IRON BASE, SOFT MAGNETIC STEEL MATERIAL

Abstract: This invention relates to an iron base, soft magnetic steel material which has high permeability and moreover, can be produced economically. The steel material of this invention uses a pure iron type component as a base and contains 0.5 to 2.5% of Al and, whenever necessary, 0.005 to 1.0% of Ti. Its ferrite crystal grain size is at least 0.5 mm, its magnetic flux density is at least 11,000 G at 0.5 Oe under the state where a lattice strain is removed sufficiently, its magnetic flux density is at least 15,500 G at 25 Oe and its coercive force is up to 0.4 Oe.

Description: TECHINCAL FIELD The present invention relates to soft magnetic ferrous materials, for instance, used as electromagnetic cores or magnetic shielding materials where good DC magnetization properties are required. TECHNICAL BACKGROUND Soft irons or pure irons, permalloy or supermalloy have been used as DC electromagnetic iron cores, or magnetic shielding materials or medical appliances, physical machinery, electronic parts or appliances, which have recently been remarkable especially in their demand development. A magnetic flux density at 1 Oe (called as "B₁ value" hereinafter) of the soft iron or the pure iron is about 3000 to 11000 G. It has been used as the magnetic shielding materials or MRI (tomogram diagnosis apparatus by a nuclear magnetic resonance) or those shielding uo to a level around several gausses of magnetic flux, or as electromagnetic iron core materials. In the usage where the DC magnetization property is important, problems of conventional techniques on the magnetic shielding will be described. The pure iron having a high saturated magnetization has been used to the magnetic shielding of MRI mainly because of its low cost and good performance. grade (concretely, JIS C2504 SUYPO) requiring the most severe properties in JIS specification on electromagnetic soft irons specifies a low limit of the B₁ value only to be 8000 G. Thus it is difficult for them to shield a level of the earth magnetism, and a shielding system of a level lower than several gausses has become bulky. An Fe-Ni alloy known as the permalloy or the supermalloy is sometimes used for more effective shielding. Those materials are possible to shield the magnetism lower than the earth magnetism but they are very expensive, and further their saturated magnetizations are as low as 1/3 to 2/3 of that of the pure iron. For shielding a high magnetic field, their thickness must be increased extremely. A good deal of their use is however difficult from an economical viewpoint. Taking the above mentioned situations into consideration, some studies have been made on heightening of the magnetic permeability without spoiling the high saturated magnetization of the pure iron materials. They are, for example, methods taught in Japanese Patent Publication No. 63-45443, Japanese Patent Laid Open No. 62-77420 or "Developments of Ultra Thick Electromagnetic Steel Plates" mentioned in No. 5 of vol. 23, (published in 1984) by Japan Metal Society. Each of these methods aims at improvement of the magnetic permeability through accompanying with coarsening of ferrite crystal grains. However, those technologies limit objects to hot rolled plates of small thickness, or they could not accomplish not less than 11000 G with the magnetic flux density at 0.5 Oe (called as "B 0.5 value" hereinafter). Thus they have not been sufficient for the use where a more severe DC magnetization property is appreciated as the present invention. Up to the present, such materials have not yet been offered that the saturated magnetization is high, and the magnetic permeabality is high, that is, the high magnetic flux density is generated in a low magnetic field corresponding to an extent of the earth magnetism. It is an object of the invention to offer such materials. DISCLOSURE OF THE INVENTION For solving the above stated problems, the inventors made investigations on industrial pure irons which were typical soft magnetic materials for the DC magnetic field. Through clearing defects thereof, we obtained knowledge under mentioned. From standpoints for obtaining the high magnetic permeability, following procedures were found to be effective. (1) The addition of Al makes an effective deoxidation possible, improves the magnetic permeability in company with decreasings of an oxygen amount and oxides, and lowers solute N which is harmful to the magnetic permeability by fixing it as AlN precipitates; (2) The addition of a certain necessary amount enables coarsening of finely scattered AlN, reduces bad influences of AlN themselves as low as possible, and considerably accelerates coarsening of ferrite grains by the annealing which is an instrument to remove the lattice strains, and each of these effects is profitable to the improvement of the permeability; (3) Especially the addition of more than 0.5 wt% raise transformation temperatures remarkably, or can provide a uniform ferrite phase, and enables annealing at temperatures exceeding 900°C without introducing strain used by the phase transformation, accordingly. The annealing brings about removal of lattice strain and the coarsening of the ferrite grains. The improvement of the magnetic permeability of solute Al itself may be also considered, but by synergetic effects thereof, very excellent permeability may be provided; (4) If Ti is added as required, the solute N is preferentially fixed by Ti and attributes to the improvement of the properties, so that an effort is not required for decreasing N content. From a standpoint of holding the saturated magnetization high, followed findings were obtained; (5) The Al addition exceeding 2.5 wt% should be avoided; (6) If C and N amounts are high, the transformation temperature lowers, or the necessary amount of Al increases. Further, the properties are deteriorated by the increment of the lattice strain by increasings of solute C and N or precipitations of carbides and nitrides The inventors found upper limits of C and N amounts for avoiding them, and accomplished the present invention. A first invention is to offer soft magnetic ferrous materials of an iron base, composed of Al: 0.5 to 2.5 wt%, Si: not more than 1.0 wt%, C+N: not more than 0.007 wt%, Mn: not more than 0.5 wt%, oxygen: not more than 0.005 wt%, the rest being Fe and unavoidable impurities; having ferrite crystal grain diameter of not less than 0.5 mm, showing magnetic flux density in 0.5 Oe being not less than 11000 G, magnetic flux density in 25 Oe being not less than 15500 G, and a coercive force of not more than 0.4 Oe under a condition that lattice strains are all removed. A second invention is to offer soft magnetic ferrous materials of an iron base, composed of Al: 0.5 to 2.5 wt%, Si: not more than 1.0 wt%, C+N: not more than 0.014 wt%, Mn: not more than 0.5 wt%, oxygen: not mroe than 0.005 wt%, Ti: 0.005 to 1.0 wt%, the rest being Fe and unavoidable impurities; having ferrite crystal grain diameters of not less than 0.5 mm, showing magnetic flux density in 0.5 Oe being not less than 11000 G, magnetic flux density in 25 Oe being not less than 15500 G, and a coercive force of not more than 0.4 Oe under a condition that lattice strains are all removed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing relation between C+N content and the DC magnetization property (B 0.5 value); and Fig. 2 is a diagram showing relation between sol.Al addition and the DC magnetization property (B 0.5 value and B₂₅ value). DETAILED DESCRIPTION OF THE INVENTION An explanation will be made to reasons for limiting the chemical composition of this invention. It is preferable to decrease C and N as low as possible for securing an excellent DC magnetization property, but an utmost decrease is difficult in industrial production since it causes an extreme cost-up. In view of raising the transformation temperature by Al addition, if the amount of C addition is not controlled to be low, the amount of Al addition should be increased, resulting in lowering the saturated magnetization, which is contrary to the intention of the invention. Fig.1 shows that the annealing is carried out under ordinary conditions at temperatures between 1000 and 1100°C, thereby to remove the lattice strains, and then a change of the DC magnetization property is taken as a change of B 0.5 value so as to study influences of C+N contents. According to this study, it is seen that the C+N content should be not more than 0.007 wt% for providing satisfactory properties. Thus C+N is determined to be not more than 0.007 wt% in the invention. Ti is added as required which is a strong nitride former as said later. Ti addition is for decerasing the above said harms of N without severely specifying an upper limit of N, resulting in high costs. Therefore, in this case, the upper limit of C+N is determined to be 0.014 wt%. Si contributes to the improvement of the magnetic permeability, but since coarse ferrite crystal grains of not less than 0.5 mm may be obtained by the Al addition after an annealing, the upper limit is 1.0 wt% for avoiding lowering of the saturated magnetization and the cost-up by much addition. Since Mn deteriorates the DC magnetization property, lower content is desirable, but an extreme lowering causes the cost-up and the increase of N content. Further, this element also suppress a hot brittleness by fixing S. It may be contained 0.5 wt% as an upper limit within a range that the Mn/S ratio is not lower than 10. Al is, as said above, the most important element of this invention. It brings about the fixing of the solute N, the coarsening of AlN, and the raising of the transformation temperature, and as results, thereby expands a ferrite phase region, so that this element accomplishes the coarsening of the ferrite grains and the decreasing of the lattice strain by the annealing. Furthermore, it is assumed that solute Al itself improves the DC magnetization proeprty. Thus, in the present invention, this element must be added for providing the excellent DC magnetization property. As is seen in Fig.2, such effect of Al may be obtained by adding not less than 0.5% in a value of sol.Al. On the other hand, it is undesirable to add exceedingly 2.5%, because B₂₅ value is lowered by decreasing the saturated magnetization. Al addition is determined to be 0.5 to 2.5% in the value of sol.Al. Ti is the strong nitride former as said above. If adding it 0.005 to 1.0%, it is possible to avoid considerable damages of the DC magnetization property by a fixing solute N even in such materials where N content is not fully decreased, that is, cheap materials. If the N content is relatively low, the generating amount of nitride is low, and the DC magnetization property may be expected to be improved more or less, accordingly. The Ti addition of more than the upper limit deteriorates the DC magnetization property. If the chemical composition is limited as above according to the invention, such ferrous materials may be produced which have the high B 0.5 value and B₂₅ value, that is, the excellent soft magnetic properties in the DC magnetic field. The ferrous materials of the invention include hot worked, cold worked and warm worked materials, and include these kinds of plates, sheets, bar, wire materials (shape steels, etc.), forged materials, and others. The ferrous materils of the invention may be produced by the hot working process of cast pieces, the warm or cold working processes of as-cast pieces, the hot working followed by cold or warm working process, the direct-rolling process, the annealing (ordinarily not lower than 450°C) between the workings in the above respective processes, and others. In such of them, a final annealing is performed at the temperatures of ordinarily not lower than 900°C, preferably 1000 to 1300°C. EXAMPLE Table 1 shows chemical compositions of the inventive and comparative examples. Steels B-G, J, L, N-T, V-X and Z belong to the composition of the invention, and Steels A, H, I, K, M, U, Y and a are the comparative examples. Table 2 shows results that the steels of Table 1 were made ingots of 110 mm thickness after melted, hot rolled into thickness of 15 mm at a temperature of 1200°C, and measured, after the annealing, with respect to the DC magnetization properties and the ferrite crystal grain diameters. The annealings were performed under ordinary conditions of heating - holding time for 1 to 3 hours and cooling rates of 100°C/hr to 500°C/hr. In Table 2, the influences of the sol.Al contents were studied in Nos.1 to 9 and No.21, and No.21 was a comparative example of the pure iron. Fig.2 shows summations thereof. Nos.10 to 13 and No.25 studied influences of the C+N content. Fig.1 shows these results, to which the result of No.4 was added. According to these results, it is recognized that in a case of no Ti-addition, when the C+N content exceeds 0.007%, B 0.5 value is deteriorated. Nos.14 to 16 studied influences of the Mn contents, where the DC magnetization was deteriorated as increasing of Mn content, but it might be assumed that a desirable property was secured in a range not exceeding 0.5%. Nos.17 to 20 studied influences of the Si contents, where the magnetic flux densities (B 0.5 value, B₁ value and B₂₅ value ) were lowered by lowering of the saturated magnetization along with the increasing of Si, but desirable properties were still secured. Further, since it is known that the Si addition increases a proper resistance of the steel as Al, the material was cold rolled into a thin sheet, and when using to soft magnetic ferrous materials to be used in AC magnetic field, the decreasing of iron losses may be expected. Nos.22 to 24, No.26 and No.27 studied influences of Ti additions. Since N was fixed by adding Ti, preferable properties were acknowledged. No.23 is an inventive steel where Ti was added to a steel equivalent to No.11 (comparative steel). No.26 is an inventive steel where Ti was added to a steel equivalent to No.25 (comparative steel). In each of them, in spite of C+N > 0.007%, N was fully fixed by Ti, and they were largely improved in comparison with the comparative ones of No.11 and No.25 Table 3 shows results that some steels of Table 1 were hot rolled, and cold rolled into thin sheets, and subjected to the ordinary annealings, and studied in the DC magnetization properties as in Table 2. The cold reduction rates shown in the inventive examples and the comparative ones were 50 to 80%. In Table 3, No.1 and No.2 were the comparative examples of Steel U, while Nos.3 to 6 were the inventive steels which reveal the desirable DC magnetization properties in comparison with the comparative examples of Nos.1 and 2. In each of Tables 2 and 3, the inventive steels have the desirable DC magnetization properties, and are all not less than 0.5 mm in the ferrite crystal grain diameters. As stated above, the soft magnetic ferrous materials of the invention have the excellent DC magnetization properties, and therefore may be easily magnetized even in very weak magnetic fields, and those are useful as iron cores of high function or magnetic shielding materials of high function. INDUSTRIAL APPLICABILITY The present invention may be applied to soft magnetic ferrous materials of iron base where the high DC magnetization properties such as an electromagnetic core and a magnetic shielding material are required.

Claims: Soft magnetic ferrous materials of an iron base, composed of Al: 0.5 to 2.5 wt%, Si: not more than 1.0 wt%, C+N: not more than 0.007 wt%, Mn: not more than 0.5 wt%, oxygen: not more than 0.005 wt%, the rest being Fe and unavoidable impurities; having ferrite crystal grain diameters of not less than 0.5 mm, showing magnetic flux density in 0.5 Oe being not less than 11000 G, magnetic flux density in 25 Oe being not less than 15500 G, and a coercive force of not more than 0.4 Oe under a condition that lattice strains are all removed. Soft magnetic ferrous materials of an iron base, composed of Al: 0.5 to 2.5 wt%, Si: not more than 1.0 wt%, C+N: not more than 0.014 wt%, Mn: not more than 0.5 wt%, oxygen: not more than 0.005 wt%, Ti: 0.005 to 1.0 wt%, the rest being Fe and unavoidable impurities; having ferrite crystal grain diameters of not less than 0.5 mm, showing magnetic flux density in 0.5 Oe being not less than 11000 G, magnetic flux density in 25 Oe being not less than 15500 G, and a coercive force of not more than 0.4 Oe under a condition that lattice strains are all removed.

Inventors: KUNISADA YASUNOBU, OMORI TOSHIMICHI, SANPEI TETSUYA, SUZUKI HARUO, TAKANO TOSHIO, KUNISADA, YASUNOBU, OMORI, TOSHIMICHI, SANPEI, TETSUYA, SUZUKI, HARUO, TAKANO, TOSHIO
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1990
Main classification: H01F1
IPC codes: C22C38, H01F1, C21D8