PROCESS FOR PRODUCING A THIN PLATE OF A HIGH FERROSILICON ALLOY

Abstract: A ferroalloy containing more than 4% by weight of silicon is produced by casting thin pieces. The manufacturing conditions consist of quickly coagulating the ferroalloy from the molten state at a cooling rate of greater than 1°C/sec but less than 10⁵⁰C/sec. A thin cast piece which is obtained is subjected to hot rolling reduction of 30% or more at a temperature of 600°C to 800°C. After being washed with acid, the cast piece is cold rolled to a desired thickness. The above manufacturing techniques permit the cast piece to be cold rolled on an industrial scale without developing cracks. After cold rolling. the cast piece is annealed so as to exhibit excellent magnetic properties.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to a method of producing thin sheets of high Si-Fe alloy having excellent properties as soft magnetic materials. BACKGROUND OF THE INVENTION Since Si steel sheets are higher in magnetic permeability and electric resistance in comparison with electrical steel sheets containing no Si, and may be produced economically, those have been mass-produced as magnetic cores for electric power. It is known that Si steel sheet shows that the more is Si content, the better is the soft magnetic characteristic, and it shows a peak at 6.5% Si. However, since, if Si content were more than 4.0%, elongation would be rapidly lowered, ordinary cold rolling could not be carried out. Therefore, it has been industrially difficult to produce thin sheets of high Si-Fe alloy containing Si more than 3%. With respect to such difficulties, it is reported that if hot rolling conditions are appropriately selected after hot forging, cold rolling would be possible to a certain extent (Ishizaka et al: Journal of Japan Institute of Metals Vol 30 (1966) No. 6). This report teaches, melting the alloys containing 1 to 7% Si by high frequency induction furnace in the air so as to produce ingots of 50mm square, hot forging the ingots until 15mm thickness, machining these ingot test pieces on the surfaces until coming to 11mm thickness, hot rolling them until 1mm thickness at the temperatures of 1000, 850 and 750°C respectively; or hot rolling the samples until 5mm thickness at 750°C, followed by hot-rolling until 1mm thickness at 600°C, and hot rolling until 5mm thickness 750°C, followed by hot rolling until 3mm thickness at 600°C, hot rolling the samples until 1mm thickness at 450°C; and pickling and cold rolling them for observing appearance of crackings, in order to investigate influences of the hot rolling conditions to the cold workability. According to this report, at Si content of not more than about 4.7%, the cold rolling is possible, irrespectively of the hot rolling conditions, and at about 5% Si the cold rolling is also possible, irrespectively of the hot rolling conditions, if the edge of the hot rolled sheet is removed by machining. However in the steel sheet of more than about 6% Si, the cold rolling property thereafter depends upon the hot rolling temperatures, and especially the steel of around 6.5% Si may be cold rolled by performing the hot rolling at the temperatures of 600 to 750°C. On the other hand, there is a rapid solidification process (cooling rate is ordinarily more than 10⁵°C/sec) for making the thin sheets, other than the above mentioned rolling procedures (for example, Patent Specification Laid Open No. 16926/84). Since the former of said procedures necessarily requires the hot forging before the rolling, the process cannot but be discontinuous due to presence of the hot forging, resulting in the complicated process and the high production cost. Further, if the cast ingot is subjected to the hot forging, cracks are generated, and therefore surface-machining prior to the hot rolling is necessary. In fact, the experiment in said report carried out the machining of about 27% (15mm thickness to 11mm) for the surface treatment. In addition, for rolling the steel at the temperatures of less than 750°C where the cold rolling property was excellent, the steel could not be rolled directly at this temperature, and the rolling was involved with inconvenienses of undertaking a pre-rolling more than 750°C, followed by a subsequent rolling. As is seen, it is very difficult to practise the above mentioned processes in the industrial scale in view of the production cost and yield. The latter of the rapid solification process spouts the molten metal from the nozzle to the surface of a cooling roller and solidifies it, and is possible to produce thin plates continuously and at high yield. In this case, the maxiumum thickness is about 100 µ, and the width is about 20cm at the maximum. Therefore the usage is limited, and the production in the industrial scale has not yet been realized. From the document DE-B-2 024 525 is known a method of producing thin sheets of high Si-Fe alloy, wherein a normally cast ingot is hot rolled at temperatures above 650°C and then continuously rolled during cooling down between temperatures of 650°C and 350°C, and wherein during the following cold rolling a heating up of the product to temperatures of above 350°C is avoided. Further it is known from US-A-4 265 682 a method of producing thin sheets of high Si-Fe alloy, wherein very thin steel strips are made by cooling the molten steel super rapidly to a thin strip having thickness of between 50 and 100 µm, by completely eliminating the hot rolling and cold rolling steps, by producing a thin strip, having a thickness of 80 µm, by super rapid cooling, annealing the strip at 1200°C and rolling the same into a thickness of 65 µm, or by producing a thin strip having a thickness of 100 µm and immideately rolling it into a thickness of 50 µm. Obviously, in the known method, strips having a thickness significantly above 100 µm cannot be achieved, and further it is necessary to cool down the strip at a cooling rate higher than a critical cooling rate of 10³°C/sec which is preferably between 10³°C and 10⁶°C to obtain a thin steel strip of satisfactority fine cristall grains and substantically ordered lattice structure. The point of the above mentioned conventional process (the former) is present in carrying out the rolling at the temperatures of 600 to 750°C for improving the cold rolling property. But the rolling cannot be done instantly at such low temperatures, and it is indispensable as said above to perform the hot forging as the pre-treatment of the hot rolling. The forging is well known as the pre-treatment for processing and rolling material with less workability, but is inferior in the production and restrained with respect to shapes of products to be obtained. It is assumed that the reason exists in this point why the above process has not been yet practised. The inventors made studies for improving the hot and cold workability of the high Si-Fe alloy, and confirmed that the hot rolling at the temperature between 600°C and 750°C was made possible by the hot forging because the structure was made fine, and found that a fine structure which was obtained by rapid solidification, might be substituted for said fine structure. Further, the inventors paid attention to a process of casting thin pieces as a method for realizing said rapid solidification. At present, the cast technical field has had interests in a thin plate casting process because processes may become simple, and many casting processes have been proposed. Thicknesses of the cast pieces thereby are 30 to 0.5mm, and the cooling rates are lower than the so-called rapid solidification process (cooling rate: more than 10⁵°C/sec) but far higher than the ingot making process, and structures of produced steels are fine and uniform in grain, and further thicknesses are larger than the rapid solification process, and since the thin plate casting process may continuously produce cast pieces having large width, it is characterized by using the conventional processes after the hot rolling. The inventors made many investigations for employing said characteristics of the thin plate casting process, that is, direct production of high Si-Fe alloy plate of fine grains from the molten metal, and found that if the material produced by the thin plate casting process was hot rolled under determined conditions, it would be possible to produce high Si-Fe alloy excellent in the cold workability continuously and low production cost. Thus, the invention comprises thin plate casting Fe alloy containing Si more than 4.0wt% from the molten condition at the cooling rate of more than 1°C/sec to less than 10⁵°C/sec by a thin plate casting process, the thickness of the cast pieces being 0.5 to 30 mm, and hot rolling the obtained thin plate at reduction of more than 30% at a temperature range between 600°C and 800°C, followed by pickling, cold rolling and annealing. The invention will be explained in detail. The invention uses the high Si-Fe alloys containing Si more than 4.0wt%, which will include such alloys of so-called sendust alloy and the like other than general high Si-Fe alloys. Ordinary high Si-Fe alloys contain around 4.0 to 7.0wt% Si for providing magnetic characteristics. As mentioned above, magnetic permeability is increased by adding Si, and it becomes the maximum value when Si content is about 6.5wt%. Further, because an electric resistance is increased by Si addition, iron loss is lowered. In the materials of less than 4.0% Si, the hot rolling and the cold rolling are easily possible in the conventional processes. The invention also includes so-called sendust alloy and high magnetic permeable alloy called as super sendust alloy. These alloys are composed of, (a) Si: 8.0 to 10.0wt%, Al: 4.0 to 7.0wt%, the rest being substantially Fe and inavoidable impurities (b) Si: 4.0 to 8.0wt%, Al: 2.0 to 6.0wt%, Ni: 1.0 to 5.0wt%, the rest being substantially Fe and inavoidable impurities. They are brittle and the conventional art has not produced thin sheets via the rolling procedures. According to the invention, it is possible to produce thin sheets in the industrial scale with respect to the high magnetic permeable alloys which are difficult to be processed and further other materials with less formability. The present invention solidifies Fe-alloy of the above said chemical composition from the melts at the cooling rate of more than 1°C/sec to less than 10⁵°C/sec in the thin plate casting process. Fig. 1 shows relationship between the cooling rate and the crystal grain size of rapidly solidified 6.5wt% Si steel. As is seen from this diagram, since the crystal grain size of the cast plate becomes larger as the cooling rate becomes slower, the hot workability is deteriorated at a subsequent hot rolling. Therefore, the invention determines the lower limit of the cooling rate at 1°C/sec for providing the fine and uniform grain structure. In order to increase the cooling rate more than 10⁵°C/sec in the thin plate casting process, the thickness of the cast piece should be not more than 0.1mm, and it will be difficult thereby to obtain practicable materials having large width. Therefore, the invention determines the upper limit of the cooling rate at less than 10⁵°C/sec. The casting of thin plates may depend upon any process which can realize the above mentioned cooling rates, and any include twin roller process, melt spinning process, spray casting process, or hazellette process. The thus produced thin cast plate is undertaken with the hot rolling at the temperatures of 600 to 800°C and the reduction of more than 30%. This hot rolling may be performed after the thin cast plate is heated at the temperatures of 600 to 800°C, or until the temperature of the produced thin cast plate does not become less than 600°C. Fig. 2 shows the relationship between the hot rolling temperatures and the possible hot-rolling reduction, and Fig. 3 shows the relationship between the hot rolling temperatures and the cold rolling reduction after the hot rolling at the reduction of 80% at said hot rolling temperatures. The 6.5wt% Si steels were used in the experiments, which were cast into thin plate (thickness: 5mm) and then, hot rolled at the reduction rate of 80%. The hot and cold workability were evaluated by the cold rolling reduction where fine cracks would be visually observed. It is seen from Fig. 2 that the hot rolling of the reduction being 80% is possible at the temperatures of more than 600°C. However, if the hot rolled steel was subjected to the cold rolling, the cold rolling of the reduction rate of more than 60% was possible with only the samples hot rolled at the temperature range between about 600°C and 800°C, as shown in Fig. 3. Fig. 4 shows the relationship between the cold rolling reduction after the hot rolling was performed at the temperature of 730°C until the determined reduction, and the hot rolling reduction rate. As is seen from Fig. 4, the cold rolling is impossible if the hot rolling reduction is less than 30%. Further, Fig. 5 shows influences of the hot rolling condition (the hot rolling reduction and the hot rolling temperatures) to the cold rolling reduction. Thus, in the invention it is necessary to perform the hot rolling of the more than 30% reduction in the temperature range of 600 to 800°C. The steel sheet is carried out, after the hot rolling, with the pickling, cold rolling and annealing. The annealing after the cold rolling is important for providing the objective magnetic characteristics. Especially, the steel of 6.5wt% Si may be imparted with anisotropy by appropriate combination of the cold rolling and the annealing, and it is possible therewith to produce grain-oriented high Si-Fe alloy. At the final annealing, it is possible to form an insulation-coating, and perform a heat treatment in the magnetic field. According to the invention, the under mentioned effects may be obtained when producing thin sheets of high Si-Fe alloy excellent in magnetic characteristisc. 1) Complicated processes such as ingot-making, reheating and hot forging are not required, and the energy may be saved as much; 2) Since the material is not processed before the hot rolling, cracks do not appear on the surface, and only pickling after the hot rolling is enough for carrying the cold rollings; 3) The products may be coiled; 4) Since the structure of the cast piece by the thin plate casting process is composed of columnar grains oriented in the thickness, the anisotrophy may be easily controlled by the heating treatment after the hot rolling; 5) High Si-Fe alloy or other materials with less workability may be produced in the industrial scale, which have been conventionally impossible to be produced in the industrial scale. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the relationship between the average cooling rate of the solidification and the average crystal grains; Fig. 2 shows the relationship between the hot rolling temperatures and the possible hot rolling reduction; Fig. 3 shows the relationship between the hot rolling temperatures and the cold rolling reduction after the hot rolling of the reduction rate being 80%; Fig. 4 shows the relationship between the hot rolling reduction rate at the temperature of 730°C and the possible cold rolling rate; and Fig. 5 shows influences of the hot rolling conditions (hot rolling rate and the hot rolling temperatures) to the cold workability. THE MOST PREFERRED EMBODIMENT FOR PRACTISING THE INVENTION EXAMPLE 1 The steel of Table 1 was molten, refined, and cast in the thin plate casting machine of the twin roller type, and formed in 500mm width and 5mm thickness. The pieces were hot rolled, aiming at the reduction of 80% as changing the rolling temperatures, and the pieces rolled at the aimed reduction rate were cold rolled, after pickling, aiming at the reduction of 60%. Table 2 shows the rolling conditions thereof. As is seen from this Table, according to the invention, the hot rolling was possible without forging prior to the hot rolling, besides without pre-rolling, and those hot rolled at the temperature range between 600°C and 800°C could be subjected to the cold rolling for producing thin sheets of 500mm width and 0.4mm width. EXAMPLE 2 The thin plates (thickness: 5mm) of Table 2 were hot rolled at the reduction of 80% at the temperature of 700°C, followed by pickling, subsequently cold rolled at the reduction of 70%, and annealed in the dry H₂ gas atmosphere of 1200°C for 30 min. followed by measuring the magnetic characteristics. Table 3 shows the measuring results. As recognized from Table 3, in the products by the thin plate casting process, the improvement of the processing property and the uniformalization by the fine structure were provided and the improvement of the magnetic characteristics was provided. Test processes Direct current magnetic characteristics B₈(KB) Hc(Oe) µm Δµm(%) Invention process 15 0.2 34000 5 Conventional process 13 0.2 23000 30 Ingot 11 0.8 10000 43

Claims: A method of producing thin sheets of high Si-Fe alloy, comprising rapidly solidifying Fe alloy containing Si more than 4.0wt% from a molten condition at cooling rate of more than 1°C/sec to less than 10⁵°C/sec by a thin plate casting process, the thickness of the cast pieces being 0.5 to 30 mm, and hot rolling the obtained thin plate at reduction of more than 30% at a temperature range between 600°C and 800°C, followed by pickling, cold rolling and annealing. A method as claimed in claim 1, wherein Fe alloy contains Si 4.0 to 7.0wt%. A method as claimed in claim 1, wherein Fe alloy contains Si 8.0 to 10.0wt% and Al 4.0 to 7.0wt%. A method as claimed in claim 1, wherein Fe alloy contains Si 4.0 to 8.0wt%, Al 2.0 to 6.0wt% and Ni 1.0 to 5.0wt%. A method as claimed in claim 1, wherein the obtained thin cast plate is heated at the temperature range between 600°C and 800°C, and subjected to the hot rolling at reduction of more than 30%. A method as claimed in claim 1, wherein the thin cast plates are obtained by the thin plate casting progress, and subjected to the hot rolling while said plates are at the temperature range between 600°C and 800°C.

Inventors: HIURA, AKIRA, INAGAKI, JUNICHI, NAKAOKA, KAZUHIDE, TAKADA, YOSHIKAZU
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1985
Main classification: C21D8
IPC codes: C21D8, H01F1