealed at 750°C for 2 hours, observed for precipitate of the steel sheet and crystal grain size, and measured for watt loss. As a result, as shown in Table 8, in Samples 12 to 15 with Mn of 0.4% or more and a heating temperature of the hot rolling of 1220-»1070°C, the sulfide densities of the stress relief annealing were 3.0xl05 or less, the average crystal grain sizes were 50 Urn or more, and good watt losses were obtained. In these samples, there were a large number of spherical sulfides of a diameter of 0.2 to 0.3 \|4m and a large number of the sulfides were confirmed to have a plurality of Ti precipitates precipitated at their outer circumferences. In the other samples, the sulfide densities of the stress relief annealing were high, the Ti precipitate densities were high, the average crystal grain sizes were also small, and the watt losses were poor. Table 8 Code Hot rolling heating temperature Mn (%) Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Urn) W15/50 (W/kg) Remarks 1 0.31 4.0xl05 2 0.45 4.1xl05 3 1070°C 0.68 3.7xl05 4 1.03 3.4xl05 5 1.44 3.3xl05 6 0.31 4.7xl05 5.5xl03 32 4.64 Comp. ex. 7 1220°C 0.45 2.7xl05 5.7xl03 30 4.51 Comp. ex. 8 0.68 l.SxlO5 6.1xl03 32 4.45 Comp. ex. 9 1.03 1.3xl05 7.7xl03 33 4.36 Comp. ex. 10 1.44 1.2xl05 7.2xl03 34 4.41 Comp. ex. 11 0.31 3.5xl05 12 0.45 2.6xl05 13 1220-»1070°C 0.68 2.2xl05 14 1.03 l.SxlO5 15 1.44 1.4xl05 We claim 1. A non-oriented magnetic steel sheet characterized by containing, by mass%, Si:1.5% or less, Mn:0.40% to 1.5%, Sol.Al:0.01% to 0.04%, Ti:0.0015% or less, N;0.0030% or less, S:0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and optionally containing one or more of Sn , Cu, Ni, REM, Ca, Mg and where Sn , Cu and Ni in total amount of 0.01% to 0.50%, and REM, Ca and Mg in a total of 0.001% to 0.05%, and the balance of Fe and unavoidable impurities, and 10% or more of the particles of sulfide containing Mn , complexly precipitating with the B precipitates, and a distribution density of a total of MnS, Cu2S, and their complex sulfides of 3.0x105/mm2 or less, and a distribution density of the Ti precipitates not reaching a diameter of 0.1 µm of 1.0xl03/ mm or less. 2. A non-oriented magnetic steel sheet as claimed in claim 1, wherein a crystal grain size of the steel sheet after stress relief annealing being 50um or more. 3. A method of production of a non-oriented magnetic steel sheet as claimed in claim 1 or 2, wherein the steel sheet is produced by a process comprising the steps of; steel making, slab heating at a temperature range at 1150°C to 1250°c and keeping the slab for 5 minutes or more, then further keeping the slab at a temperature range at 1050°C to less than 1150°C for 15 minutes or more, immediately commencing hot rolling and making a finish outlet temperature of 800°C or more, pickling, and cold rolling. 4. A method of production of a non-oriented magnetic steel sheet as claimed in claim 3, wherein when the steel sheet contains Sn as an indispensable element, the finish outlet temperature T(°C) is defined as T(°C)>900-.1000xSn[mass%] 5. A non-oriented magnetic steel sheet and a method of production of a non-oriented magnetic steel sheet substantially such as herein described with reference to foregoing examples.

Full Text

TECHNICAL FIELD
The present invention relates to a nonoriented magnetic steel sheet used as a core material of electrical equipment and a method of production of the same, more particularly relates to a nonoriented magnetic steel sheet excellent in stampability and magnetic characteristics of stress relief annealing and a method of production of the same.
BACKGROUND ART
In recent years, the global rise in energy saving in electrical equipment has led to demands for higfier performance characteristics in nonoriented magnetic steel sheet used as core materials of rotating machines. The motors for "high efficiency model" electrical products are now using high grade materials increased in the content of Si and Al to raise the specific resistance and increased in crystal grain size. On the other hand, general use model motors are also being required to be improved in performance, but due to severe cost restrictions, it is difficult to switch to high grade materials such as with high efficiency models.
The steel..sheet used for general use models has been strikingly improved in watt loss by giving it an Si content of 1.5% or less and promoting the crystal grain growth at the time of the stress relief annealing performed after stamping out the motor core. Further, recently, an increasing number of users are utilizing the scrap produced when stamping out cores as materials for castings. From the viewpoint of securing the castability of the scrap, it has become necessary to reduce the content of Al in the steel sheet to less than 0.05%.

To improve the crystal grain growth at the time of stress relief annealing, it is important to reduce or render harmless the precipitates unavoidably mixed in the steel. The designs of materials of nonoriented magnetic steel sheet with Al: less than 0.05% may be roughly divided into two groups. One, as seen in Japanese Patent Publication (A) No. 54-163720, is the method of adding 0.002% or so of B to Al-killed steel (Sol. Al: 0.02% or so) to form a nitride constituted by BN and suppress the precipitation of A1N harmful to crystal grain growth.
On the other hand, as seen in Japanese Patent Publication (A) No. 7-150248, the method of controlling the ratios of the oxides of Si02 and MnO included in the Si-killed steel (Sol. Al As a method for suitably controlling the nitrides or oxides produced along with the deoxidation and further reducing or rendering harmless the sulfides to improve the magnetic characteristics, for example Japanese Patent Publication (A) No. 58-117828 discloses a method of limiting the S to 0.005% or less to obtain an effect of improvement of the magnetic characteristics, Japanese Patent Publication (A) No. 58-164724 discloses a method of adding Ca or a rare earth element to fix the S when including S: 0.01 to 0.02%, and Japanese Patent Publication (A) No. 4-63228 discloses a method of limiting S to 0.0050% or less and limiting the heating temperature of the slab 1100°C or less so as to prevent fine precipitation of MnS during the hot rolling.
DISCLOSURE OF THE INVENTION
With further reduction of the watt loss required, it is becoming difficult to produce the desired sheet sufficiently and stably by the above techniques. The present invention was made in consideration of this

problem and provides a steel sheet with a good crystal grain growth of stress relief annealing, low watt loss, and high magnetic flux density by means of improvement focusing on the B added for the purpose of suppressing precipitation of A1N and the S as an element unavoidably mixed in the steel.
The present invention was made to solve the above problem and has as its gist the following:
(1) A nonoriented magnetic steel sheet
characterized by containing, by mass%, Si: 1.5% or less, Mn: 0.4% to 1.5%, Sol. Al: 0.01% to 0.04%, Ti: 0.0015% or less, N: 0.0030% or less, S: 0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and the balance of Fe and unavoidable impurities, 10% or more of the particles of sulfide containing Mn, in terms of ratio of number, complexly precipitating with the B precipitates.
(2) A nonoriented magnetic steel sheet
characterized by containing, by mass%, Si: 1.5% or less, Mn: 0.4% to 1.5%, Sol. Al: 0.01% to 0.04%, Ti: 0.0015% or less, N: 0.0030% or less, S: 0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and the balance of Fe and unavoidable impurities, a crystal grain size of the steel sheet being 30 urn or less and a crystal grain size of stress relief annealing of 750°Cx2 hours being 50 pm or more.
(3) A nonoriented magnetic steel sheet as set forth
in (1) or (2), characterized in that a distribution
density of a total of MnS, CuaS, and their complex
sulfides is 3.0xl05/mm2 or less.
(4) A nonoriented magnetic steel sheet as set forth
in any one of (1) to (3), characterized in that a
distribution density of the Ti precipitates not reaching
a diameter of 0.1 jjm is l.OxloVmm2 or less.
(5) A nonoriented magnetic steel sheet as set forth
in any one of (1) to (4), characterized by further
containing one or more of Sn, Cu, and Ni in a total mass%
of 0.01% to 0.50% and/or one or more of REM, Ca, and Mg

in an amount of 0.001 to 0.5%.
(6) A method of production of a nonoriented
magnetic steel sheet as set forth in any one of (1) to
(5), characterized, during finish annealing after steelmaking, hot rolling, pickling, and cold rolling, by heating a slab for hot rolling by keeping it in a range of 1150°C to 1250°C for 5 minutes or more, then keeping it in a range of 1050°C to less than 1150°C for 15 minutes or more, then immediately hot rolling it.
(7) A method of production of a nonoriented
magnetic steel sheet as set forth in any one of (1) to
(6), characterized, during finish annealing after steelmaking, hot rolling, pickling, and cold rolling,
l
making an outlet temperature of finish rolling of the hot rolling 800°C or more.
(8) A method of production of a nonoriented
magnetic steel sheet as set forth in (6), characterized
by making a finish outlet temperature T (°C) of the hot
rolling, in steel sheet containing Sn, T>900-
1000xSn[mass%].
BEST MODE FOR WORKING THE INVENTION
The inventors ran against the problem that in steel with an Si of 1.5% or less, even if reducing the unavoidably included element S to 0.0010% or so and reducing the slab heating temperature to 1100°C or less like in Japanese Patent Publication (A) No. 4-63228, the watt loss of the stress relief annealing fluctuates and will not stabilize. The inventors investigated the causes and learned that despite the amount of S and the slab heating temperature being low, the steel has a large amount of MnS, CU2S, and their complex sulfides finely dispersed in it and the crystal grain growth of the stress relief annealing is remarkably suppressed. Observed more closely, these sulfides form spheres of diameters of 0.1 to 0.3 (jm or so, but the centers contain Ti precipitates of diameter equivalents of 0.05 (am or so.

The reason why the sulfides take this form of precipitation is that the TiN initially precipitating after the casting or by the heating of the hot rolled slab finely disperses and forms the nuclei for precipitation of the sulfides.
To deal with this situation, the inventors took note of sulfides with a faster speed of growth and easier roughening compared with TiN. That is, as opposed to the current situation of using TiN as the nuclei for complex precipitation of sulfides, the inventors experimented with using sulfides as the nuclei for complex precipitation of TiN and as a result discovered that stable, low watt loss is obtained. Further, they discovered that a low watt loss is also obtained stably by complex precipitation of sulfides for the BN formed fixing the excess N explained later and thereby completed the present invention. Below, the results of experiments leading to the present invention will be explained.
(Experiment 1)
A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 0.6%, Mn: 0.1 to 0.8%, Sol. Al: 0.03%, Ti: 0.0012%, N: 0.0021%, S: 0.0005 to 0.0025%, B: 0.0020%, and Sn: 0.08%. These slabs were soaked at 1200°C for 20 minutes, raised in temperature over 10 minutes to 1100°C/ soaked for 30 minutes, then hot rolled to a sheet thickness of 2.5 mm, pickled, and cold rolled to a sheet thickness of 0.50 mm.
The thus obtained cold rolled sheets were finish annealed at 800°C for 10 seconds, then were stress relief annealed at 750°C for 2 hours and measured for crystal grain size and watt loss. As a result, as shown in Table 1, in Samples 8, 9, 11, and 12 with Mn of 0.4% or more and with S of 0.0012 and 0.0025%, the crystal grain sizes of the stress relief annealing were 50 |om or more and good watt losses were obtained.
Next, the stress relief annealed samples were

observed for precipitates, whereupon in Samples 8, 9, 11, and 12 with good watt loss, complex precipitation with the B precipitate was observed at a numerical ratio of 10% or more of the sulfides including the Mn. On the other hand, in the other samples with the poor watt loss, a large number of fine precipitates of less than a diameter of 0.1 um thought to be precipitates of B was observed.
The effect of improvement of the crystal grain growth of the stress relief annealing and watt loss appearing at Mn: 0.4% or more and S: 0.0012 and 0.0025% is believed to appear as follows: First, by increasing the Mn, the precipitation start temperature of MnS rises. Due to this, MnS precipitates at the time of heating of the hot rolling before the BN and, further, the later precipitating BN precipitates complexly about MnS nuclei. Due to this, it is possible to suppress the formation of fine precipitates of B and obtain good crystal grain growth of the stress relief annealing and watt loss.
On the other hand, with Mn of less than 0.4%, at the stage where BN precipitates, the precipitation of MnS is not sufficient. Further, with S is 0.0005%, the amount of precipitation of MnS itself is small, so probably in both cases the nuclei for precipitation of BN are insufficient and the characteristics of stress relief annealing are not improved by the sole, fine precipitation of B.
7

Table 1

Mn S Crystal Complex
Code (* U grain size (Urn) W15/50 (W/kg) precipitation ratio (%) Remarks
1 0.0005 30 4.88 0 Comp . ex .
2 0.1 0.0012 25 5.02 0 Comp. ex.
3 0.0025 19 5.21 5 Comp . ex .
4 0.0005 34 4.78 5 Comp . ex .
5 0.2 0.0012 29 4.89 5 Comp . ex .
6 0.0025 21 5.05 5 Comp . ex .
7 0.0005 35 4.72 40 Comp . ex .
8 0.4 0.0012 62 3.65 30 Inv. ex.
9 0.0025 54 4.03 20 Inv. ex.
10 0.0005 43 4.55 50 Comp . ex .
11 0.8 0.0012 65 3.51 45 Inv. ex.
12 0.0025 58 3.88 40 Inv. ex.
{Experiment 2)
A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0034%, Si: 0.75%, Mn: 0.15 to 0.72%, Sol. Al: 0.019%, Ti: 0.0008 to 0.0017%, N: 0.0018%, S: 0.0023%, B: 0.0025%, Sn: 0.03%, Cu: 0.01%, and Ni: 0.02%. These slabs were soaked at 1200°C for 5 minutes, lowered in temperature to 1100°C, soaked for 30 minutes/ then hot rolled to a sheet thickness of 2.7 mm, pickled, then cold rolled to a sheet thickness of 0.50 mm. The thus obtained cold rolled sheets were finish annealed at 800°C for 10 seconds, then were stress relief annealed at 750°C for 2 hours then observed for the precipitates and crystal grain sizes of the steel sheets and measured for watt loss.
As a result, as shown in Table 2, in Samples 4, 5, If and 8 with Mn of 0.4% or more and with Ti of 0.0015% or less, the sulfide densities of the stress relief annealing were 3.0xl05 or less, the average crystal grain sizes were 50 ^m or more, and the watt losses were good. In these samples, it was confirmed that there were many spherical sulfides with diameters of 0.2 to 0.3 jam and a

large number had a plurality of fine Ti precipitates with diameter equivalents less than 0.1 pm precipitated at their outer circumferences. On the other hand, in Samples 1 to 3 with Mn of low 0.15%, the sulfide densities of the stress relief annealing were 4.5xl05/mm2 or more, the average crystal grain sizes were small 35 jjm or less, and the watt losses were poor. It was confirmed that the sulfides seen in these samples had diameters of small 0.1 pm or less and a large number of sulfides contained Ti precipitates at their centers. Further, in Samples 6 and 9 with Mn of 0.4% or more and with Ti of over 0.0015% as well, the sulfide densities of the stress relief annealing were high 3.8xl05/mm2 or more, the average crystal grain sizes were also 45 |im or less, and the watt losses were relatively poor. In these samples, the sulfides varied in diameter in a broad range of 0.05 to 0.3 pm or less. The form of complexing with the Ti precipitates also varied including the outer circumferences or the centers. Note that the distribution density of Ti precipitates with diameter equivalents of less than 0.1 jam was a low less than 1.0xl03/mm2 in all samples.
The effect of raising the Mn to 0.4% or more and limiting Ti to 0.0015% or less in this way is believed to be as follows: Further, by raising the Mn, the precipitation start temperature of the MnS rises, while by lowering the Ti, the precipitation start temperature of the TiN falls. Due to this, the order of precipitation inverts from the usual and MnS precipitates before the TiN. Next the MnS rising in precipitation start temperature precipitates and grows at the 1200°C of the first stage of the hot rolling heating. On the other hand, the TiN dropping in precipitation start temperature is believed to precipitate about the nuclei of the
already roughened MnS at the 1100°C of the second stage of

the hot rolling heating. Table 2

Code Mn Ti Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Mm) W15/50 (W/kg) Remarks

(%)

I 0.15 0.0008 4.5xl05 2
0.0013 4.8xl05 3
0.0017 5.4xl05 4 0.42 0.0008 2.8xl05 5
0.0013 2.7xl05 6
0.0017 3.9xl05 7 0.0008 l.SxlO5 8 0.72 0.0013 1.9xl05 9
0.0017 3.8xl05 (Experiment 3)
Next, to investigate the effects of the heating conditions of the hot rolling, slabs of Experiment 1 with Mn: 0.4% and S: 0.0025% were hot rolled at heats of two levels of soaking at 1200°C for 60 minutes and soaking at 1100°C for 60 minutes, then were pickled and further processed by the same process as in Experiment 1 to fabricate and compare and evaluate samples. As a result, as shown in Table 3, compared with Sample 1 with the good results (Sample 9 in Experiment 1), in Sample 2 soaked at 1200°C, the ratio of complex precipitation was substantially zero, a large number of independently fine B precipitates were observed, and the crystal grain size of the stress relief annealing and watt loss were the worst. On the other hand, in Sample 3 soaked at 1100°C, while not to the extent of the 1200°C heated material (Sample 2), the ratio of complex precipitation was a low 5% and the crystal grain size and watt loss were not that good. This result is interpreted as follows. First, when

soaking at 1200°C, since Mn is a high 0.4-2% and MnS precipitates, the B is hot rolled in the unprecipitated state, so during the hot rolling or stress relief annealing, independent fine B precipitates are formed and the crystal grain growth and watt loss remarkably deteriorate. Next, when soaked at 1100°C, the distribution of the MnS becomes rough and the number of nuclei for complex precipitation of BN becomes insufficient, so it is believed that part of the B ends up precipitating independently and finely and therefore a good crystal grain size and watt loss cannot be obtained. Table 3

Code Hot rolling heating temperature Crystal grain size (Urn) W15/50 (W/kg) Complex precipitation ratio (%) Remarks
1 1200°C-»1100°C 54 4.03 25 Inv. ex.
2 1200°C 19 5.68 0 Comp. ex.
3 1100°C 35 4.51 5 Comp . ex .
(Experiment 4)
Next, to investigate the effects of the heating cycle of the hot rolling, slabs of Experiment 2 with Mn: 0.42% and Ti: 0.0013% were hot rolled at heats of two
levels of soaking at 1200°C for 60 minutes and soaking at 1100°C for 60 minutes, then were pickled and further processed by the same process as in Experiment 2 to fabricate and compare and evaluate samples. As a result, as shown in Table 4, compared with Sample 1 with the good results (Sample 5 in Experiment 2), in Sample 2 soaked at 1200°C, the sulfide density was low, but the distribution density of the Ti precipitates was high and the crystal grain size of the stress relief annealing and watt loss were the worst. On the other hand, in Sample 3 soaked at 1100°C, the Ti precipitate density was low, but the sulfide density was high and the crystal grain size and the watt loss were both excellent. This result is interpreted as follows. First, when soaking at 1200°C, the

Mn was a high 0.42% and roughening of the MnS proceeded, but the slab was hot rolled with the TiN not yet precipitated, independent fine Ti precipitates were formed during the hot rolling or at the time of stress relief annealing, and the crystal grain growth and watt loss remarkably deteriorated. Next, when soaked at 1100°C, the growth of MnS was insufficient, so it is believed the number of sulfides increased and good crystal grain size and watt loss could not be obtained. Table 4

Code Hot rolling heating temperature Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Mm) W15/50 (W/kg) Remarks
1 1200°C-»1100°C 2.7xl05 2 1200°C l.SxlO5 5.6xl03 25 5.44 Comp. ex.
3 1100°C 4.8xl05 Summarizing the above, the present invention discovers that by optimizing the amount of Mn and the heating cycle of the hot rolling so as to cause MnS to preferentially and optimally precipitate and simultaneously cause fine BN to complexly precipitate, both MnS and BN are simultaneously made harmful and the crystal grain growth and watt loss are improved. To realize this, the amount of Mn has to be raised and simultaneously the amount of Al has to be lowered. The reason is that Al consumes N as A1N and the AlN itself inhibits crystal grain growth. On the other hand, with the method of production of not adding any Al at all, the amount of Al is kept extremely low, which is perfect for formation of BN, but the large number of SiC>2*MnO inclusions remaining in the steel are flattened along with the increase in the amount of Mn and conversely the grain growth ends up being worsened. Therefore, the present invention discovered that the optimal range of Al for suppression of flattening due to modification of the

inclusions and for preferential precipitation of BN is 0.01% to 0.04%.
Further, the present invention discovers that by optimizing the precipitation temperatures of MnS and TiN and optimizing the heating cycle of the hot rolling so as to first cause the coarse precipitation of MnS and then cause the complex precipitation of fine TiN, both precipitates are rendered harmless and the crystal grain growth and watt loss are improved.
To realize this, it is necessary to raise the amount of Mn, raise the precipitation temperature of MnS, further lower the amount of Al to suppress the formation of A1N, and promote the precipitation of TiN. As the method of keeping the amount of Al extremely low, the method of using Si for deoxidation in steelmaking may be mentioned, but in this case, if increasing the amount of Mn, it is learned that the inclusions are flattened and conversely the watt loss is made worse. Therefore, in the present invention, by controlling the amount of Al to the relatively small range of 0.01 to 0.04%, it is possible to maintain the precipitation of TiN while modifying the inclusions to thereby increase the amount of Mn. Further, in this range of amount of Al, the excess N after precipitation of TiN results in easy fine precipitation of A1N harmful to grain growth, so this is avoided by adding a small amount of B to obtain BN.
This technical idea was first discovered by the present invention. For example, Japanese Patent Publication (A) No. 58-117828 defines a content of Si: 0.1 to 1.0%, Al: less than 0.1%, Mn: 0.75 to 1.5%, N/B: 0.7 to 1.2 of B, but does not define complex precipitation of MnS and BN and the amount of Sol. Al for realizing this or the heating temperature of the hot rolling, so completely differs from the present invention in technical idea and could not have enabled deduction of the present invention. Further, Japanese Patent Publication (A) No. 2000-248344 defines a content of Si:

1.8% or less, Sol. Al: 0.05 to 0.20%, and Mn: 0.05 to 1.5%, but if Sol. Al is over 0.04%, A1N ends up precipitating with priority over BN, so the technical idea of the present invention of precipitating BN around MnS does not stand.
Next, the reasons for numerical limitation of the ingredients and products in the present invention will be explained.
Si is an element effective for increasing the electrical resistance, but if added over 1.5%, the hardness rises, the magnetic flux density falls, and the cost rises, so the content was limited to 1.5%.
Mn is an important element for realizing the present invention. The present invention has as its main gist the precipitation of BN and/or TiN around sulfide containing MnS. Therefore, it is necessary to make MnS sufficiently precipitate before the precipitation temperature of BN and/or TiN. In the present invention containing Ti: 0.0015% or less, N: 0.0030% or less, and B/N of 0.5 to 1.5 of B, the object is achieved by making the Mn 0.4% or more. Further, if added over 1.5%, the drop in the saturated magnetic flux density becomes remarkable. In addition, the y-Mx transformation temperature falls and control of the structure of the hot rolled sheet becomes difficult, so 1.5% was made the upper limit.
Al is an element required for deoxidation of steel. If less than Sol. 0.01%, the unremoved oxygen remains in the steel and oxides of SiC^'MnO are formed. These are flattened along with the effects of the Mn added in an amount of 0.4% or less and thereby inhibit crystal grain growth, so the lower limit of Sol. Al was made 0.01%. Further, if Sol. Al exceeds 0.04%, A1N precipitates instead of BN and achievement of the present invention becomes difficult. Further, there is also the viewpoint of securing the precipitation of TiN and the utilization of scrap in the demand sector. From this, the upper limit of Sol. Al was made 0.04%.

Ti forms TIN and remarkably degrades the grain growth. It is an unavoidably included element, so reducing it to zero is industrially difficult. In the present invention, the upper limit was set to 0.0015% as the allowable amount able to be rendered harmless by complex precipitation with MnS, Cu2S, and their complex sulfides etc. If Ti exceeds 0.0015%, the precipitation start temperature of TiN becomes higher and the preferential precipitation of MnS can no longer be controlled.
N forms TiN and A1N in addition to BN. In the present invention containing Sol. Al: 0.01 to 0.04%, if A1N is formed, the crystal grain growth remarkably deteriorates, so it is necessary to add B to suppress the formation of A1N. Therefore, if the N becomes higher, the amount of B added has to be increased, so excess addition of B invites embrittlement of the steel sheet and reduces the productivity, so the upper limit of N was made 0.0030%.
S is necessary for forming the sulfide forming the nuclei for precipitation of BN and/or TiN. By including 0.0010% or more, the object of the present invention is achieved. However, if exceeding 0.0040%, the amount of precipitation of sulfide itself increases and the crystal grain growth is inhibited, so 0.0040% was made the upper limit.
B is an element which has to be added to suppress the formation of A1N harmful to crystal grain growth, but 0.5 or more in terms of B/N has to be added to satisfy this object. Even if added in excess to N, the effect is saturated, so the upper limit was made 1.5 in terms of B/N.
Sn, Cu, and Ni have the effect of suppressing nitridation and oxidation of the surface of the steel sheet during annealing, particularly stress relief annealing. The steel of the present invention containing Sol. Al: 0.01 to 0.04% is particularly easily nitrided,

so these are preferably added. If the amount of addition is less than 0.01%, there is no effect. Further, if over 0.50%, even if added, the effect becomes saturated and the cost increases, so the range of the amount of addition was made 0.01% to 0.50%. Note that Sn, Cu, and Ni are equal in the effects of suppressing nitridation and oxidation, so may be added alone or in combination to meet the above range of the amount of addition. In addition, one or more of REM, Ca, and Mg may be added in an amount of 0.001 to 0.5%.
In particular, Sn is an element extremely effective for improvement of the magnetic flux density in the present invention. The reason is that in the present invention, Mn is increased, so inevitably, the y-»cx transformation temperature becomes lower and the grain growth of the hot rolled sheet cannot be sufficiently promoted, so this has to be made up for. If the amount of addition is less than 0.01%, there is no effect, while even if added over 0.50%, the effect becomes saturated and the cost increases, so the range of the amount of addition was made 0.01% to 0.50%. Further, Sn also has the effect of suppressing nitridation and oxidation of the surface of the steel sheet during stress relief annealing. Addition is preferable from this viewpoint.
Regarding the complex precipitation characterizing the present invention, the ratio of the number of sulfides including Mn with which B precipitates complexly precipitate is limited to 10% or more. This is based on the results of observation in samples with almost no independent fine B precipitates.
The crystal grain size is an important factor for achieving both stampability and magnetic characteristics. In steel sheet used for stamping, if the grain size exceeds 30 |om, the stampability deteriorates, so the crystal grain size was made 30 jam or less. Further, in an electrical product, if the grain size is less than 50 nm,

the required watt loss is not satisfied, so the crystal grain size at the generally performed stress relief annealing of 750°Cx2 hours was set to 50 (am or more.
If the MnS, Cu2S, and their complex sulfides are too great, the crystal grain growth is inhibited. To obtain a crystal grain size of 50 ^m or more in the stress relief annealing, the distribution density must be made 3.0xl05/mm2 or less. The "distribution density" referred to here is the number of precipitates from which Mn, S or Cu, S, or Mn, Cu, and S are detected when observing a mirror polished, then chemically polished sample by a scan type or transmission type electron microscope, divided by the area of the observation field (the total area when observing a plurality of fields).
Ti precipitates complexly precipitate about nuclei of sulfides so can be rendered harmless even if so fine so as to be less than 0.1 jjm converted to diameter. A crystal grain size of 50 JJITI or more is obtained in stress relief annealing and a good watt loss is obtained when the distribution density of the Ti precipitates with a diameter equivalent of less than 0.1 jam is l.OxloVmm2 or less, so this was made the upper limit.
Next the reasons for limitation of the production conditions in the present invention will be explained.
The slab heating in the hot rolling has to be made a two stage continuous cycle in order to make the BN and/or TiN precipitate around nuclei of sulfides including MnS, Cu2S, and their complex sulfides to render them harmless. In the steel of the present invention with Mn increased to 0.4% or more, the precipitation and growth of MnS becomes remarkable in the range of temperature of 1150°C or more, but even over 1250°C, this ends up entering into solid solution, so the first stage of the heating temperature was made 1150°C to 1250°C. Note that the speed of growth of the MnS is fast, so the residence time in this temperature range need only be 5 minutes or more.

Next, TiN and BN complexly precipitate over the MnS at a temperature of less than 1150°C/ so the second stage of the heating temperature was made less than 1150°C. From the viewpoint of securing reliability etc., the lower limit temperature was made 1050°C. Note that with a second stage of the heating temperature of less than 1050°C, the TiN precipitates, but the combination with sulfides is insufficient, so independent, fine precipitates increase in the middle of the hot rolling. The second stage heating time was made 15 minutes or more considering the precipitation times of TiN and BN. More preferably, it is 30 minutes or more.
The higher the outlet temperature of the finish rolling of the hot rolling above 800°C but not more than the y-MX transformation temperature, the higher the magnetic flux density, but that temperature is eased depending on the amount of addition of Sn, so considering the degree of this relaxation when Sn is added, T>900-lOOOxSn[mass%] was set.
Example 1
A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 0.55%, Mn: 0.12 to 0.96%, Sol. Al: 0.033%, Ti: 0.0008%, N: 0.0025%, S: 0.0032%, B: 0.0017%, and Sn: 0.02 to 0.09%. These slabs were raised in temperature to 1230°C, then immediately lowered in temperature to 1120°C, held there for 30 minutes, then hot rolled to a sheet thickness of 2.5 mm. Note that the outlet temperature of the finish rolling was 855°C. The hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finish annealed at 825°C for 10 seconds, and stress relief annealed at 750°C for 2 hours. The thus obtained samples were measured for crystal grain size and watt loss and for magnetic flux density and were observed for precipitates by a transmission type electron microscope.

As a result, as shown in Table 5, in Samples 7 to 12 with Mn of 0.4% or more, the average crystal grain sizes of the stress relief annealing were 50 jam or more, good watt losses were obtained, and the ratios of the number of complex precipitates were 10% or more. Further, in Samples 8, 9, 11, and 12 satisfying 855>900-1000xSn, high approximately 0.02T magnetic flux densities were obtained.
Table 5

Code Mn Sn Crystal grain size (Urn) W15/50 (W/kg) B50 (T) Complex precipitation ratio (%) Remarks

(%)

1 0.02 18 5.51 1.71 0 Comp . ex .
2 0.12 0.05 19 5.48 1.74 0 Comp . ex .
3 0.09 21 5.47 1.74 0 Comp . ex .
4 0.02 29 5.01 1.71 5 Comp . ex .
5 0.26 0.05 30 4.92 1.74 5 Comp . ex .
6
0.09 31 4.87 1.74 5 Comp . ex .
7 0.43 0.02 55 4.12 1.70 15 Inv. ex.
8
0.05 57 4.09 1.73 16 Inv. ex.
9
0.09 59 4.08 1.73 15 Inv. ex.
10 0.96 0.02 61 3.96 1.70 35 Inv. ex.
11
0.05 64 3.92 1.73 42 Inv. ex.
12
0.09 65 3.90 1.73 46 Inv. ex.
Example 2
A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.003%, Si: 1.3%, Mn: 0.29 to 1.08%, Al: 0.027%, Ti: 0.0013%, N: 0.0019%, S: 0.0026%, B: 0.0024%, and Sn: 0.07%. These slabs were raised in temperature to 1230°C, then immediately lowered in temperature to 1090°C, held there for 30 minutes, then hot rolled. In this heating, the time the slabs were kept at a temperature of 1150°C or more was 15 minutes, and the outlet temperature of the finish rolling was 840°C. In addition, the slabs were tested by being heated at a constant temperature of 1230°C or 1090°C for 60 minutes, then immediately hot rolled. The thus obtained hot rolled

sheets were pickled, then cold rolled to a sheet thickness of 0.50 ram, finish annealed at 850°C for 10 seconds, stress relief annealed at 750°C for 2 hours, measured for crystal grain size and watt loss and for magnetic flux density, and observed for precipitates by a transmission type electron microscope. As a result, as shown in Table 6, in Samples 10 to 12 with Mn of 0.4% or more and with hot rolling temperatures of a two-stage cycle of 1230-»1090°C, the average crystal grain sizes of the stress relief annealing were 50 (jm or more, good watt losses were obtained, and the ratios of the number of complex precipitates were also 10% or more. Further, in all samples, 840>900-1000xSn(=0.07%) was satisfied and high magnetic flux densities were obtained. Table 6

Code Hot rolling heating temperature Mn (%) Crystal grain size (Jim) W15/50 (W/kg) B50 (T) Complex precipitation ratio (%) Remarks
1 1090°C 0.29 44 4.11 1.73 5 Comp. ex.
2
0.42 40 4.03 1.72 5 Comp. ex.
3
0.71 42 4.00 1.72 5 Comp. ex.
4
1.08 44 3.96 1.72 5 Comp. ex.
5 0.29 32 4.66 1.73 0 Comp. ex.
6 1230°C 0.42 30 4.53 1.73 0 Comp. ex.
7
0.71 32 4.38 1.73 5 Comp. ex.
8
1.08 33 4.34 1.73 5 Comp. ex.
9 0.29 44 4.01 1.72 5 Comp. ex.
10 i Q^nT1 ki nQfiT 0.42 57 3.39 1.72 20 Inv. ex.
11 0.71 59 3.35 1.72 30 Inv. ex.
12 1.08 64 3.21 1.72 50 Inv. ex.
Example 3

A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0038%, Si: 0.51%, Mn: 0.12 to 0.84%, Sol. Al: 0.025%, Ti': 0.0008 to 0.0024%, N: 0.0025%, S: 0.0035%, and B: 0.0016%. These slabs were raised in temperature to 1240°C, then immediately lowered in temperature to 1120°C, held there for 30 minutes, then hot rolled to a sheet thickness of 2.7 mm. In this heating, the time during which the slabs were kept at 1150°C or more in temperature was 22 minutes. Further, the outlet temperature of the finish rolling was 820°C. This hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finish annealed at 825°C for 10 seconds, stress relief annealed at 750°C for 2 hours, then observed for the precipitate and crystal grain size of the steel sheets and measured for watt loss.
As a result, as shown in Table 7, in Samples 7, 8, 10, and 11 with Mn of 0.4% or more and Ti of 0.0015% or less, the sulfide densities of the stress relief annealing were S.OxlO5 or less, the average crystal grain sizes were 50 (Jin or more, and good watt losses were obtained. In these samples, there were many spherical sulfides with a diameter of 0.2 to 0.3 pm and a large number of the sulfides were confirmed to have a plurality of Ti precipitates precipitated at their outer circumferences. On the other hand, in Samples 1 to 6 with low Mn of 0.12 and 0.25%, the sulfide densities of the stress relief annealing were high, the average crystal grain sizes were small, and the watt losses were poor. The sulfides seen in these samples had small diameters of 0.1 pm or less. A large number of the sulfides could be confirmed to contain fine Ti precipitates. Further, in Samples 9 and 12 with Mn of 0.4% or more, but Ti of over 0.0015% as well, the sulfide densities of the stress relief annealing were high, the average crystal grain sizes were also small, and the watt losses were

relatively poor. In these samples, the sulfides varied in diameter over a broad range of 0.05 to 0.3 pm. The forms of combination with the Ti precipitates were also varied including the outer circumferences and centers of the sulfides. Note that the distribution densities of the Ti precipitates with diameters of less than O.lpm were low ones of less than 1.0xl03/mm2 in all samples. Table 7

Code Mn Ti Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Urn) W15/50 (W/kg) Remarks

(* )

1 0.0006 6.3xl05 2 0.12 0.0011 7.7xl05 3 0.0016 8.9xl05 4 0.0006 3.5xl05 5 0.25 0.0011 4.3xl05 6 0.0016 4.9xl05 7 0.0006 1.3xl05 8 0.48 0.0011 l.SxlO5 9 0.0016 3.6xl05 10 0.0006 1.2xl05 11 0.84 0.0011 l.SxlO5 12 0.0016 3.3xl05 Example 4
A laboratory vacuum melting furnace was used to prepare slabs containing, by mass%, C: 0.0022%, Si: 1.2%, Mn: 0.31 to 1.44%, Sol. Al: 0.03%, Ti: 0.0013%, N: 0.0016%, S: 0.0031%, B: 0.0021%, and Sn: 0.02%. The slabs were raised in temperature to 1220°C, then immediately lowered in temperature to 1070°C, held there for 20 minutes, then hot rolled. In this heating, the time during which the slabs were kept at 1150°C or more in

temperature was 15 minutes. In addition to this, the slabs were tested by heating at a constant temperature of 1220°C or 1070°C for 45 minutes then immediately hot rolled. The thus obtained hot rolled sheets were pickled, then cold rolled to a sheet thickness of 0.50 mm, finished annealed at 850°C for 5 seconds, stress relief annealed at 750°C for 2 hours, observed for precipitate of the steel sheet and crystal grain size, and measured for watt loss. As a result, as shown in Table 8, in Samples 12 to 15 with Mn of 0.4% or more and a heating temperature of the hot rolling of 1220-»1070°C, the sulfide densities of the stress relief annealing were 3.0xl05 or less, the average crystal grain sizes were 50 Urn or more, and good watt losses were obtained. In these samples, there were a large number of spherical sulfides of a diameter of 0.2 to 0.3 |4m and a large number of the sulfides were confirmed to have a plurality of Ti precipitates precipitated at their outer circumferences. In the other samples, the sulfide densities of the stress relief annealing were high, the Ti precipitate densities were high, the average crystal grain sizes were also small, and the watt losses were poor.

Table 8

Code Hot rolling heating temperature Mn (%) Sulfide density (/mm2) Ti precipitate density (/mm2) Crystal grain size (Urn) W15/50 (W/kg) Remarks
1 0.31 4.0xl05 2 0.45 4.1xl05 3 1070°C 0.68 3.7xl05 4 1.03 3.4xl05 5 1.44 3.3xl05 6 0.31 4.7xl05 5.5xl03 32 4.64 Comp. ex.
7 1220°C 0.45 2.7xl05 5.7xl03 30 4.51 Comp. ex.
8
0.68 l.SxlO5 6.1xl03 32 4.45 Comp. ex.
9 1.03 1.3xl05 7.7xl03 33 4.36 Comp. ex.
10 1.44 1.2xl05 7.2xl03 34 4.41 Comp. ex.
11 0.31 3.5xl05 12 0.45 2.6xl05 13 1220-»1070°C 0.68 2.2xl05 14 1.03 l.SxlO5 15 1.44 1.4xl05

We claim
1. A non-oriented magnetic steel sheet characterized by containing, by mass%, Si:1.5% or less, Mn:0.40% to 1.5%, Sol.Al:0.01% to 0.04%, Ti:0.0015% or less, N;0.0030% or less, S:0.0010% to 0.0040%, B in a B/N of 0.5 to 1.5, and optionally containing one or more of Sn , Cu, Ni, REM, Ca, Mg and where Sn , Cu and Ni in total amount of 0.01% to 0.50%, and REM, Ca and Mg in a total of 0.001% to 0.05%, and the balance of Fe and unavoidable impurities, and 10% or more of the particles of sulfide containing Mn , complexly precipitating with the B precipitates, and a distribution density of a total of MnS, Cu2S, and their complex sulfides of 3.0x105/mm2 or less, and a distribution density of the Ti precipitates not reaching a diameter of 0.1 µm of 1.0xl03/ mm or less.

2. A non-oriented magnetic steel sheet as claimed in claim 1, wherein a crystal grain size of the steel sheet after stress relief annealing being 50um or more.
3. A method of production of a non-oriented magnetic steel sheet as claimed in claim 1 or 2, wherein the steel sheet is produced by a process comprising the steps of; steel making, slab heating at a temperature range at 1150°C to 1250°c and keeping the slab for 5 minutes or more, then further keeping the slab at a temperature range at 1050°C to less than 1150°C for 15 minutes or more, immediately commencing hot rolling and making a finish outlet temperature of 800°C or more, pickling, and cold rolling.
4. A method of production of a non-oriented magnetic steel sheet as claimed in claim 3, wherein when the steel sheet contains Sn as an indispensable element, the finish outlet temperature T(°C) is defined as T(°C)>900-.1000xSn[mass%]
5. A non-oriented magnetic steel sheet and a method of production of a non-oriented magnetic steel sheet substantially such as herein described with reference to foregoing examples.

Documents:

5896-DELNP-2006-Abstract-(14-07-2009).pdf

5896-delnp-2006-abstract.pdf

5896-DELNP-2006-Claims-(14-07-2009).pdf

5896-delnp-2006-claims.pdf

5896-delnp-2006-correspondence-others.pdf

5896-DELNP-2006-Corresponence-Others-(14-07-2009).pdf

5896-delnp-2006-description (complete).pdf

5896-DELNP-2006-Form-1-(14-07-2009).pdf

5896-delnp-2006-form-1.pdf

5896-delnp-2006-form-18.pdf

5896-DELNP-2006-Form-2-(14-07-2009).pdf

5896-delnp-2006-form-2.pdf

5896-delnp-2006-form-26.pdf

5896-DELNP-2006-Form-3-(14-07-2009).pdf

5896-delnp-2006-form-3.pdf

5896-delnp-2006-form-5.pdf

5896-delnp-2006-gpa.pdf

5896-delnp-2006-pct-304.pdf

5896-delnp-2006-pct-308.pdf

5896-delnp-2006-pct-search report.pdf

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Patent Number

240825

Indian Patent Application Number

5896/DELNP/2006

PG Journal Number

24/2010

Publication Date

11-Jun-2010

Grant Date

03-Jun-2010

Date of Filing

10-Oct-2006

Name of Patentee

NIPPON STEEL CORPORATION

Applicant Address

6-3, OTEMACHI 2-CHOME, CHIYODA-KU, TOKYO 100-8071, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	KOICHI KIRISHIKI	C/O NIPPON STEEL CORPORATION YAWATA WORKS, 1-1, TOBIHATACHO, TOBATA-KU, KITAKYUSHU-SHI, FUKUOKA 804-8501, JAPAN
2	YUTAKA MATSUMOTO	C/O NIPPON STEEL CORPORATION YAWATA WORKS, 1-1, TOBIHATACHO, TOBATA-KU, KITAKYUSHU-SHI, FUKUOKA 804-8501,JAPAN
3	YOSHIHIRO ARITA	C/O NIPPON STEEL CORPORATION YAWATA WORKS, 1-1, TOBIHATACHO, TOBATA-KU, KITAKYUSHU-SHI, FUKUOKA 804-8501, JAPAN
4	HIDEKUNI MURAKAMI	C/O NIPPON STEEL CORPORATION YAWATA WORKS, 1-1, TOBIHATACHO, TOBATA-KU, KITAKYUSHU-SHI, FUKUOKA 804-8501, JAPAN

PCT International Classification Number

C21D 8/12

PCT International Application Number

PCT/JP2005/007653

PCT International Filing date

2005-04-15

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2005-045222	2005-02-22	Japan
2	2004-121446	2004-04-16	Japan