Electrical steel sheet

Abstract: An electrical steel sheet contains 0.01 wt.% or less C, 4 to 10 wt.% Si, 0.5 wt.% or less Mn, 0.01 wt.% or less P, 0.01 wt.% or less S, 0.2 wt.% or less Sol. Al, 0.01 wt.% or less N, 0.02 wt.% or less O, and the balance being Fe and inevitable impurities, the electrical steel sheet containing crystal grains and grain boundaries, said grain boundary having an oxygen content of 30 atomic % or less. The electrical steel sheet may contain 4 to 10 wt.% (Si + Al) instead of 4 to 10 wt.% Si. The grain boundaries may have the oxygen content of 30 atomic % or less and a sulfur content of 0.2 atomic %.

Description: Background of the Invention Field of the Invention The present invention relates to a high silicon electrical steel sheet used for a core material of transformers and electric motors. Description of the Related Arts An electrical steel sheet is widely used as a core material of electric motors and transformers. The electrical steel sheet generally contains silicon to control a texture and to improve specific resistance. An iron alloy containing 6.5 wt.% silicon exhibits best soil magnetic property owing to their magnetostriction substantially near to zero. However, increase of silicon content makes steel brittle, and high silicon steel containing silicon at 4 wt.% or more can not be formed to a thin steel sheet using an ordinary rolling method. To cope with this issue, several methods to obtain a high silicon thin steel sheet have been proposed. One of them is direct production by rapid solidification which produces the high silicon thin steel sheet directly from melt by casting, which method, for example, is disclosed in Japanese Examined Patent Publication No. 60-32705. Another one is application of a special rolling method, which method, for example, is disclosed in Japanese Examined Patent Publication No. 3-80846 and a further one is a siliconizing method where silicon is enriched into a low silicon steel sheet prepared by rolling, which method, for example, is disclosed in Japanese Examined Patent Publication 2-60041. Among them, the siliconizing method has already been brought into commercial use. A high silicon steel sheet prepared by either one of the methods described above is necessary to be processed by punching, shearing, and bending before applying to the electric motors and the transformers. The high silicon steel sheet has, however, a problem of brittleness which induces cracks and chips at punched or sheared corners and tends to generate breaks during a bending process. Aiming to improve the processing of high silicon steel sheet, various proposals were made. Japanese Examined Patent Publication No. 61-15136 discloses a method to obtain a high silicon steel sheet having superior workability and magnetic characteristic by controlling a grain size to a range of from 1 to 100 µ m and by controlling the crystal grains to have columnar crystals grown vertically to a thin sheet surface while substantially eliminating ordered lattice. Japanese Unexamined Patent Publication No. 62-270723 discloses a method to obtain a high silicon steel sheet having substantially high workability by forming a steel having as-rolled texture into a product shape followed by annealing. Japanese Unexamined Patent Publication No. 4-165050 discloses a method to obtain a high silicon grain-oriented steel sheet having high workability by adding Mn to suppress bad influence of solid solution of sulfur and by increasing grain orientation. Nevertheless, the method of Japanese Examined Patent Publication No. 61-15136 does not give its target effect when the grain diameter increases to 100 µm or more, and, for a high silicon steel, the method needs to employ a quenching step such as water-quenching from a high temperature of 900°C or more to substantially eliminate a ordered grain phase. Consequently, the method faces a difficulty in practical application. The method of Japanese Examined Patent Publication No. 62-270723 employs a working of steel having rolled texture so that the method needs high temperature annealing after the working. Thus, the method has a disadvantage of adding an extra step to the manufacturing processes of the transformers and the electric motors. The method of Japanese Unexamined Patent Publication No. 4-1605050 needs to use highly grain oriented steel. Obtaining the high grain orientation is difficult owing to poor stability of secondary re-crystallization using inhibitor. In addition, this method has a disadvantage that it can not be applied to a non-oriented silicon steel sheet. JP-A-2 267 246 discloses a process for manufacturing an electrical steel similar to that in claim 1 but it does not disclose controlling the dew point of nitrogen by stipulating a dew point of -70°C during siliconization. Summary of the Invention An object of the invention is to provide a process for preparing an electrical steel sheet having high workability. The present invention provides a process for preparing an electrical steel sheet having a thickness of 0.5 mm or less and which comprises 0.01 wt.% or less of C, 4-10 wt.% of Si a part of which may be substituted by Al, 0.5 wt.% or less of Mn, 0.01 wt.% or less of P, 0.01 wt.% or less of S, 0.01 wt.% or less of N, 0.02 wt.% or less of O, and a balance of Fe and inevitable impurities, the sheet containing (i) crystal grains having an average diameter of 2 mm or less, and (ii) grain boundaries having an oxygen content of 30 atomic percent or less; the process comprising the step of: diffusing additional silicon into a silicon steel sheet by treating the sheet with a mixture of gases comprising a siliconizing gas and high purity nitrogen gas having a dew point of -70°C. Preferably, the siliconizing gas is SiCl 4 . It is preferred that the steel sheet should comprise 0.2 wt.% or less of soluble Al. In this case, it is further preferred that the steel sheet comprises 0.0005 to 0.02 wt.% of oxygen. It is further preferred that the grain boundaries should have an oxygen content of 15 atomic % or less. It is further preferred that the grain boundaries should have a carbon content of at least 0.5 atomic %, and more preferably of at least 0.8 atomic %. It is further preferred that the grain boundaries should have a sulphur content of 0.2 atomic % or less. Brief Description of the Drawings Fig. 1 is a graph showing a relation between degree of vacuum in a final heat treatment atmosphere and three points bending characteristic of a steel sheet according to the present invention; Fig. 2 is a graph showing a relation between an oxygen content in grain boundaries and an elongation of the steel sheet according to the present invention; Fig. 3 is a graph showing a relation between a grain boundary intensity parameter and a three points bending characteristic of the steel sheet according to the present invention; Fig. 4 is a graph showing a relation between an average grain diameter and the three points bending characteristic of the steel sheet according to the present invention; Fig. 5 is an illustration of a three points bending test method which evaluates workability of a steel sheet; Fig. 6 is a graph showing a relation between the oxygen content in the grain boundaries and the three points bending characteristic of a steel sheet in Example 1; Fig. 7 is a graph showing a relation between a sulfur content in the grain boundaries and the three points bending characteristic of a steel sheet according to the present invention; Fig. 8 is a graph showing a relation between the sulfur content in the grain boundaries and the elongation of the steel sheet according to the present invention; Fig. 9 is a graph showing a relation among a sulfur content, the oxygen content in the grain boundaries and the elongation of the steel sheet according to the present invention; Fig. 10 is a graph showing a relation between an average grain diameter and the three points bending characteristic of the steel sheet according to the present invention; and Fig. 11 is a graph showing a relation between the sulfur content in the grain boundaries and the three points bending characteristic of a steel sheet in Example 2. Detailed Description Since a mother phase of a high silicon steel sheet is brittle in nature, the sheet has been considered substantially unable to improve the workability. Meanwhile, the inventors have carried a series of experiment aiming at improvement of the workability of the high silicon steel sheet focusing on the workability under various levels of dew points and oxygen concentrations in an atmosphere of final heat treatment, and found that there exist a steel sheet which shows relatively high workability among others while containing the same percentage of silicon. Fig. 1 shows a test result of the workability of the steel sheet. Degree of vacuum was varied in the test to change the dew point and the oxygen concentration in an annealing atmosphere. A horizontal axis represents the degree of vacuum, and a vertical axis represents a bend amount of a specimen in a three points bending test (the test to determine the maximum stroke before breaking the specimen under a condition of pressing down shown in Fig. 5) as an index of the workability. The annealing was carried out at 1,200°C for 15min. The test result showed that the higher the degree of vacuum is, the more the workability improved. Those tested samples were investigated to clarify a fracturing mechanism of the high silicon steel sheet, and a strong relation between working performance and a state of fractured surface was revealed. In concrete terms, a high silicon steel sheet which gave poor workability showed much intergranular fractured surface, and a high silicon steel sheet which gave superior workability showed much cleavage fractured surface. When a sample of high workability and a sample of poor workability were studied to determine the oxygen content on the intergranular fractured surface using Auger electron spectroscopy, the sample of high workability gave low oxygen content in grain boundaries, and the sample of poor workability gave high oxygen content in the grain boundaries. Further study of an obtained Auger spectrum showed that there are relations not only between the oxygen content in the grain boundaries and the workability but also between a carbon content in the grain boundaries and the workability. Since the tests described above did not define a condition to control quantity of carbon, change in the amount of grain boundary carbon is considered to be a joint phenomenon with a behavior of grain boundary oxygen. Detail of a mechanism is, however, not known. In addition, it was found that a change of annealing temperature easily controls a grain diameter and largely varies the workability. Consequently, the inventors found that the workability of the high silicon steel sheet which were considered to have substantially poor workability in nature has, actually, extremely high correlation with a characteristics of grain boundaries and that a high silicon steel sheet having excellent workability is obtained by controlling the characteristics of grain boundaries. The present invention will be now described in more detail, with preferable range of elements. Carbon is a harmful element for soil magnetic property, and when C content exceeds 0.01 wt.%, the soil magnetic property degrades with time, which phenomenon is called "age degrading". To avoid such a disadvantage, the C content of 0.01 wt.% or less is required. Silicon content of approximately 6.5% makes the magnetostriction zero and shows the best soil magnetic property. When Si content is less than 4 wt.%, the silicon steel sheet does not show a wanted magnetic property, and the workability of the steel sheet raises no specific problem. When Si content exceeds 10 wt.%, saturation flux density significantly decreases. Therefore, the Si content is specified as in a range of from 4 to 10 wt.%. A part of Si is able to be substituted with Al. In that case, total quantity of Si + Al is necessary to be specified. When the total quantity of Si + Al is less than 4 wt.%, the magnetic characteristics being aimed by the present invention can not be attained, and the workability of the steel sheet raises no specific problem. When the Si content exceeds 10 wt.%, the saturation flux density significantly decreases. Consequently, when a part of Si is substituted by Al, the total quantity of Si + Al is specified as in a range of from 4 to 10 wt.%. Manganese combines with S to form MnS to improve hot workability in a slab stage. When the Mn content exceeds 0.5 wt.%, however, reduction of the saturation flux density becomes significant, which is not preferable. Accordingly, the Mn content of 0.5 wt.% or less is required. Phosphorus is an element of degrading the soil magnetic property, and the content is preferred to decrease as far as possible. Since the P content of 0.01 wt.% or less raises substantially no bad influence and is preferred from economy, the P content is specified as 0.01 wt.% or less. Sulfur is an element to increase brittleness during a hot rolling stage and to degrade the soft magnetic property. Consequently, the content is preferred to decrease as far as possible. Since the S content of 0.01 wt.% or less raises substantially no bad influence and is preferred from the economy, the S content of 0.01 wt.% or less is desirable. Aluminum has an ability to clean steel by deoxidation and, from a viewpoint of the magnetic property, has a function to increase electric resistance. With a steel containing 4 to 10 wt.% of Si, an improvement of the magnetic property is performed by Si, and Al is expected only to perform the deoxidation. Therefore, the Sol. Al content is preferably specified as 0.2 wt.% or less. On the other hand, when a part of Si is substituted by Al, a total Si + Al content is specified as in a range of from 4 to 10 wt.%, as described above. Nitrogen is an element of degrading the soft magnetic property and also of inducing an age change of the magnetic property, so the content is preferred to decrease as far as possible. Since the N content of 0.01 wt.% or less raises substantially no bad influence and is preferred from the economy, the N content of 0.01 wt.% or less is required. Oxygen is an element of degrading the soft magnetic property, and the content is preferred to decrease as far as possible. As described later, the most important factor in the present invention is the oxygen content in the grain boundaries, and the O content is total quantity of O in both the grain boundaries and inside of grains. The present invention provides superior workability by controlling the content of oxygen in the grain boundaries which is unavoidably existing in the steel sheet, which is described later. When the O content in the steel sheet exceeds 0.02 wt.%, oxygen exists in both the grain boundaries and the inside of grains under all heat treatment conditions, and the state makes difficult to decrease the Oxygen content in the grain boundaries to 30 at.% or less. In other words, only the O content of 0.02 wt.% or less makes a selective control of oxygen existing region (inside of the grains or the grain boundaries) possible. Therefore, the O content is specified as 0.02 wt.% or less. On the other hand, a lower limit of the O content is not specifically defined. Simple reduction of the O content does not induce a decrease of the O concentration in the grain boundaries. However, excess reduction of O increases production costs. Consequently, from a economical reason, it is not preferable to reduce the O content to below 0.0005 wt.%. Other than elements described above, impurities of steel may include Cr, Ni, Cu, Sn, and Mo. Presence of each of these elements at approximately 0.03 wt.% does not affect the effect of the present invention. The O content in the grain boundaries (the O content in elements segregated to the grain boundaries) of the steel of the present invention is required to be 30 at.% (atomic percent) or less. This is the most important condition of the present invention. The O content at the grain boundaries means the oxygen content (at.%) in the elements segregated to the grain boundaries. Generally, Auger electron spectroscopy is employed to determine the oxygen content. According to the spectroscopy, a specimen is broken in a vacuum chamber which is held at 1 × (1/10 9 ) Torr or less, and the Auger electron spectrometry is applied while observing an intergranular fractured surface where is not polluted by atmospheric air. This procedure allows an elemental analysis on the clean intergranular fractured surface. The following is a general method of element determination using Auger electron spectrometry. (Refer to "Practical Auger Electron Spectroscopy for Users" Kyoritsu Shuppan, 1989.) When the element determination on a material surface is carried out, the measured Auger electron intensity (which is obtained by differentiation against energy and is represented by a peak height) and relative sensitivity, an index of emission efficiency of Auger electron for each element, are substituted into the equation below. [X] (at.%) = {(X/x)/[(A/a)+(B/b)+(C/c)+........+(X/x)+....]}×100 where A, B, C, and D .... Auger electron intensity of each element a, b, c, and d .... relative sensitivity of each element An energy position to measure the Auger electron intensity is defined for each element. For example, Fe uses a peak on a highest energy side among three LMM transitions, O uses a KLL transition, C uses the KLL transition, and S uses a LVV transition. The relative sensitivity was already known for each element transition, and these values are described in a literature cited above. According to Phai's unit, a value is 0.220 for Fe, 0.140 for C, 0.400 for O, and 0.750 for S. In this manner, the Auger electron spectroscopy has been widely used to determine an element quantitative value. Accordingly, the present invention also adopted this method to determine the elements at the grain boundaries. As described above, the inventors employed this method to determine the grain boundaries on each of the materials of superior workability and of inferior workability, and carried out the elemental analysis at the grain boundaries, and found that the O content at the grain boundaries has an extremely strong relation with a degree of easiness of workability. As an example, Fig. 2 shows a relation between elongation and the O content in the grain boundaries determined by the Auger electron spectroscopy using a high silicon steel sheet having chemical composition listed in Table 1 and having plate thickness of 0.1 mm. According to the Fig. 2, a steel sheet containing less O in the grain boundaries shows better elongation. Among steel sheets tested, the ones showing 3% or higher elongation gave plastic deformation. In addition, an observation of scanning electron microscope on a fractured surface revealed that the steel sheet having superior elongation gave rather a cleavage fracture than a grain boundary fracture and that the steel sheet having inferior elongation showed a tendency toward the grain boundary fracture. In the past, this type of high silicon steel sheet was accepted to induce no plastic deformation. However, it was found that when the O content in the grain boundaries is 30 at.% or less, the plastic deformation occurs. Consequently, the present invention specifies the O content as 30 at.% or less. From Fig. 2, the O content of 15 at.% or less in the grain boundaries provides better workability. (wt.%) C Si Mn P S Sol.Al N O 0.0023 6.65 0.15 0.0023 0.0012 0.001 0.0032 0.0089 In addition to the effect of the O content in the grain boundaries, which is described above, the carbon in the grain boundaries has been found to clearly play an important role on workability. In concrete terms, even in the O content range of 30 at.% or less in the grain boundaries, when the C content in grain boundaries (the C content in the elements segregated to the grain boundaries) is 0.5 at.% or more, the elongation is further improved and workability was improved. The Carbon in the grain boundaries are considered to have a suppression function against grain boundary cracks, though a detailed mechanism is not known yet. Accordingly, to obtain favorable workability of the high silicon steel sheet, it was found that the limitation of oxygen in the grain boundaries is necessary to be incorporated by an action of the carbon in the grain boundaries. To validate the effect, the inventors prepared a grain boundary intensity parameter which is expressed by following equation from the oxygen and carbon existing at the grain boundaries, and corresponded the equation to actual three points bending test results. Grain boundary intensity parameter = [C(284)]/[O(531)] Terms C(284) and O(531) in the equation represent signal intensity which is determined by differentiating the Auger spectrum with energy, and figures within parentheses indicate the energy where peak of C and O appears, respectively. Fig. 3 shows a relation between a grain boundary intensity parameter and a three points bending characteristic. According to the figure, the workability shown in the three points bending characteristic has an extremely strong relation with the grain boundary intensity parameter which was described above. At this point, when the bend amount in three point bending test is 5 mm or more, the workability is judged improved compared with prior art materials, so values of (C/Fe)/(O/Fe) (grain boundary intensity parameter) is 0.5 or more suggests the improved workability. When the bend amount in three point bending test is 10 mm or more, the workability is concluded to be significantly improved. Therefore, the grain boundary intensity parameter is preferred to specify as 1.0 or more. Regarding the C content in the grain boundaries, when the O content in the grain boundaries exceeds 30 at.%, there appears no effect of improvement of workability, and when the O content in the grain boundaries is below 30 at.%, if the C content in the grain boundaries is 0.5 at.% or more, the grain boundary intensity increases and the workability is improved. Consequently, the C content in the grain boundaries is preferred to specify as 0.5 at.% or more. For further improvement of workability, the C content in grain boundaries is preferably specified as 0.8 at.% or more. Adding to such a finding of the effect of the oxygen and carbon in the grain boundaries, it was found that the grain diameter affects the workability and that an average grain diameter viewed from a sheet surface at 2.0 mm or less further improves the workability. The reason for this effect is speculated that when the O content and C content in the grain boundaries are limited to strengthen the grain boundaries, the intensity inside of the grains relatively decreases and enhances the cracks across the grains, so an excessive grain diameter degrades the workability. Among the high silicon steel sheets (thickness: 0.1 mm) having the chemical composition listed in Table 1, the high silicon steel sheet having the O content in the grain boundaries of approximately 5 at.% and the C content in the grain boundaries of approximately 1 at.% were tested to determine the relation between the average grain diameter viewed from the sheet surface and the three points bending characteristic under various grain diameters. The result is shown in Fig. 4. According to the figure, the workability is improved by specifying the average grain diameter as 2.0 mm or less. The high silicon steel sheet employed in the experiments described above has extremely good crystal grain growth to form coarse grains by heat treatment, and tend to form a bamboo structure where the crystal grains penetrate in the sheet thickness direction. Nevertheless, as described before, the grain diameter of the steel sheet is specified at 2.0 mm or less from a viewpoint of the workability, and the steel sheet needs to select a controlled heat treatment condition not to yield excessively coarse grains. When a crystal structure of the high silicon steel sheet forms the bamboo structure, the present inventors found that the growth of crystal grains substantially stops at the diameter of approximately 3 to 4 times of the steel sheet thickness. Consequently, to hold the grain diameter at 2.0 mm or less, the sheet thickness may be selected as 0.5 mm or less. In this case, there is no need for care of the heat treatment condition. From these reasons, the specified thickness of the steel sheet is 0.5 mm or less. The effect of the present invention is obtained independent of the grain orientation distribution in the silicon steel sheet. Therefore, the present invention does not specify either an oriented silicon steel sheet or a non-oriented silicon steel sheet. An ordinary electrical steel sheet is coated by film for insulation, and the present invention is independent of presence of the coating film. The present invention does not specify a method of preparing a thin sheet, and the present invention is applicable to the high silicon steel sheet manufactured by the special rolling method, the siliconizing method, which were described before, and other adequate methods. Examples Example 1 A silicon steel sheet (thickness: 0.3 mm) having a chemical composition listed in Table 2 was prepared. The sheet was subjected to siliconization - diffusion treatment (Si diffusion and penetration treatment) to manufacture a 6.5% silicon steel sheet. The siliconization was carried by employing two different mixed gases as a carrier gas: one was mixed with high purity nitrogen gas (dew point: -70°C) and the other was mixed with ordinary nitrogen gas (dew point: -30°C). Obtained specimens were subjected to the three points bending test, and residual specimens were analyzed by the Auger electron spectrometer to determine the O content in the grain boundaries. All specimens showed the C content in the grain boundaries in a range of from 0.2 at.% to 1.2 at.% and the average grain diameter at approximately 0.89 mm. The result is summarized in Fig. 6. The figure shows that the limitation of the O content in the grain boundaries is effective for the improvement of workability even for the high silicon steel sheets obtained by the Si diffusion - penetration process. (wt.%) C Si Mn P S Sol.Al N O 0.0031 3.02 0.15 0.0010 0.0010 <0.001 0.0021 0.0025 With more detailed study, the inventors clarified factors, other than the O content in the grain boundaries, relating to the workability by close observation of the grain boundaries. In concrete terms, the present inventors investigated effects of various elements segregated to the grain boundaries on the workability using samples having constant O content in the grain boundaries, and found that sulfur has a significant effect on the workability separately from an effect of oxygen. Fig. 7 shows a relation between the three points bending characteristic (the stroke of pressing-in of the specimen in the three points bending tester described before) and the S content detected at the grain boundaries using the Auger electron spectrometer. The specimen used here had composition of 6.49 wt.% Si, 0.005 wt.% Mn, 0.0015 wt.% S, and 0.0022 wt.% O, having a thickness of 0.35 mm, and was heat-treated in N 2 atmosphere containing 0.1 vol.% of H 2 S. The specimens after the heat treatment gave nearly same quantity of total S within an analytical error. The O content in the grain boundaries determined by the Auger electron spectrometer gave in a range of from 3 to 5 at.% for all specimens tested. According to Fig. 7, the workability has a strong relation with the S content in the grain boundaries, which suggests that the S in the grain boundaries degrades the workability. Although a detail mechanism of the strong relation of the S content in the grain boundaries and the workability is not known yet, a fact that the grain boundaries do not contain elements such as Mn to form sulfide suggests that the S presumably exists in the grain boundaries in a form of solid solution. The control of the grain size is easily done by changing the annealing temperature. The tests described above showed, however, that the change of annealing temperature largely changes the workability. As described above in detail, the inventors found that the workability of the high silicon steel sheet which had long been considered to be inferior in nature for working has an extremely strong correlation to the characteristics of the grain boundaries and that control of the characteristics provides the high silicon steel sheet having excellent workability. As an example, the high silicon steel sheet having thickness of 0.1 mm and having chemical composition listed in Table 3, and having almost the same O content in the grain boundaries determined by the Auger electron spectrometer were employed to determine a relation between the elongation and the S content in the grain boundaries measured by the Auger electron spectrometer. The result is shown in Fig. 8. The figure shows that a specimen having less S content in the grain boundaries gives high elongation. During the test, specimens which gave the elongation of 3% or more yielded plastic deformation. Observation of the fractured surface by the scanning electron microscope revealed that the specimens giving high elongation gave rather cleavage fracture than grain boundary fracture and that the steel sheet having inferior elongation showed the tendency toward the grain boundary fracture. In the past, this type of high silicon steel sheet was accepted to induce no plastic deformation. However, it was found that when the S content in the grain boundaries is 0.2 at.% or less, the plastic deformation occurs. Consequently, the present invention specifies the S content as 0.2 at.% or less. (wt.%) C Si Mn P S Sol.Al N O 0.0024 6.51 0.03 0.0021 0.0009 0.001 0.0012 0.0019 The present invention needs to specify not only the S content in the grain boundaries as described above but also the O content in the grain boundaries (the O content in the elements segregated to the grain boundaries) to 30 at.% (atomic percent) or less. In other words, an effect of a reduction of the S content in the grain boundaries is acquired only when the O content in the grain boundaries is sufficiently low. For that purpose, the O content in the grain boundaries is necessary to decrease as low as 30 at.% or less. Fig. 9 shows a relation between the three points bending characteristic (the stroke of pressing-in of the specimen in the three points bending tester described before) and the S content in the grain boundaries. A specimen used here had composition of 6.66 wt.% Si, 0.001 wt.% S, 0.001 wt.% Sol. Al, and 0.0025 wt.% O, having a thickness of 0.35 mm, and containing different O content in the grain boundaries. The figure shows that the O content in the grain boundaries of 30 at.% or less gives a correlation between the S content in the grain boundaries and the three points bending characteristic and that the O content in the grain boundaries of above 30 at.% gives very little change of the three points bending characteristic even when the S content in the grain boundaries is 0.2 at.% or less. Therefore, the present invention specifies the O content in the grain boundaries as 30 at.% or less, most preferably as 15 at.% or less. In addition to the effect of the S content and the O content in the grain boundaries, the grain diameter was found to affect the workability. It was found that when the average grain diameter viewed from the sheet surface was 2.0 mm or less, the workability was further improved. The reason for the effect is speculated that the intensity within the grains relatively decreased corresponding to the increase of intensity of the grain boundaries owing to the effect of S at the grain boundaries, which enhances the cracks across the grains, so an excessive grain diameter degrades the workability. Among the high silicon steel sheet having the chemical composition listed in Table 1, the high silicon steel sheets having the O content in the grain boundaries of approximately 5 at.%, the C content in the grain boundaries of approximately 1 at.%, and the S content in the grain boundaries of approximately 0.05 at.% were tested to determine a relation between the average grain diameter viewed from the sheet surface and the three points bending characteristic under various grain diameters. The result is shown in Fig. 10. According to the figure, the workability is improved by specifying the average grain diameter as 2.0 mm or less. The high silicon steel sheets employed in experiments described above have extremely good crystal grain growth to form coarse grains, and tend to form the bamboo structure where the crystal grains penetrate in the sheet thickness direction. Nevertheless, as described before, the grain diameter of the steel is preferably at 2.0 mm or less from the viewpoint of workability, and the steel needs to select the controlled heat treatment condition not to yield excessively coarse grains. When the crystal structure of the high silicon steel sheet forms the bamboo structure, the present inventors found that the growth of crystal grains substantially stops at the diameter of approximately 3 to 4 times the steel sheet thickness. Consequently, to hold the grain diameter at 2.0 mm or less, the sheet thickness may be selected as 0.5 mm or less. In this case, there is no need for the care of heat treatment condition. From these reasons, the preferable thickness of the steel sheet is 0.5 mm or less. The effect of the present invention is obtained independent of the grain orientation distribution in the silicon steel sheet. Therefore, the present invention does not specify either a oriented silicon steel sheet or a non-oriented silicon steel sheet. An ordinary electric steel sheet is coated by film for insulation, and the present invention is independent of presence of the coating film. The present invention does not specify the method of preparing a thin sheet, and the present invention is applicable to the high silicon steel sheet manufactured by the special rolling method, the siliconizing method, which were described before, and other appropriate methods. Example 2 A silicon steel sheet (thickness: 0.3 mm) having a chemical composition listed in Table 4 was prepared. The sheet was subjected to the siliconization - diffusion treatment (Si diffusion and penetration treatment) at 1,200°C. The siliconization was carried by employing two different mixed gases as a carrier gas: one was SiCl 4 gas mixed with high purity nitrogen gas (dew point: -70°C) and the other was SiCl 4 gas mixed with ordinary nitrogen gas (dew point: -30°C). Obtained specimens were subjected to the three points bending test, and residual specimens were analyzed by the Auger electron spectrometer to determine the O content, the C content, and the S content in the grain boundaries. Among the specimens tested, the ones having the O content in the grain boundaries of approximately 10 at.%, the C content in the grain boundaries of approximately 0.7 at.%, and the average grain diameter of approximately 0.80 mm were selected to determine the relation between the S content in the grain boundaries and the bend amount of three point bending test. The result is summarized in Fig. 11. The figure shows that the limitation of the S content in the grain boundaries is effective for the improvement of workability even for the high silicon steel sheet obtained by the Si diffusion-penetration process. (wt.%) C Si Mn P S Sol.Al N O 0.0035 2.99 0.02 0.0015 0.0012 <0.001 0.0031 0.0021

Claims: A process for preparing an electrical steel sheet having a thickness of 0.5 mm or less and which comprises 0.01 wt.% or less of C, 4-10 wt.% of Si a part of which may be substituted by Al, 0.5 wt.% or less of Mn, 0.01 wt.% or less of P, 0.01 wt.% or less of S, 0.01 wt.% or less of N, 0.02 wt.% or less of O, and a balance of Fe and inevitable impurities, the sheet containing (i) crystal grains having an average diameter of 2 mm or less, and (ii) grain boundaries having an oxygen content of 30 atomic percent or less; the process comprising the step of: diffusing additional silicon into a silicon steel sheet by treating the sheet with a mixture of gases comprising a siliconizing gas and high purity nitrogen gas having a dew point of -70°C. A process according to Claim 1, wherein the siliconizing gas is SiCl 4 . A process according to Claim 1, wherein the steel sheet comprises 0.2 wt.% or less of soluble Al. A process according to Claim 3, wherein the steel sheet comprises 0.0005 to 0.02 wt.% of oxygen. A process according to Claim 1, wherein the grain boundaries have an oxygen content of 15 atomic % or less. A process according to Claim 1, wherein the grain boundaries have a carbon content of at least 0.5 atomic %. A process according to Claim 6, wherein the grain boundaries have a carbon content of at least 0.8 atomic %. A process according to any preceding claim, wherein the grain boundaries have a sulfur content of 0.2 atomic % or less.

Inventors: ABE MASAHIRO, HIRATANI TATSUHIKO, ISHIKAWA MASARU, KASAI KATSUJI, OKADA KAZUHISA, TANAKA YASUSHI, ABE, MASAHIRO, HIRATANI, TATSUHIKO, ISHIKAWA, MASARU, KASAI, KATSUJI, OKADA, KAZUHISA, TANAKA, YASUSHI, ABE, MASAHIRO, C/O INTELLECTUAL PROP. DEPT., HIRATANI, TATSUHIKO, C/O INTELLECTUAL PROP. DEPT., ISHIKAWA, MASARU, C/O INTELLECTUAL PROP. DEPT., KASAI, KATSUJI, C/O INTELLECTUAL PROP. DEPT., OKADA, KAZUHISA, C/O INTELLECTUAL PROP. DEPT., TANAKA, YASUSHI, C/O INTELLECTUAL PROP. DEPT.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1993
Main classification: C22C38
IPC codes: C22C38, C21D1, C21D8