PROCESS FOR PRODUCING THIN MAGNETIC STEEL PLATE HAVING HIGH PERMEABILITY

Abstract: A thin steel plate manufactured through ordinary steps is placed in an atmosphere which contains SiCl₄, and a silicon permeation treatment is effected at a silicon permea­ tion temperature of 1100 to 1200°C for a predetermined period of time. At this moment, the heating rate in the SiCl₄-containing atmosphere is set to at least 50°C per minute at a temperature of 1000°C or above. This makes it possible to produce a thin magnetic steel plate of a high permeability without internal flaw.

Description: This invention relates to a method for producing thin steel sheets of high magnetic permeability, and is to produce thin steel sheets of high Si magnetism without internal defects by diffusing and penetrating Si into low Si thin steel sheets. In Fe-Si alloy and Fe-Si-Al alloy, there are Fe-6.5%Si alloy and Fe-9.6%Si-5.4%Al alloy (sendust) which have very high magnetic permeability and excellent soft magnetic characteristics. Especially, the sendust has been applied to electronic instrumentalities such as dust cores, magnetic heads and others since its invention in 1973. With respect to the magnetic head, a high coercive field strength of recording media has advanced nowadays, accompanying with high density of magnetic recording media, and the sendust of high saturated magnetization has been interesting since this material is more suitable to the recording than ferrite headsused conventionally. Since Fe-6.5%Si alloy has high saturation flux density, this material is considered to be applied to iron cores of transformers, or other electric, electronic instrumentalities. A problem when these high Si alloys excellent in the soft magnetic characteristics are used for electronic parts, is that they could not be rolled in thin shape, since they have brittleness. Therefore, the sendust is sliced after forging to produce thin pieces for the magnetic heads, which is, however, a process very inferior in efficiency in the production of the heads. Besides, the sendust is easily caused with cracks or pinholes during solidification after casting, and those defects should be removed for which, however, a process is required. For solving the problems involved with the above mentioned manufacturing process, the below mentioned processes have been proposed. 1) Rolling and deforming in hot work 2) Improvement of workability by addition of elements 3) Direct production by rapid solidification 4) Composition control after rolling The above mentioned process (1) is made possible by super slow strain rate at the temperature of more than 1000°C, however it would introduce much difficulties in practising such a condition industrially. The attempt (2) more or less improves the workability by adding the elements, but the material is brittle, and an application to the thin sheet is difficult and the added elements deteriorate the magnetism. The process (3) directly casts the molten metal into the thin shape, and is very useful to the brittle material in regard to production of the thin sheets without the rolling process. The control (4) comprises, melting low Si or low Al steel, rolling it in thin shape, enriching Si or Al by penetration from the surface thereof, and finally producing high Si thin steel sheets. However, since conventionally proposed penetrating processes take penetration treating time as long as more than 30 minutes and temperatures as high as about 1230°C, the shapes after the penetrating treatment are undesirable. Further, the most fatal phenomenon in the prior art to the production of the high magnetic permeable materials is to generate large voids called kirkendall voids in accompany with the penetration, which remain in spite of the sintering treatment, so that the magnetic permeability is considerably declined. The reason why a process of producing high Si thin steel sheet by the Si penetration has not yet been realized, is difficult in removing the voids. From DE-A-1945 298 is known a method for treating a thin steel sheet in a SiCl₄ atmosphere to increase the Si-content of the steel sheet for high magnetic permeability. In the known method the steel sheet is heated and subjected for predetermined period of time to the SiCl₄ atmosphere for Si to penetrate into the steel sheet and to increase the Si-content thereof, and then is subjected to a diffusion treatment in an inert gas atmosphere as a homogenisation treatment. For carrying out the method continuously, the steel sheet is preheated in a preheating zone up to about 1038°C and then introduced into a zone of SiCl₄ atmosphere of which the temperature is between 1038 and 1260°C. A similar method for increasing the Si-content of a steel sheet is known from GB-A 1 083 290, where the steel sheet is subjected to a SiCl₄ atmosphere at a temperature between 1100 and 1250°C and then subjected to a homogenizing annealing process at a temperature of about 800°C and then cooled at above 100°C/min in a magnetic field. The object of the present invention is to provide a producing method, where a composition control process after rolling is improved for providing a desired content of Si in a short period of time and inhibiting the generation of voids. According to a first effect of the invention, this object is achieved by a method for treating a thin steel in a SiCl₄ atmosphere to increase the Si-content of the steel sheet for high magnetic permeability, including heating the steel sheet and subjecting the heated steel sheet for a predetermined period of time to said SiCl₄ atmosphere to cause Si to penetrate into said steel sheet, and subjecting the steel sheet to a diffusion treatment in an inert gas atmosphere, characterised in that heating is carried out at a heating rate of more than 50°/min at temperatures of more than 1000°C and essentially carrying out Si penetration between 1100°C and 1200°C during heating said steel sheet. According to a second aspect of the invention, this object is achieved by a method of treating a thin steel in a SiCl₄ atmosphere to increase the Si-content of the steel sheet for high magnetic permeability, including heating the steel sheet and subjecting the heated steel sheet for a predetermined period of time to said SiCl₄ atmosphere to cause Si to penetrate into said steel sheet, and subjecting the steel sheet to a diffusion treatment in an inert gas atmosphere, characterised by heating the thin steel in a furnace at temperatures between 1100°C and 1200°C, subsequently introducing the SiCl₄ atmosphere into the furnace and performing Si penetrating treatment for a determined period of time. The present invention has been realized to improve shortcomings of the conventional techniques, and is to provide a producing method, where a composition control process after rolling is improved for providing a desired content of Si in a short period of time and restraining the generation of voids. The inventors studied in detail the Si penetrating conditions in the prior art, and found a condition which accelerated the Si penetrating speed, and did not allow voids residual after the Si penetrating treatment and the diffusion treatment. The desired Si content was accomplished by the Si penetrating treatment, and subsequently thin sheets of high Si having very high magnetic permeability could be produced. The inventors made tests and studies, and found the best range where the voids were not generated with regard to the heating rate and the Si penetrating temperatures in the atmosphere bearing SiCl₄, and further found the best range with respect to partial pressure of Si compounds in said atmosphere. In the invention, thin steel sheets (thickness: 10mm to 10µm) are at first produced through an ordinary process. Kinds of magnetic thin sheets of high magnetic permeability available by the invention include 3 - 6.5%Si-Fe alloy and sendust alloy, and it is preferable to determine as mentioned under the composition of the thin steel sheets for Si penetration. 1) In a case of 3 - 6.5%Si-Fe alloy C: not more than 0.01%; Si: 0 - 4.4%; Mn: not more than 2%; and inavoidable impurities being preferably as little as possible 2) In a case of sendust alloy C: not more than 0.01%; Si: not more than 4%; Al: 3 - 8% Ni: not more than 4%; Mn: not more than 2%; elements increasing corrosion resistance such as Cr, Ti and others: not more than 5%; and inavoidable impurities being preferably as little as possible. These thin steel sheets are placed in an atmosphere bearing SiCl₄ for penetrating treatment. That is, the material is heated up to Si penetration treating temperature, and subsequently, the material is heated in the atmosphere bearing SiCl₄, and effected with soaking and penetration . This treating condition is, according to the invention, limited to the Si penetrating temperatures between 1100°C and 1200°C (temperature of the sheet). Fig. 1 shows the relationship between the Si penetrating temperature and the number of generated voids. As is seen from this graph, the number of the voids is almost zero above 1100°C after a diffusion treatment (later mentioned). Therefore, the lower limit is 1100°C. On the other hand, Fe₃Si to be formed in the Si penetrating layer will be molten away above 1200°C, and this temperature is an upper limit. High temperature as possible is advantageous for restraining the generation of voids. With respect to the number of the voids of the graph in Fig. 1, the cross section of the test piece having thickness of 0.4mm was measured over the width of 2.4mm, and the void number was counted (same also in Figs.2 and 5). According to the first aspect of the invention, the heating rate is limited to more than 50°C/min, coming to said penetrating temperatures in the SiCl₄ atmosphere at the temperature of more than 1000°C. The reason for limiting the heating rate is for avoiding generation of kirkendall voids by the Si penetration at the temperature between 1000°C and the determined temperature during heating. Fig. 2 shows the relationship between said heating rate and the void number. The higher the heating rate is, the more the void number decreases, and since the voids almost fade away, this rate is determined as the lower limit. Said heating rate is, to the end, in the SiCl₄ atmosphere at the temperature of more than 1000°C, and not a few ways are available for providing the heating rate of more than 50°C/min. For example, the most ordinary manner is to place the thin steel sheet made by the ordinary process as at the room temperature into the heating furnace of the SiCl₄ atmosphere, and heat it to the determined penetrating temperature. If it is difficult to obtain the heating rate of more than 50°C/min by the above mentioned manner, according to the second aspect of the invention, it is possible that the thin steel sheet is in advance heated to the set temperature of 1100 to 1200°C in the furnace of an inert gas atmosphere, and SiCl₄ steam is introduced into said furnace. In this case, since the heating is not performed in the atmosphere of SiCl₄ at the temperature between more than 1000°C and not more than 1100°C, the heating rate can be made infinite. A compromise manner thereof may be assumed variously as preheating the thin steel sheet more than 1000°C, introducing it in the heating furnace of the atmosphere of SiCl₄, and heating to the set temperature. When the steel sheet is preheated, oxidation should be avoided as possible as could. Because the oxidation of the thin steel accelerated forming of Fe-Si oxides of low melting point during Si penetration, and obstacles the intention of the invention. Fig. 1 is a graph showing the relationship between Si penetrating temperature and the number of voids; Fig. 2 is a graph showing the relationship between the heating rate and the number of voids; Fig. 3 is microscopic photographs of metal structures in cross section, showing differences in generation of the voids by the cooling rates; Fig. 4 is a graph showing the relationship between time for Si penetrating treatment and weight change of the steel sheet, where the amount of SiCl₄ is a parameter; Fig. 5 is a graph showing the relationship between the amount of SiCl₄ and the number of the voids; Fig. 6 is a graph showing the relationship between the amount of SiCl₄ and the coercive force; Fig. 7 is an arrangement for practising the invention; Figs.8 and 9 are microscopic photographs of metal structures in cross section; and Fig.10 is a graph showing iron loss W17/50 before and after the penetrating treatment. When the Fe-5.4%Al thin steels (thickness: 0.40mm) were undertaken with the Si penetrating treatment in the SiCl₄ atmosphere at the temperature of 1190°C for 30 minutes, the heating rates up to 1190°C from 1000°C were 10°C/min, 50°C/min and 300°C/min, respectively. Fig. 3 shows respective structures in cross section after Si penetration. Apparently, it is seen that the generation of the voids (black part in centers of the photograph) is restrained in the higher heating rate. The inventors, through many tests and studies, found that the partial pressure of Si compound was a large factor concerning the speed of Si penetration from the outer atmosphere, and the higher the partial pressure of Si compound is, the faster is the speed of the Si penetration, while the higher the partial pressure is, the more increases the void number, on the other hand. Fe-5.4%Al steels were treated in the SiCl₄ atmosphere, and Fig. 4 shows weight changes of the thin steels when the amounts of SiCl₄ in the introduced gas were changed 10%, 16% and 55% for changing the partial pressure of SiCl₄. The weight change is a parameter which shows the degree of the Si penetration, according to which the larger the weight change is, the more is the Si penetration. This phenomenon is assumed to depend upon the reaction of 5Fe + SiCl₄ → Fe₃Si + 2FeCl₂ where FeCl₂ is out of the solid. It is seen from Fig. 4 that the higher Si partial pressure is, the faster is the speed of Si penetration. However, with respect to the void amount, it is recognized that when Si partial pressure becomes higher, the void amount increases. Fig. 5 is the relationship between the amount of SiCl₄ and the amount of voids after the Si penetration treatment and the diffusion treatment, and clearly shows that when Si partial pressure becomes higher, the void amount increases. This reason is not cleared, but would be assumed as follows. When the amount of SiCl₄ in the introducing gas is made less, the amount of Si decreases which penetrates from the outside per time unit and the unit surface area , and this fact shows that the amount of Si atoms also decreases which penetrate into the interior through kirkendall surface, and porosities, that is, generation of kirkendall voids decreases. Under such circumstances, since the diffusion of Fe and Si atoms which are caused by thermal activity of test pieces, progress in order together with the Si penetration, said diffusion is easily absorbed or extinguished by dislocations or the like in the interior, before the generated kirkendall voids gather and turn out stable voids Therefore, if the Si penetrating speed is lowered, the voids are restrained from residual. The inventors studied the Si partial pressure and the magnetic permeable characteristics of the products and found that, as shown in Fig. 6, the less the amount of SiCl₄ is, the lower is the coercive field strength. By this finding, it is preferred that the amount of SiCl₄ in the atmosphere is not more than 25%. That is, as seen from Fig. 5, the voids are not generated when SiCl₄ is less than 25%. Fig. 6 shows that the lowering of the coercive field strength is saturated at less than 25%SiCl₄. From these two viewpoints, it is preferred to limit the amount of SiCl₄ to not more than 25% in the atmosphere of Si penetrating treatment. A limitation is not especially made to the time of Si penetrating treatment, and it may be appropriately determined in view of the amount of Si in the product, Si content in the atmosphere bearing SiCl₄, the penetration treating temperature, Si content in the starting steel sheet, and others. After Si has been penetrated at a desired amount by the above treatment, the chemical elements are uniformallzed by the diffusion treatment. The diffusion treatment may be continuously carried out by switching the atmosphere to an inert gas, instead of cooling the base sheet, otherwise it may be done after the base sheet has been once cooled to room temperature. When the base sheet is once cooled to room temperature, the cooling should be carried out in inert gas atmosphere or in SiCl₄ atmosphere for avoiding oxidation. When cooling in SiCl₄ atmosphere, it is necessary to shorten the passing time of the temperature range of more than 1000°C (especially 1000 to 1100°C), as similarly in the heating, for controlling the generation of the voids, and the cooling rate at the temperature of more than 1000°C should be more than 50°C/min. The diffusion treatment is carried out at a determined temperature in relation with the treating time, and it is done in inert gas atmosphere for avoiding oxidation. The diffusion treating time is appropriately selected in response to said treating temperature, thickness and Si content of an objective product. If the material produced by the invention shows the effect of magnetic annealing (e.g., Fe-6.5%Si, or Fe-Si-Al-Ni alloys), the soft magnetism may be improved by igniting the magnetic field in the course of cooling during the diffusion treatment. This manner has the advantage that the heating treatment is performed at the same as the diffusion treatment without requiring an independent heating treatment with respect to the cooling in the magnetic field, thereby improving magnetism. A condition of cooling in the magnetic field is to cool the magnetic field of more than 1G at the cooling rate of not more than 30°C/sec from the temperature of more than 800°C. The cooling effect of the magnetic field could not be expected in the outside of said range. THE MOST PREFERRED EMBODIMENT OF THE INVENTION EXAMPLE 1 Alloy of the chemical composition shown below was subjected to the hot and cold rollings so as to produce a thin sheet of 0.40mm thickness as a base sheet. This base sheet was performed with Si penetrating treatment through the device shown in Fig. 7, where the numeral 1 is a round bottom flask filled with SiCl₄, the numeral 2 is a thermostat bath, 3 is a furnace, and (X) is a test piece. SiCl₄ in the introducing gas was changed by controlling the temperature of the thermostat bath of a SiCl₄ vaporizer. The conditions of the penetrating treatment each depended upon the condition where Si penetrated up to 9.6% The furnace 3 for the Si penetrating treatment had a heating element of silicon carbide. A core tube of the furnace was made of ceramics and 40mm in inner diameter. A carrier gas of SiCl₄ was Ar and its flow amount was 0.5ℓ/min. When the test pieces subjected to the Si penetrating treatment were chemically analyzed, it was found that each of them contained the objective Si content (9.6%). Figs.8 and 9 are the photographs of the structure in cross section of the test pieces A to D after Si penetrating treatment and after the diffusion treatment in the inert gas atmosphere at the temperature of 1200°C for one hour. It is seen that the more SiCl₄ is in the introducing gas, the more distinguished is the generation of the voids after Si penetrating treatment as well as after the diffusion treatment. In the structures after the diffusion treatment, the test piece D has large and many residual voids while the test pieces A to C show scarecely voids. EXAMPLE 2 Fe-6.5%Si thin steel sheet was produced from the base sheet (thickness: 0.4mm) of the chemical composition down below. The penetrating treatments were performed by variously changing the conditions as below. Subsequently to these test pieces, the test pieces were subjected to the diffusion treatment of 1200°C x 3h in the Ar flow, and thereafter formed into rings of 10mm inner diameter and 20mm outer diameter by an electric discharging process, and coiled with 30 turns of a primary windings and 40 turns of a secondary windings for carrying out DC magnetism measurement. The results are shown in Table 5. From the above, it is seen that the test pieces A and B show more satisfactory magnetic characteristics than the test pieces C and D of the comparative processes. EXAMPLE 3 The base sheet of Fe-3%Si thin steel of the same chemical composition as EXAMPLE 2 were subjected to the Si penetrating treatment and the diffusion treatment under the following conditions for producing Fe-6.5%Si thin sheet. SiCl₄: 25% Penetration treating condition: 1190°C x 6min Heating rate: 300°C/min Diffusion treatment: 1200°C x 3h in Ar Cooling conditions: Cooling from not more than 1200°C to 800°C at 50°C/min and cooling from not more than 800°C to the following 10°C/min by the DC magnetic field of 8 Oe. When the magnetic characteristics were measured in the above treated materials, they showed preferable values of the maximum magnetic permeability of 38000. EXAMPLE 4 Fe-6.5%Si thin steels were produced from Si steel of grain oriented property (thickness: 0.30mm) prepared by GOSS process known per se. The chemical composition of the steel and the Si penetrating treatment conditions are shown in Tables 6 and 7. Subsequently to each of the test pieces, the test pieces were subjected to the diffusion treatment of 1200°C x 2 in Ar flow, and iron loss was sought at ignition of 50Hz and 17 kG by a single magnetic tester. Fig.10 shows iron loss value W17/50 before and after the penetrating treatments. The test pieces by the invention show more satisfactory magnetic characteristics than the comparative examples.

Claims: Method for treating a thin steel in a SiCl₄ atmosphere to increase the Si-content of the steel sheet for high magnetic permeability, including heating the steel sheet and subjecting the heated steel sheet for a predetermined period of time to said SiCl₄ atmosphere to cause Si to penetrate into said steel sheet, and subjecting the steel sheet to a diffusion treatment in an inert gas atmosphere, characterized in that said heating is carried out at a heating rate of more than 50°C/Min at temperatures of more than 1000°C and essentially carrying out Si penetration between 1100°C and 1200°C during heating said steel sheet. Method as claimed in claim 1, characterized by specifying the amount of SiCl₄ at not more than 25 Vol% in the SiCl₄ atmosphere. Method for treating a thin steel sheet in a SiCl₄ atmosphere to increase the Si-content of the steel sheet for high magnetic permeability, including heating the steel sheet and subjecting the heated steel sheet for a predetermined period of time to said SiCl₄ atmosphere to cause Si to penetrate into said steel sheet, and subjecting the steel sheet to a diffusion treatment in an inert gas atmosphere, characterized by heating the thin steel in a furnace at temperatures between 1100°C and 1200°C, subsequently introducing the SiCl₄ atmosphere into the furnace and performing Si penetrating treatment for a determined period of time. Method as claimed in claim 1 or 2, characterized by preheating the thin steel at the temperatures of more than 1000°C, leading it into the SiCl₄ atmosphere, and performing the Si penetrating treatment for a determined period of time. Method as claimed in one of claims 1 to 4, characterized by performing the Si penetrating treatment, cooling the thin steel in an inert gas atmosphere, and carrying out a diffusion treatment at a determined temperature in the inert gas atmosphere. Method as claimed in one of claims 1 to 4, characterized by leading the thin steel into the inert gas atmosphere just after the Si penetrating treatment. Method as claimed in one of claims 1 to 6, characterized by performing the Si penetrating treatment, cooling the thin steel in the SiCl₄ atmosphere at a cooling rate of more than 50°C/min at the temperature of more than 1000°C, and carrying out a diffusion treatment at a determined temperature in the inert atmosphere. Method as claimed in one of claims 1 to 7, characterized by cooling the thin steel in a magnetic field in the diffusion treatment. Method as claimed in claim 8, characterized by cooling the thin steel in the magnetic field of more than 1G at a cooling rate of more than 30°C/sec from a temperature of more than 800°C.

Inventors: NAKAOKA, KAZUHIDE, TAKADA, YOSHIKAZU, TANAKA, YASUSHI
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1985
Main classification: C23C10
IPC codes: C23C10, H01F1, C21D8