Abstract:
A magnetic steel plate for use as a magnetic shielding member and a method for the manufacture thereof are disclosed, and the steel composition consists essentially of, by weight %; 
     C: not greater than 0.05%, 
     Si: greater than 0.30%, but not greater than 1.50%, 
     Mn: not greater than 0.50%, sol Al: less than 0.005%, with the balance being Fe and incidental impurities. The magnetic steel plate has a ferrite grain size of 0 (zero) or smaller.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to magnetic steel plates exhibiting satisfactory magnetic properties, including magnetic plates which can be used for magnetic shielding from leakage magnetic flux. This invention also relates to a method of manufacturing such steel plates. 
     In recent years, many high-technology devices which utilize a strong magnetic field have been developed. One typical apparatus which uses very strong magnetic fields is a magnetic resonance imaging apparatus (hereunder referred to as an &#34;MRI apparatus&#34;). 
     During the operation of an MRI apparatus there is a large amount of leakage magnetic flux. As the leakage magnetic flux can adversely affect electrical equipment outside the MRI apparatus, it is important to shield the surroundings from the leakage magnetic flux. There are two methods of providing magnetic shielding. One is to cover the MRI apparatus itself with magnetic shielding members, and the other is to surround the room where the MRI apparatus is installed with magnetic shielding members. In either method, the shielding members are usually steel plates with a high degree of magnetic permeability. Such steel plates are called magnetic leakage-shielding steel plates, and are also used as covering members and structural members of large-scale equipment for scientific research such as cyclotrons in order to carry out magnetic shielding. 
     Therefore, such magnetic steel plates must have satisfactory mechanical properties, and there is a strong need for a material which has not only good mechanical properties but also good magnetic properties such as permeability and magnetic flux density. 
     Soft magnetic steel plates have been used as magnetic flux-shielding members. The most-widely used one is a thin plate for use in transformers. The steel plates defined in JIS C 2504 are thin plates with a thickness of 0.6-4.5 mm. JIS C 2503 defines steel bars having a diameter of 1.0-16 mm. 
     There are also cases in which a steel plate such as S10C steel which is defined in JIS G 4051 as a mechanical structural carbon steel plate is employed as a magnetic steel. 
     In addition, Japanese Published Unexamined Patent Application No. 96749/1985, Japanese Published Examined Patent Application No. 45442/1988, and Japanese Published Examined Patent Application No. 45443/1988 disclose a thick steel plate for use in direct current magnetization, which contains a rather large amount of sol. Al, e.g. 0.005-1.00% of sol. Al and as little of Si as possible. This steel plate is made from a low carbon steel which has been deoxidized with Al. 
     However, the magnetic properties of these conventional magnetic steel plates are not adequate for the plates to shield the leakage flux such as is experienced in MRI apparatuses. 
     (i) Soft magnetic bars and plates such as defined in JIS C 2503 and 2504 are intended to be used as small-sized parts. They are not intended to be used as structural members and their mechanical properties are poor. Therefore, if such a magnetic plate is to be applied to an MRI apparatus, it is necessary to laminate about 10 steel sheets in order to obtain adequate rigidity. This manufacturing method is impractical because of high manufacturing costs and poor quality of the laminated product. 
     (ii) The carbon steels for mechanical and structural use which are defined in JIS G 4051 have a maximum permeability (μ max ) of 1800 or smaller. This is because magnetic properties are not regarded as important for such materials. 
     The magnetic steel plate disclosed in Japanese Published Unexamined Patent Application No. 96749/1985 has a maximum permeability (μ max ) which extends over a wide range of 12850 to 4260. The permeability of that steel is not adequate for the steel to be used as a magnetic steel plate for shielding the leakage magnetic flux from an MRI apparatus. 
     According to the methods disclosed in Japanese Published Examined Patent Application No. 45442/1988 and No. 45443/1988, it is possible to increase the maximum permeability (μ max ) of a steel plate to 2000-5000. However, this level of permeability is inadequate for the steel plate to be used in an MRI apparatus. 
     Thus, it is not possible to obtain a satisfactory magnetic steel plate for use as a magnetic shielding member in devices such as MRI apparatuses. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a magnetic steel plate for use as a magnetic shielding member and a method for the manufacture thereof, the steel plate having not only improved magnetic properties for shielding leakage magnetic flux but also good mechanical properties. 
     The inventors of the present invention found that a low carbon steel plate which has been deoxidized with Si has extremely good magnetic properties compared with a low carbon steel plate deoxidized with Al, which is disclosed in Japanese Published Unexamined Patent Application No. 96749/1985. 
     Thus, according to the findings of the present inventors, in order to provide a magnetic steel plate having improved magnetic properties it is important to minimize the content of elements which increase the demagnetizing factor. It is also important to increase the uniformity of magnetic properties in the thickness direction of the steel plate and for the steel to have extremely coarse crystal grains. 
     Elements which increase the demagnetizing factor include C, S, Cu, Cr, and sol. Al. Of these elements sol. Al has a great influence on magnetic properties, and it is desirable that the content of sol. Al be minimized. On the other hand, an example of an element which can increase permeability is Si, and it is possible to greatly improve the magnetic properties of a steel plate when a suitable amount of Si is added. 
     FIG. 1 is a graph showing the relationship between the content of sol. Al and the magnetic flux density at 1 Oe (B 1 ) for steels having substantially the same composition except for sol. Al. It can be seen from the graph that the content of sol. Al should be restricted to less than 0.005% in order to ensure B 1  ≧10000. The steel composition of FIG. 1 was C: 0.003%, Si: 0.60%, Mn: 0.09%, sol. Al: 0.002-0.021%, P: 0.006%, and S: 0.005%. 
     FIG. 2 is a graph showing the relationship between the content of Si and the magnetic property (B 1 ) as well as tensile strength (TS) of steels having substantially the same composition except for different amounts of Si. It can be seen from this graph that the content of Si should be restricted to greater than 0.30% in order to ensure that B 1  ≧10000 and TS≧35 (kgf/mm 2 ). The steel composition was C: 0.003%, Si: 0.009-0.97%, Mn: 0.12%, sol. Al&lt;0.003%, P: 0.006%, and S: 0.006%. 
     FIG. 3 is a graph showing the relationship between the content of Si and the magnetic property (B 1  and maximum permeability) for steels having substantially the same composition except for different amounts of Si. Substantially the same tendency as in FIG. 2 can be seen. The steel composition was the same as for FIG. 2. 
     In order to ensure uniformity of magnetic properties, it is important to decrease the content of elements which easily form non-metallic inclusions as well as elements which easily segregate. It is also helpful to make the size of crystal grains as uniform as possible in the thickness direction of the steel plate. 
     Furthermore, in order to make the crystal grains coarse, it is effective to impart strains to the crystal grains during hot working, and to heat the steel to a temperature not higher than the Ac 1  point after hot working. 
     According to the findings of the present inventors, it is also effective if after casting and hot working, the resulting steel plate is subjected to heat treatment at a temperature not lower than 700° C. or not lower than the Ac 3  point, i.e. the transformation temperature in order to adjust the crystal grain size, to remove strains induced by deformation, and to improve magnetic properties such as permeability without degrading mechanical properties. 
     Thus, the present invention is a magnetic steel plate for shielding magnetic flux which consists essentially of, by weight %; 
     C: not greater than 0.05%, 
     Si: greater than 0.30%, but not greater than 1.50%, 
     Mn: not greater than 0.50%, sol Al: less than 0.005%, and a balance of Fe and incidental impurities. 
     Preferably, the ferrite grain size number is 0 (zero) or smaller. 
     In another aspect, the present invention is a method of manufacturing a magnetic steel plate for shielding magnetic flux, which comprises heat treating, after hot working, a steel plate having the above-described composition in a temperature range of 700° C.--the Ac 3  point or in a temperature range of higher than the Ac 3  point. 
     The heating time is preferably defined by the following formula: 
     
         (273+T)(log K+20)≧22.9×10.sup.3 
    
     T: heat treating temperature (° C.), T≧700° C. 
     K: heating time (h), wherein K≧t/25.4+0.1 
     In a still another aspect, the present invention is a method of manufacturing a magnetic steel plate for shielding magnetic flux, which comprises hot working a steel having the above-described composition after heating it to the Ac 3  point or higher, finishing the hot working with a total reduction of 20% or larger in a temperature range of the Ar 1  point or lower temperatures, and heating, after cooling, the resulting steel plate to a temperature of from 850° C. to the Ac 1  point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between the content of sol. Al and magnetic flux density; 
     FIG. 2 is a graph showing the relationship between the content of Si and magnetic flux density in a magnetic field of 1 Oe (B 1 ) as well as tensile strength; 
     FIG. 3 is a graph showing the relationship between the content of Si and magnetic flux density as well as the maximum permeability; 
     FIG. 4 is a graph showing the relationship between the magnetic flux density and the ferrite grain size number; and 
     FIG. 5 is a graph showing the relationship between the maximum permeability and the ferrite grain size number. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in further detail. In the description, percent (%) refers to weight % unless otherwise indicated. 
     The reasons for the above-mentioned limits on the steel composition are as follows. 
     Carbon (C) greatly increases the demagnetizing factor of steel, so the content of C is preferably reduced to as low a level as possible. However, many steps are required to reduce the C content, resulting in an increase in manufacturing costs. Thus, according to the present invention the C content is restricted to not larger than 0.05%. Preferably it is 0.01% or smaller. 
     Silicon (Si) is a very important element to achieve the intended purpose of the present invention. The addition of Si promotes orientation of crystal grains and an improvement in magnetic properties. Si also serves as a deoxidizing agent. For these purposes the Si content is restricted to greater than 0.30%. However, the incorporation of an excess amount of Si makes the steel brittle, and the resulting steel cannot be used as a thick steel plate for structural use. Therefore, the Si content is restricted to greater than 0.30% but not greater than 1.50%. Preferably the Si content is greater than 0.30% but not greater than 1.0%. 
     Manganese (Mn) is an element which should not be present in large amounts because it adversely affects magnetization, as does carbon. However, when a thick steel plate is used as a structural member it is necessary to have not only satisfactory magnetic properties but also a minimum level of mechancial strength. Therefore, the upper limit of the Mn content is defined as 0.50%. 
     Aluminum (Al) is an extremely important element for achieving the purpose of the present invention. Al increases the demagnetizing factor, and it combines with N in steel to form aluminum nitrides which accelerate the formation of a mixed grain structure. Therefore, it is desirable to reduce the Al content. When the content of sol. Al is 0.005% or greater, both the maximum permeability and the magnetic flux density at a magnetic field of 1 Oe are decreased and satisfactory magnetic properties cannot be obtained. The sol. Al content is therefore restricted to less than 0.005% in the present invention. 
     P and S are included as impurities. Both P and S easily form non-metallic inclusions in steel, and so it is desirable to reduce the content of P and S. However, since it is very costly to do so, it is desirable in the present invention that the P content be defined as 0.10% or less and the S content be defined as 0.01% or less. 
     At least one additional element selected from the group consisting of Cr, Mo, Cu, N, and oxygen may be present in the above-described steel of the present invention. However, in order to attain satisfactory magnetic properties, it is desirable that the content of these elements be as low a level as possible. 
     Namely, since an element such as Cr, Mo, Cu and N increases the demagnetizing factor, and in particular, as mentioned above, nitrogen easily reacts with Al to form nitrides which promote refining of crystal grains, it is desirable that the content of these elements be minimized. This is also desirable in order to remove segregation of added elements. However, since it is impossible to avoid contamination of Cr, Mo, and Cu from refractory bricks during melting and refining, it is rather difficult to reduce the content of these elements to an extremely low level. Therefore, Cr may be present in an amount of 0.20% or less, Mo in an amount of 0.02% or less, Cu in an amount of 0.10% or less, and N in an amount of 0.01% or less. 
     Oxygen contained in steel easily forms non-metallic inclusions which segregate to prevent movement of magnetic domain walls. Thus, the more the oxygen content the more the coercive force with a fear of degradation of magnetic properties. So it is desirable to reduce the oxygen content as much as possible, i.e., 0.003% or less. 
     According to a preferred embodiment of the present invention the ferrite grain size number is restricted to zero or smaller. When the number is larger than zero, i.e., finer, both the maximum permeability (μ max ) and the magnetic flux density (B 1 ) are decreased, and satisfactory magnetic properties cannot be obtained. 
     According to the present invention, it is desirable that the ferrite grain size number be determined by the intercept method which is defined in JIS G 0552 in which the number of ferritic grains cut by any segment of a line is determined and this number is converted into the number of ferrite grains within a 25×25 mm area in the field of view when the magnification is ×100. According to the present invention, the ferrite grains are greatly coarsened. Needless to say, the comparison method can be employed for this purpose. When the comparison method is employed, it is desirable that the ferrite grain size number be restricted to zero or smaller. 
     The magnetic steel plate of the present invention has very satisfactory magnetic properties. Of the magnetic properties which should be possessed by a magnetic steel plate for shielding leakage magnetic flux, the maximum permeability (μ max ) and the magnetic flux density are critical. High-technology equipment now requires that the minimum level for the maximum permeability (μ max ) be 10000 or larger, and preferably 30000 or larger, while the magnetic flux density (B 1 ) at a magnetic field of 1 Oe must be 10000 or higher, and preferably 14000 or higher. The properties of the magnetic steel plate of the present invention easily surpass such requirements. 
     Next, a method of manufacturing the magnetic steel plate for shielding leakage magnetic flux of the present invention will be further described. 
     Melting and refining can be carried out using either a converter or electric furnace. If necessary, refining with a ladle or refining by vacuum degassing may be employed so as to further remove elements which markedly increase the demagnetizing factor such as C, Al, Cr, Mo, Cu, and N. In order to minimize the formation of non-metallic inclusions as well as their segregation, P and S are also removed. Oxygen can be removed by the addition of Si. 
     The resulting slab steel is then subject to hot working. Pre-treatment or any other special treatments for the hot working are not always necessary, and the hot working may be carried out by either, rolling with a rolling mill or forging with a forging machine. 
     According to a preferred embodiment of the present invention, prior to the hot working, the steel is heated to a temperature higher than the Ac 3  point, and preferably higher than the Ac 3  point but lower than 1200° C. As a result of heating to a temperature higher than the Ac 3  point, the steel structure becomes a single austenitic structure on which hot working is carried out. During hot working, the temperature of the steel decreases so that the steel comprises an austenitic-ferritic dual phase. Strains are indroduced uniformly during hot working, and a desirble mixed grain structure can be obtained when the steel is subjected to the below-mentioned recrystallization. Therefore, the only necessary limit as to the heating temperature is that the heating temperature of the slab steel be the Ac 3  point or higher. Although an upper limit on the heating temperature is not mandatory, the upper limit is preferably 1200° C. from the viewpoint of practicality since there is a fear that damage to facilities such has damage to a refractory lining of the heating furnace when the heating temperature is higher than  1200° C. 
     After heating the slab steel to a temperature higher than the Ac 3  point, hot working is carried out to form a desired shape. According to a preferred embodiment of the present invention the hot working is carried out in such a manner that the reduction in a temperature range not exceeding the Ar 1  point is 20% or more. The reduction in a temperature range not exceeding the Ar 1  point is defined by the following equation in which Δh is the difference between the initial thickness of the plate and the final thickness of the plate at the finishing point, and Δhα is the difference between the thickness of the plate at the Ar 1  point and the thickness at the finishing point: ##EQU1## 
     The reason for defining the temperature range as not exceeding the Ar 1  point is that a single ferrite phase is prepared so that the same amount of strain can be imparted uniformly to each of the ferrite grains. 
     The purpose of defining the reduction as 20% or more is to make sure that strains can be imparted to ferrite grains at the center of the thickness of the plate. From this viewpoint the higher the reduction the better. However, when the reduction in a temperature range not exceeding the Ar 1  point is higher than 70%, the reduction in a low temperature range increases, resulting in overloading of equipment such as a rolling mill. This creates the danger of premature damage or collapse of equipment. Thus, it is desirable that the reduction in a temperature range not exceeding the Ar 1  point be 70% or less. From the standpoint of the uniformity of the strains which are introduced into ferritic crystal grains, it is not necessary to set a lower limit on the temperature during hot working i.e. the hot working finishing temperature. However, when the hot working is continued at a temperature lower than 650° C., the rolling mill is subject to overloading, and the wear of components such as rolls is accelerated. Therefore, it is desirable that the lower limit of hot working temperature be defined as 650° C. 
     For the purpose of introducing a lot of strains into ferrite crystal grains uniformly it is preferable that a conventional high aspect ratio rolling method be employed. 
     There is no limit on the thickness of the magnetic steel plate of the present invention, but it is usually at least 20 mm since it is intended to be used as a structural plate. 
     After hot working, heat treatment is performed to further arrange the crystal grains and to remove hot work-induced strains with a resulting improvement in magnetic properties, such as permeability and flux density. 
     Namely, the hot worked steel plate may be directly heat treated, but if necessary it can be cooled to room temperature so as to remove hydrogen. In order to thoroughly remove hydrogen, it is desirable to cool the hot worked plate to a temperature of 300° C. or lower. By cooling to such a low level, it is possible to ensure sufficient time to effect removable of hydrogen. 
     At the next stage the steel plate is subjected to heat treatment for the purposes of orientation of grains and removal of strains. In particular, annealing is effective to further improve magnetic properties. It is desirable that the annealing temperature be restricted to a temperature not lower than 850° C. but not higher than the Ac 1  point in order to form a recrystallized texture structure with well-grown ferritic grains. When the steel plate is heated to a temperature higher than the Ac 1  point, the once-formed recrystallized texture is changed into a transformed texture structure with a remarkable degradation in magnetic properties. On the other hand, a temperature lower than 850° C. is not high enough to give a sufficient amount of energy to promote the growth of ferrite grains. 
     During annealing, it is preferable that the steel plate be heated for a period of time of t/25 hours or longer (t: thickness of final product, mm) in order to uniformly heat the steel plate to the center of its thickness. In general, it is preferable that annealing treatment be carried out at 880° C. for about one hour. 
     After the completion of annealing, the steel plate may be cooled by natural cooling, air cooling, slow cooling, water cooling, quenching, etc. with substantially no change in the properties of the final product. In the present invention there is no restriction on the cooling method. 
     According to another preferred embodiment of the present invention, the hot worked steel plate is heated at a temperature of 700° C. or higher for a given period of time so that satisfactory magnetic as well as mechanical properties can be obtained. 
     In this embodiment the length of the heating period (K) is given by the following formula: 
     
         K≧(t/25.4+0.1) 
    
     
         (273+T)(log K+20)≧22.9×10.sup.3 
    
     wherein T stands for the heating temperature (° C.). 
     Thus, according to this embodiment when the steel plate is heated to a temperature not higher than the Ac 3  point, the resulting structure has a recrystallized texture and the grain growth is promoted to enlarge the magnetic domains, resulting in a remarkable improvement in magnetic properties. 
     However, in this embodiment, there is a slight degree of degradation in mechanical properties including toughness, and this process can be applied when such a degradation is tolerable. 
     On the other hand, if the steel plate is heated at a temperature higher than the Ac 3  point for the above-defined period (K), the resulting structure has a transformed texture with refined crystal grains. The magnetic properties are degraded to a slight extent, but the mechanical properties can be improved remarkably. Therefore, a relatively high heating temperature of greater than the Ac 3  point can be employed when the mechanical properties are particularly important. 
     The present invention will be further described in conjunction with the following working examples which are presented merely for illustrative purposes. 
     EXAMPLE 1 
     Slab steels having the compositions shown in Table 1 were prepared by carrying out melting and refining using an electric furnace. 
     The resulting slab steels were formed into given shapes, and annealing was performed under the conditions shown in Table 1. 
     Samples No. 1-13 were prepared from the annealed steels. The maximum permeability (μ max ) and the magnetic flux density (B 1 ) of the samples in a magnetic field of 1 Oe were determined. 
     The test results are also shown in Table 1. 
     In Samples No. 1 through No. 3, the Si content was varied from 0.37% to 0.95% while the steel compositions were otherwise the same. The heat treatment conditions were also substantially the same for these Samples. The maximum permeability was 15300-17600, and the magnetic flux density was 12200-14000 (Gauss). These values are double or more those obtained in the prior art. These values increased as the Si content increased. 
     EXAMPLE 2 
     Slab steels having the compositions shown in Table 2 were obtained using an electric furnace melting method. From these slab steels, JIS No. 5 type test pieces were cut to make Samples No. 1-No. 4. The steel compositions of Samples No. 1-No. 3 corresponded to those of Table 1. The test results are also shown in Table 2. 
     As is apparent from Table 2, Samples No. 1 through No. 3 of the present invention had values much higher than in the prior art in respect to Y.P., T.S., and vEo. It is said that the T.S. should be higher than 25 kgf/mm 2  for a soft magnetic thick steel plate. The samples of the present invention had values much higher than 25 kgf/mm 2 . Thus, the material of the present invention is strong enough to be used as a structural member for an MRI apparatus. 
     EXAMPLE 3 
     Steel A through Steel C having the compositions shown in Table 3 and having a thickness of 230 mm were heated to 1100°-1160° C., as shown in Table 4, and then hot rolling was carried out. 
     During hot rolling, the reduction in a temperature range not exceeding the Ar 1  point was adjusted to be 0-50% and the hot rolling was finished at 760°-911° C. followed by cooling to 150° C. to give a hot-rolled steel plate with a thickness of 20 mm. 
     The resulting steel plates were annealed by heating at 880° C. to obtain Samples No. 1 through No. 36 in Table 4. 
     The ferrite crystal grain size number of these samples was determined by means of the before-mentioned intercept method, and the maximum permeability. (μ max ) and the magnetic flux density (B 1 ) were also determined. 
     The test results are shown in Table 4, and the relationship between the ferrite grain size number and μ max  is illustrated in FIG. 5. The relationship between the ferrite grain size number and B 1  is illustrated in FIG 4. 
     As is apparent from Table 4, FIG. 4, and FIG. 5, when the ferrite grain number is zero or smaller, μ max  is 30000 or larger and B 1  is 14000 or greater, as shown for Samples No. 1-No. 8. These high values indicate that the material of the present invention can exhibit excellent magnetic properties. 
     EXAMPLE 4 
     Slab steels having the compositions shown in Table 5 were heated to 1160° C. and then subjected to hot rolling. The hot rolling was carried out with the reduction shown in Table 5. After finishing hot rolling at the finishing temperature shown in Table 5, the resulting hot rolled steel plates were cooled to the temperatures indicated in Table 5 to produce hot rolled steel plates with a thickness of 20 mm or 80 mm. Thereafter, annealing was performed at the heating temperature and heating time indicated in Table 5, and after cooling to room temperature Samples No. 1 through No. 30 were obtained. 
     The following properties of the resulting steel plates were determined: 
     (i) Ferrite grain size number by the intercept method in accordance with JIS G 0552. 
     (ii) Maximum permeability (μ max ) and magnetic flux density (B 1 , Gauss) in a magnetic field of 1 Oe. 
     (iii) Average Charpy absorbed energy for V-notched test pieces at 0° C., vEo AVE  (kgf.m), and tensile strength, TS (kgf/mm 2 ). 
     The test results are shown in Table 5. 
     EXAMPLE 5 
     Slab steels having the compositions shown in Table 6 were formed into plates having a thickness of 20-160 mm. The resulting steel plates were then subjected to heat treatment under the conditions shown in Table 6 to produce the thick steel plates of Samples No. 1 to No. 21. The maximum permeability and the magnetic flux density at the magnetic field of 1 Oe (B 1 , Gauss) were determined for each of the samples. 
     The test results are shown in Table 6. 
     The indication &#34;Calculation&#34; means values obtained by calculation of the left-hand side of the following formula: 
     
         (273+T)(log K+20)≧22.9×10.sup.3 
    
     wherein K=t/25.4+0.1 
     The above note will apply to Tables 7 and 8. 
     EXAMPLE 6 
     In this example, slab steels having the compositions shown in Table 7 were hot worked in the same manner as in Example 5 to produce hot worked steel plates having a thickness of 20-160 mm. The resulting steel plates were subjected to heat treatment under the conditions shown in Table 7. 
     The magnetic and mechanical properties of the thus prepared samples of the present invention are shown in Table 7. 
     Table 8 shows experimental data of comparative samples having steel compositions falling outside the range of the present invention. 
     Samples No. 1 of Table 8 had a carbon content higher than that of the present invention. The maximum permeability and magnetic flux density were decreased. 
     Sample No. 2 of Table 8 shows the importance of the presence of Si. Its Si content was lower than that of the present invention. The maximum permeability and magnetic flux density were both decreased. 
     Sample No. 3 of Table 8 had an Al content higher than that of the present invention. The maximum permeability and magnetic flux density were greatly decreased. 
     Sample No. 4 of Table 8 has an Mn content higher than that of the present invention. Both the maximum permeability and magnetic flux density were decreased. 
     
                                           TABLE 1__________________________________________________________________________                                MaximumSample    Steel composition (wt %)                    Heat Treatment                                Permea-                                      B.sub.1No. C  Si  Mn  P  S  sol. Al                    Temp. (°C.)                           Time (h)                                bility                                      (Gauss)                                           Remarks__________________________________________________________________________1   0.002  0.37      0.16          0.006             0.003                0.003                    880    1    15300 12200                                           Present2   0.003  0.68      0.12          0.007             0.007                0.002                    880    1    17600 14000                                           Invention3   0.004  0.95      0.12          0.006             0.006                0.003                    880    1    17400 126004   0.008  0.58      0.09          0.006             0.006                0.003                    950    1    14500 117005   0.004  0.62      0.10          0.032             0.004                0.002                    950    1    14200 112006   0.007  0.61      0.12          0.067             0.007                0.004                    950    1    11400 106007   0.010  0.58      0.09          0.082             0.007                0.004                    950    1    10800 100008   0.009  0.32      0.18          0.009             0.007                0.003                    880    1    13700 116009   0.004  0.63      0.47          0.007             0.004                0.002                    950    1    10200 1000010  0.06*  0.60      0.12          0.026             0.008                0.002                    880    1     6800  5700                                           Com-11  0.006   0.21*      0.09          0.009             0.007                0.002                    880    1     9700  8200                                           parative12  0.003  0.65      0.11          0.007             0.006                 0.021*                    880    1     7100  670013  0.006  0.62       0.72*          0.006             0.003                0.004                    880    1     4800  4400__________________________________________________________________________ Note: *Outside the range of the present invention 
    
     
                                           TABLE 2__________________________________________________________________________Sample    Steel Composition (wt %)                   Y.P.     T.S.     vE.sub.0No. C  Si  Mn P  S  sol. Al                   (kgf/mm.sup.2)                            (kgf/mm.sup.2)                                     (kgf · m)                                          Remarks__________________________________________________________________________1   0.002  0.37      0.16         0.006            0.003               0.003                   19.6     31.8     31.2 Present2   0.003  0.68      0.12         0.007            0.007               0.002                   20.2     32.7     33.4 Invention3   0.004  0.95      0.12         0.006            0.006               0.003                   22.3     35.8     36.24   0.002  0.004*      0.09         0.005            0.004               0.003                   12.3     24.5     22.8 Com-                                          parative__________________________________________________________________________ Note: *Outside the range of the present invention 
    
     
                                           TABLE 3__________________________________________________________________________                                  Transformation Temp.Steel Composition (wt %)               Ar.sub.1                                        Ac.sub.1                                              Ac.sub.3Steel   C  Si Mn P  S  sol. Al                 Cr Mo Cu N   O   Point (°C.)                                        Point (°C.)                                              Point__________________________________________________________________________                                              (°C.)A  0.003 0.68    0.12       0.007          0.007             0.002                 0.05                    0.01                       0.01                          0.0038                              0.0016                                  856   907   926B  0.004 0.59    0.14       0.005          0.003             0.001                 0.20                    0.05                       0.18                          0.0047                              0.0018                                  853   906   921C  0.003 0.62    0.47       0.004          0.006             0.002                 0.06                    0.01                       0.01                          0.0027                              0.0022                                  861   904   916__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________  Heating       Initial              Finishing                   Cooling                        Heating                             Heating                                  Ferrite                                        MaximumSample Temp.       Temp.           *1 Temp.                   Temp.                        Temp.                             Time Grain Size                                        Perme-                                              B.sub.1No. Steel  (°C.)       (°C)           (%)              (°C.)                   (°C.)                        (°C.)                             (min)                                  Number                                        ability                                              (Gauss)__________________________________________________________________________ 1  A  1160 1148           50 768  150  880  60   -1.0  35200 14600 2  A  1140 1136           &#34;  764  &#34;    &#34;    &#34;    -1.0  40000 15400 3  A  1120 1112           30 760  &#34;    &#34;    &#34;    -0.3  35200 15400 4  B  1140 1132           50 763  &#34;    &#34;    &#34;    -0.5  37600 15400 5  B  1120 1108           30 761  &#34;    &#34;    &#34;    0     37000 15300 6  B  1100 1093           &#34;  760  &#34;    &#34;    &#34;    -1.0  37200 15400 7  C  1160 1157           50 766  &#34;    &#34;    &#34;    -1.0  36000 14200 8  C  1140 1135           30 764  &#34;    &#34;    &#34;    -1.0  39200 15400 9  A  1160 1157           15 825  &#34;    &#34;    &#34;    1.0   24200 1400010  B  &#34;    1156           &#34;  815  &#34;    &#34;    &#34;    1.0   20000 1340011  C  &#34;    1156           &#34;  817  &#34;    &#34;    &#34;    1.6   20000 1360012  A  &#34;    1151           10 806  &#34;    &#34;    &#34;    2.1   17400 1360013  A  &#34;    1157           &#34;  &#34;    &#34;    &#34;    &#34;    2.2   17400 1380014  B  &#34;    1152           &#34;  800  &#34;    &#34;    &#34;    2.2   17200 1220015  B  &#34;    1156           &#34;  &#34;    &#34;    &#34;    &#34;    2.3   16200 1220016  C  &#34;    1155           &#34;  855  &#34;    &#34;    &#34;    2.4   15600 1310017  C  &#34;    1153           &#34;  &#34;    &#34;    &#34;    &#34;    2.4   15200 1300018  A  &#34;    1156            0 860  &#34;    &#34;    &#34;    2.5   16000 1320019  B  &#34;    1151           &#34;  867  &#34;    &#34;    &#34;    2.8   15400 1300020  C  &#34;    1154           &#34;  865  &#34;    &#34;    &#34;    2.8   16000 1340021  A  1160 1156            0 888  150  880  60   3.0   14100 1240022  A  &#34;    1155           &#34;  876  &#34;    &#34;    &#34;    3.0   12000 1130023  B  &#34;    1154           &#34;  884  &#34;    &#34;    &#34;    3.0   18500 1390024  B  &#34;    1156           &#34;  882  &#34;    &#34;    &#34;    3.0   17700 1310025  A  &#34;    1157           &#34;  891  &#34;    &#34;    &#34;    3.1   15400 1320026  B  &#34;    1153           &#34;  896  &#34;    &#34;    &#34;    3.1   13800 1240027  C  &#34;    1154           &#34;  893  &#34;    &#34;    &#34;    3.1   16400 1300028  A  &#34;    1154           &#34;  894  &#34;    &#34;    &#34;    3.4   17400 1400029  B  &#34;    1152           &#34;  894  &#34;    &#34;    &#34;    3.4   18000 1360030  C  &#34;    1155           &#34;  901  &#34;    &#34;    &#34;    3.4   13300 1240031  A  &#34;    1157           &#34;  907  &#34;    &#34;    &#34;    3.5   14200 1270032  B  &#34;    1153           &#34;  906  &#34;    &#34;    &#34;    3.5   20000 1400033  A  &#34;    1154           &#34;  904  &#34;    &#34;    &#34;    3.7   18600 1390034  B  &#34;    1154           &#34;  902  &#34;    &#34;    &#34;    3.8   12200 1220035  A  &#34;    1157           &#34;  911  &#34;    &#34;    &#34;    4.2   13300 1220036  A  &#34;    1152           &#34;  908  &#34;    &#34;    &#34;    4.2   11500 11000__________________________________________________________________________ Note: *1 Reduction within a Temperature Range of ≦ Ar.sub.1 Point 
    
     
         TABLE 5  Transformation  Temp. Heat-  Finish- Cool- Plate Heat Ferrite Magnetic Mechanical  Sam-  Ar.sub.1 Ac.sub.1 Ac.sub.3 ing  ing ing Thick- Treatment Grain Properties Properties  ple Steel Composition (wt %) Point Point Point Temp. *1 Temp. Temp. ness Temp. Time Size  B.sub.1 vE.sub.0.sup.Ave T.S. Re- No. C Si Mn P S sol. Al (°C.) (°C .) (°C.) (°C.) (%) (°C.) (°C.) (mm) (° C.) (h) Number μ  max (Gauss) (kgf·m) (kgf/mm.sup.2) marks       1 0.002 0.37 0.16 0.006 0.003 0.003 844 898 907 1160 50 740 Room 80 850 4 -0.1 34000 14400 12.1 34.1 Pre-              Temp.         sent (24° C.)         Inven-  2 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 880 &#34; 0 34800 14700 11.8 34.2 tion              Temp.              (24° C.)  3 0.003 0.68 0.12 0.007 0.007 0.002 856 907 926 &#34; &#34; &#34; Room 20 850 &#34; 0 37700 14800 11.9 32.4              Temp.              (24° C.) 4 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 880 &#34; -0.3 39800 15100 11.4 33.8  Temp.              (24° C.)  5 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 900 &#34; -1.0 40000 15200 10.1 33.6              Temp. (24° C.)  6 0.004 0.59 0.14 0.005 0.003 0.001 853 906 921 &#34; 30 &#34; Room 80 850 &#34; -0.8 36600 14800 10.2 33.1              Temp. (24° C.)  7 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;  &#34; &#34; &#34; &#34; Room &#34; 880 &#34; -1.0 36800 15500 12.1 33.6              Temp.              (24° C.)  8 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 900 &#34; -1.5 40000 15400 10.4 33.4 Temp.              (24° C.)  9 0.004 0.95 0.12 0.005 0.003 0.001 878 923 937 &#34; 60 &#34; Room &#34; 850 &#34; 0 32200 14100  9.8 35.1 Temp.              (24° C.) 10 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 880 &#34; -1.0 38200 15400 10.6 33.1              Temp.              (24° C.) 11 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 900 &#34; -1.6 41200 15400 10.4 33.8       Temp.              (24° C.) 12 0.003 1.42 0.13 0.004 0.003 0.001 881 931 947 &#34; 50 780 Room &#34; 880 &#34; -0.7 37300 14300 10.1 34.1         Temp.              (24° C.) 13 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 900 &#34; -1.1 39500 14700 10.3 34.8              Temp.  (24° C.) 14 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 920 &#34; -1.3 41800 15100  8.7 34.3              Temp.              (24° C.) 15 0.003 0.62 0.47 0.004 0.006 0.002 861 904 916 &#34; &#34; 740 Room &#34; 850 &#34; -1.1 31100 14200 21.8 37.7              Temp.              (24° C.) 16 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 880 &#34; -0.4 34300 14300 20.4 36.2 Temp.              (24° C.) 17 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; Room &#34; 900 &#34; -0.4 37700 14900 17.7 36.8              Temp.              (24° C.) 18 0.002 0.37 0.16 0.006 0.003 0.003 844 898 907 1160  0 870 Room 20 880 1 2.4 15300 13800 31.2 31.8              Temp.              (24.degre e. C.) 19 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 12 820 Room &#34; &#34; 1 1.8 16800 13900 29.9 31.4              Temp.              (24° C.) 20 0.003 0.68 0.12 0.007 0.007 0.002 856 907 926 &#34;  0 910 Room &#34; &#34; 1 2.2 17600 14000 33.4 32.7              Temp.              (24° C.) 21 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 12 820 Room &#34; &#34; 1 1.7 18800 14200 31.1 32.6              Temp.   (24° C.) 22 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 50 740 Room 80 950 4 2.8 16600 12300 32.9 32.4              Temp.              (24° C.) 23 0.007 0.61 0.13 0.006 0.003 0.001 852 904 927 &#34;  0 910 Room &#34; 880 4 2.3 15400 13200 28.8 32.2              Temp.              (24° C.) 24 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34;  0 910 Room &#34; 950 4 3.2 13800 12400 33.8 35.8  Temp.              (24° C.) 25 0.004 0.95 0.12 0.005 0.003 0.001 878 923 937 &#34;  0 910 Room 20 880 2 3.2 13200 11800 31.1 34.9   Temp.              (24° C.) 26 &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; &#34; 10 800 Room 80 &#34; 4 2.6 14100 14000 30.2 34.4              Temp. (24° C.) 27 0.003 1.42 0.13 0.004 0.003 0.001 881 931 947 &#34;  0 910 Room 20 &#34; 1 4.2 12800 11400 35.1 37.7              Temp.  (24° C.) 28 0.06* 0.60 0.12 0.026 0.008 0.002 796 824 909 &#34; 50 740 Room &#34; 880 1 4.4  7900  6200  6.6 41.2 Com-              Temp.   para-              (24° C.)         tive 29 0.003 0.65 0.11 0.007 0.006  0.021* 849 902 925 &#34; &#34; &#34; Room &#34; 880 1 6.2  7100  6700 24.5 34.1              Temp.              (24° C.) 30 0.006 0.62 0.72* 0.006 0.003 0.004 844 897 911 &#34; &#34; &#34; Room &#34; &#34; 1 5.9  6900  6800 12.2 40.2              Temp.              (24° Note: *Outside the range of the present invention *1 Reduction within a Temperature Range of ≦ Ar.sub.1 Point 
    
     
                                           TABLE 6__________________________________________________________________________                 Heat   Magnetic Mechanical   Plate                                                  Cal-Sam-                  Treatment                        Properties                                 Properties   Thick-                                                  cula-ple   Steel Composition (wt %)                 Temp.                     Time   B.sub.1                                 vE.sub.0.sup.Ave                                     T.S.  Ac.sub.3                                              ness                                                  tion                                                      Re-No.   C  Si Mn P  S  sol. Al                 (°C.)                     (h)                        μ max                            (Gauss)                                 (kgf·m)                                     (kgf/mm.sup.2)                                           (°C.)                                              (mm)                                                  (×10.sup.3)                                                      marks__________________________________________________________________________ 1 0.002 0.37    0.16       0.006          0.003             0.003                 950 1  10700                            10100                                 30.8                                     35.2  907                                              20  24.5                                                      Pre- 2 0.003 0.68    0.12       0.007          0.007             0.002                 950 1  12200                            11000                                 30.6                                     35.7  926                                              20  &#34;   sent 3 0.004 0.95    0.12       0.006          0.006             0.003                 950 1  12800                            11800                                 30.8                                     37.6  937                                              20  &#34;   Inven- 4 0.008 0.58    0.09       0.006          0.006             0.003                 950 1  12400                            11400                                 31.3                                     35.8  920                                              160 &#34;   tion 5 0.004 0.62    0.10       0.032          0.006             0.003                 950 1  12000                            11200                                 30.7                                     36.6  925                                              80  &#34; 6 0.007 0.61    0.12       0.062          0.007             0.004                 950 1  11400                            10600                                 33.6                                     36.2  927                                              80  &#34; 7 0.010 0.58    0.09       0.082          0.007             0.003                 950 1  10800                            10000                                 32.9                                     36.6  929                                              80  &#34; 8 0.009 0.32    0.18       0.009          0.007             0.003                 950 1  10800                            10200                                 30.6                                     35.1  907                                              20  &#34; 9 0.004 0.63    0.47       0.007          0.004             0.002                 950 1  11100                            10800                                 33.3                                     39.2  916                                              20  &#34;10 0.018 0.61    0.09       0.018          0.005             0.003                 950 1  11200                            10800                                 33.5                                     39.7  920                                              20  &#34;11 0.004 0.95    0.12       0.006          0.006             0.003                 920 1  18400                            14400                                 27.7                                     32.2  937                                              20  23.912 0.008 0.58    0.09       0.006          0.006             0.003                 900 1  16400                            13800                                 32.6                                     31.7  920                                              160 23.513 0.004 0.62    0.10       0.032          0.006             0.003                 900 1  16100                            13200                                 27.8                                     30.8  925                                              80  &#34;14 0.007 0.61    0.12       0.062          0.007             0.004                 900 1  16200                            13400                                 27.4                                     31.1  927                                              80  &#34;15 0.010 0.58    0.09       0.082          0.007             0.003                 900 1  15800                            13100                                 28.1                                     31.1  929                                              80  &#34;16 0.009 0.32    0.18       0.009          0.007             0.003                 900 1  12300                            12100                                 30.6                                     31.0  907                                              20  &#34;17 0.004 0.63    0.47       0.007          0.004             0.002                 900 1  12100                            12000                                 30.4                                     34.4  916                                              20  &#34;18 0.018 0.61    0.09       0.018          0.005             0.003                 900 1  12100                            12000                                 31.8                                     34.7  920                                              20  &#34;19 0.006 0.21*    0.09       0.009          0.007             0.002                 950 1   9400                             7800                                 18.9                                     31.7  905                                              20  24.5                                                      Com-20 0.003 0.65    0.11       0.007          0.006             0.021*                 950 1   7000                             6800                                 29.4                                     36.6  925                                              20  &#34;   para-21 0.006 0.62    0.72*       0.006          0.003             0.004                 950 1   7600                             7400                                 30.1                                     35.8  911                                              20  &#34;   tive__________________________________________________________________________ Note: *Outside the range of the present invention 
    
     
                                           TABLE 7__________________________________________________________________________                 Heat   Magnetic Mechanical   Plate                                                  Cal-Sam-                  Treatment                        Properties                                 Properties   Thick-                                                  cula-ple   Steel Composition (wt %)                 Temp.                     Time   B.sub.1                                 vE.sub.0.sup.Ave                                     T.S.  Ac.sub.3                                              ness                                                  tion                                                      Re-No.   C  Si Mn P  S  sol. Al                 (°C.)                     (h)                        μ max                            (Gauss)                                 (kgf·m)                                     (kgf/mm.sup.2)                                           (°C.)                                              (mm)                                                  (×10.sup.3)                                                      marks__________________________________________________________________________ 1 0.002 0.37    0.16       0.006          0.003             0.003                 700 5  13200                            12100                                 32.1                                     34.2  907                                              20  20.1                                                      Pre- 2 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   800 2  14100                            12800                                 32.3                                     33.7  &#34;  &#34;   21.7                                                      sent 3 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   850 1  14700                            13500                                 32.1                                     31.9  &#34;  &#34;   22.5                                                      Inven- 4 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   880 1  15300                            13800                                 31.2                                     31.8  &#34;  &#34;   23.1                                                      tion 5 0.003 0.68    0.12       0.007          0.007             0.002                 800 1  14700                            13100                                 33.3                                     33.8  926                                              20  21.7 6 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   880 1  17600                            14000                                 33.4                                     32.7  &#34;  &#34;   23.1 7 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   900 1  17800                            14700                                 31.6                                     31.1  &#34;  &#34;   23.5 8 0.004 0.95    0.12       0.006          0.006             0.003                 850 1  15700                            12300                                 30.7                                     34.1  937                                              20  22.5 9 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   880 1  17400                            12600                                 31.3                                     33.8  &#34;  &#34;   23.110 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   920 1  18400                            14400                                 27.7                                     32.2  &#34;  &#34;   23.911 0.008 0.58    0.09       0.006          0.006             0.003                 850 1  15200                            12700                                 32.3                                     32.9  920                                              160 22.512 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   900 1  16400                            13800                                 32.6                                     31.7  &#34;  &#34;   23.513 0.004 0.62    0.10       0.032          0.006             0.003                 850 1  14900                            12600                                 28.2                                     32.4  925                                              80  22.514 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   900 1  16100                            13200                                 27.8                                     30.8  &#34;  &#34;   23.515 0.007 0.61    0.12       0.062          0.007             0.004                 850 1  14800                            12700                                 27.8                                     32.7  927                                              80  22.516 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   900 1  16200                            13400                                 27.4                                     31.1  &#34;  &#34;   23.517 0.010 0.58    0.09       0.082          0.007             0.003                 850 1  14400                            12800                                 27.8                                     32.2  929                                              80  22.518 &#34;  &#34;  &#34;  &#34;  &#34;  &#34;   900 1  15800                            13100                                 28.1                                     31.1  &#34;  &#34;   23.519 0.009 0.32    0.18       0.009          0.007             0.003                 900 1  12300                            12100                                 30.6                                     31.0  907                                              20  &#34;20 0.004 0.63    0.47       0.007          0.004             0.002                 900 1  12100                            12000                                 30.4                                     34.4  916                                              20  &#34;21 0.018 0.61    0.09       0.018          0.005             0.003                 900 1  12100                            12000                                 31.8                                     34.7  920                                              20  &#34;__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________                 Heat   Magnetic Mechanical   Plate                                                  Cal-Sam-                  Treatment                        Properties                                 Properties   Thick-                                                  cula-ple   Steel Composition (wt %)                 Temp.                     Time   B.sub.1                                 vE.sub.0.sup.Ave                                     T.S.  Ac.sub.3                                              ness                                                  tion                                                      Re-No.   C  Si Mn P  S  sol. Al                 (°C.)                     (h)                        μ max                            (Gauss)                                 (kgf·m)                                     (kgf/mm.sup.2)                                           (°C.)                                              (mm)                                                  (×10.sup.3)                                                      marks__________________________________________________________________________1  0.06* 0.60    0.12       0.026          0.008             0.002                 880 1  6800                            5700  6.8                                     41.2  909                                              40  23.1                                                      Com-2  0.006 0.21*    0.09       0.009          0.007             0.002                 880 1  9700                            8200 21.4                                     24.4  905                                              20  &#34;   para-3  0.003 0.65    0.11       0.007          0.006             0.021*                 880 1  7100                            6700 24.5                                     34.1  925                                              &#34;   &#34;   tive4  0.006 0.62    0.72*       0.006          0.003             0.004                 880 1  4800                            4400 10.7                                     40.6  911                                              &#34;   &#34;__________________________________________________________________________ Note: *Outside the range of the present invention