grain-oriented magnetic steel sheet made by the method-including hot rolling and final finish annealing, wherein
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1. A method of manufacturing a grain-oriented magnetic steel sheet, comprising the steps of hot-rolling a steel slab containing up to about 0.12 wt % C, from about 1.0 to 8.0 wt % Si, and from about 0.005 to 3.0 wt % Mn, optionally applying annealing to the resulting hot-rolled steel sheet, subjecting said annealed sheet or said hot-rolled sheet to one or more runs of cold rolling including intermediate annealing in between, optionally applying to the intermediate annealed cold-rolled sheet decarburization annealing, optionally coating the intermediate annealed cold-rolled sheet or the decarburized annealed sheet with an annealing separator and then applying final finish annealing; wherein:
(1) the O content of said steel slab is limited to up to about 30 wtppm;
(2) for the entire steel sheet having final thickness including oxide film before final finish annealing, the Al content is limited to up to about 100 wtppm, and the respective contents of B, V, Nb, Se, S, P, and N, are limited to up to about 50 wtppm each; and
(3) during final finish annealing, the N content in the steel is limited within a range of from about 6 to 80 ppm, at least in a temperature region of from from 850 to 950° C.
2. The method according to
(a) increasing the nitrogen partial pressure in the atmosphere at least in the temperature region of from about 850 to 950° C. during final finish annealing; and
(b) adding a nitrification accelerating agent to said annealing separator.
3. The method according to
4. The method according to
5. The method according to
6. The method according to
7. The method according to
said steel slab has a composition further comprising one or more elements selected from the group consisting of:
Ni: from about 0.005 to 1.50 wt %,
Sn: from about 0.02 to 0.50 wt %,
Sb: from about 0.01 to 0.50 wt %,
Cu: from about 0.01 to 0.50 wt %,
Mo: from about 0.01 to 0.50 wt %, and
Cr: from about 0.01 to 0.50 wt %.
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1. Field of the Invention
The present invention relates to a grain-oriented magnetic steel sheet having a low iron loss, suitable for use as an iron core material mainly for electric power transformers and rotary machines.
2. Description of the Related Art
When manufacturing a grain-oriented magnetic steel sheet, it is a common practice to use a precipitate known as an inhibitor to produce secondary recrystallized grains of a Goss orientation ({110}<001>) during final finish annealing.
Representative methods so far disclosed include a method using AlN and MnS as disclosed in Japanese Patent publication No. 40-15644 and a method using MnS and MnSe as disclosed in Japanese Patent Publication No. 51-13469, both having already been industrialized.
Apart from the above, adding CuSe and Bn as disclosed in Japanese Patent Publication No. 58-42244, using nitrides such as those of Ti, Zr and V as disclosed in Japanese Patent Publication No. 46-40855 and many other methods are known.
These methods using inhibitors are useful for stably producing secondary recrystallized grains. However, because precipitates must be finely dispersed, it is necessary that the slab heating temperature before hot rolling is at least 1,300° C. Heating of the slab to a high temperature requires higher equipment cost, and in addition, results in an increase in the quantity of scale produced during hot rolling. This leads to many problems such as a lower product yield and a more complicated equipment maintenance.
Another problem involved in the methods using inhibitors is that these inhibitors constituents, if remaining after the final finish annealing, cause deterioration of the magnetic properties. For the purpose of eliminating these inhibitor constituents Al, N, Be, Se and S, therefore, purification annealing is carried out for several hours in a hydrogen atmosphere at a temperature of at least 1,100° C. after completion of secondary recrystallization. However, purification annealing carried out at such a high temperature leads to problems of a lower mechanical strength of the steel sheet, bucking of the lower part of the coil, and a considerably lower product yield.
It is true that, as a result of this high-temperature purification annealing, the contents of Al, N, B, Se and S in steel are reduced to up to 50 ppm, respectively. These constituents are however concentrated in the forsterite film; in the interface between the film and the iron substrate these constituents remain inevitably as single substances or as compounds. Presence of these substances prevents movement of a magnetic domain wall and causes an increase in iron loss. Further, these substances present in the film/iron interface inhibit grain boundary displacement of the crystal grains directly under the film. As a result, fine grains not completely encroached by secondary recrystallized grains are often present directly below the surface layer. Presence of such fine grains also causes deterioration of the magnetic properties. Moreover, it is still difficult to eliminate Nb, Ti and V even by high-temperature purification annealing, and this is also a cause of deterioration of iron loss.
The manufacturing methods of a grain-oriented magnetic steel sheet using inhibitors faces the problem of a high cost as described above; achievement of a lower iron loss is also limited. In order to avoid these problems, we have considered use of a method not using an inhibitor.
There are known manufacturing methods of a grain-oriented magnetic steel sheet without using an inhibitor such as those disclosed in Japanese Unexamined Patent Publication No. 64-55339, No. 2-57635, No. 7-76732 and No. 7-197126. One of the features common to these techniques is that it is intended to preferentially cause growth of grains having the {110} orientation, using the surface energy as a driving force.
In order to effectively utilize the difference in surface energy, it is necessarily required to use a thin sheet for increasing the contribution of the surface. For example, the technique disclosed in Japanese Unexamined Patent Publication No. 64-55339 limits the thickness to up to 0.2 mm, and the technique disclosed in Japanese ah Unexamined Patent Publication No. 2-57635, to up to 0.15 mm. The technique disclosed in Japanese Unexamined Patent Publication No. 7-76732, not particularly limiting the thickness, reveals a very poor orientational integration as typically represented by a magnetic flux density of up to 1.700 T for B8, for a thickness of 0.30 mm according to Example 1 presented in the specification thereof. In the examples shown therein, the thickness giving a satisfactory magnetic flux density is limited to 0.10 mm. In a technique disclosed in Japanese Unexamined Patent Publication No. 7-197126 also, the thickness is not limited, but the technique is for applying a tertiary cold rolling of from 50 to 75%. The thickness necessarily becomes smaller: a thickness of 0.10 mm is proposed in an example shown in the Publication.
Most of the thicknesses of grain-oriented magnetic steel sheet now commonly use at least 0.20 mm. That is, it is difficult to obtain a product generally in use by a method using the surface energy as described above.
Further, in order to utilize surface energy, it is necessary to carry out the final finish annealing at a high temperature in a state in which the growth of surface oxides is inhibited. For example, Japanese Unexamined Patent Publication No. 64-55339 discloses a technique of using a vacuum, an inert gas, or a mixed gas of hydrogen and nitrogen as an annealing atmosphere at a temperature of at least 1,180° C. Japanese Unexamined Patent Publication No. 2-57635 recommends using an inert gas, hydrogen or a mixed gas of hydrogen and an inert gas as an annealing atmosphere at a temperature of from 950 to 1,100° C., and further, reducing the pressure of the atmospheric gases. Japanese Unexamined Patent Publication No. 7-197126 discloses a technique of carrying out final finish annealing at a temperature within a range of from 1,000 to 1,300° C. in a non-oxidizing atmosphere having an oxygen partial pressure of up to 0.5 Pa or in vacuum.
When desiring to obtain satisfactory magnetic properties by the use of surface energy, as described above, the atmosphere for the final finish annealing must be an inert gas or hydrogen, and a vacuum is suggested as a recommended condition. However, it is very difficult to use a high temperature and a vacuum simultaneously in equipment, further leading to high cost.
When utilizing surface energy, only the grains having the {110} plane are selected to grow. In other words, unlike secondary recrystallization using an inhibitor, it is not always possible that a Goss grain growth with the <001> orientation aligned with the rolling direction is selected. Magnetic properties of a grain-oriented electromagnetic steel sheet are improved only when the easy axis of magnetization <001> is aligned to the rolling direction. Satisfactory magnetic properties are unavailable in principle with the selection of grains having the {110} plane alone. That is, satisfactory magnetic properties are available only under very limited rolling conditions or annealing conditions in a method using surface energy. As a result, magnetic properties of a steel sheet available by use of surface energy are inevitably very unstable.
In a method using surface energy, furthermore, formation of a surface oxide layer must be inhibited during final finish annealing. In other words, an annealing separator such as MgO cannot be coated for annealing. It is therefore impossible to form an oxide film similar to that of an ordinary grain-oriented magnetic steel sheet manufactured using an inhibitor after final finish annealing. For example, a forsterite film is an oxide film formed on an ordinary grain-oriented magnetic steel sheet surface made by using an inhibitor upon coating an annealing separator mainly comprising MgO. The forsterite film not only imparts a tension to the steel sheet surface, but also exerts a function of ensuring adhesion of an insulating tensile coating mainly comprising a phosphate to be coated and baked. In the absence of a forsterite film, therefore, there is a large iron loss.
More specifically, the use of surface energy, known as a manufacturing technique of a grain-oriented magnetic steel sheet not using an inhibitor, encounters problems of a limited thickness of steel sheet, a poor accumulation of secondary recrystallized grain orientations, and iron loss caused by the absence of a surface oxide film.
The present invention provides a manufacturing method not using an inhibitor, which permits avoidance of the problems encountered when using an inhibitor, resulting from the high-temperature slab heating before hot rolling and the high-temperature purification annealing after secondary recrystallization. The invention has an object to provide a favorable solution of the problems necessarily resulting from non-use of an inhibitor but using surface energy, including limited range of steel sheet thickness, poor accumulation of the secondary recrystallized grain orientation, and iron loss caused by the absence of a surface oxide film.
More particularly, an object of the invention is to create a grain-oriented magnetic steel sheet which, even when an inhibitor is not used, does not limit the steel sheet thickness, is free from deterioration of the accumulation of secondary recrystallized grain orientation, and permits reduction of iron loss through positive formation of a surface oxide film.
This invention also proposes the creation of a secondary recrystallized grain texture and secondary recrystallization annealing conditions which permit achievement of the aforementioned object.
The proposed secondary recrystallized grain texture comprises extra-fine crystal grains produced in coarse secondary recrystallized grains, and the proposed secondary recrystallization annealing conditions are materialized by the use of a temperature gradient.
More specifically, the invention provides a manufacturing method of a grain-oriented magnetic steel sheet, comprising the steps of hot-rolling a steel slab containing up to about 0.12 wt % C, from about 1.0 to 8.0 wt % Si, and from 0.005 to 3.0 wt % Mn, applying annealing to the resultant hot-rolled steel sheet as required, subjecting the resultant annealed sheet to one or more runs of cold rolling including intermediate annealing into a final thickness, then applying decarburization annealing as required, coating an annealing separator as required, and then applying final finish annealing; wherein:
Further, the invention provides a method of manufacturing a grain-oriented magnetic steel sheet, wherein:
The invention provides also a grain-oriented magnetic steel sheet having a low iron loss having a composition containing from about 1.0 to 8.0 wt % Si, and an oxide film mainly comprising forsterite (Mg2SiO4), wherein the contents of Al, B, Se and S in the entire steel sheet including the oxide film are up to about 50 ppm, respectively.
We have carried out extensive studies on the mechanism of secondary recrystallization of Goss orientation grains. As a result, we have discovered that grain boundaries having an orientational differential angle within a range of from about 20 to 45° in the primary recrystallization texture played an important role, and reported our findings in a paper (Acta Material, vol. 45 (1997), p. 85).
According to the experimental data reported by C. G. Dunn et al. (AIME Transactions, vol. 188 (1949), p. 368), a grain boundary having an orientational differential angle of from 20 to 45° is a high-energy grain boundary. This high-energy grain boundary, having a large free space within the boundary and a complicated structure, permits easy displacement of atoms. That is, the grain boundary diffusion, which is a process of displacement of atoms through a grain boundary, is more rapid in a higher-energy grain boundary.
Secondary recrystallization is known to take place along with the growth of precipitates called inhibitors. Growth of precipitates proceeds under the control of diffusion. Because precipitates on the high-energy grain boundaries preferentially grain boundaries is preferentially released, and the high-energy grain boundaries begin moving.
From the aforementioned findings, we have determined that, in a grain-oriented magnetic steel sheet, Goss grains exhibiting a high frequency of occurrence relative to easily moving high-energy grain boundaries were subjected to secondary recrystallization.
As a result of further studies, we found that an essential factor of secondary recrystallization of Goss orientation grains lies in the state of distribution of high-energy grain boundaries in the primary recrystallized grain texture; and the role of the inhibitor is only to produce a difference in the speed of displacement between the high-energy grain boundaries and the other grain boundaries. We have found therefore that even without the use of an inhibitor, generation of a difference in speed of displacement of grain boundaries, if possible, would cause occurrence of secondary recrystallization.
Impurity elements present in steel tend to easily segregate in grain boundaries, particularly in high-energy grain boundaries. When many impurity elements are present, therefore, a difference is considered to have been eliminated in the displacement speed between the high-energy grain boundaries and the other grain boundaries. If the effect of such impurity elements can be excluded by purifying the material, therefore, it is considered possible to cause secondary recrystallization of Goss orientation grains through actualization of difference in displacement speed between high-energy grain boundaries primarily dependent upon the texture of the high-energy grain boundaries and the other grain boundaries.
We have further studied and have discovered that, in a composition not containing an inhibitor constituent, secondary recrystallization proceeds under the effect of purification of the material and action of trace nitrogen, and have thus completed the present invention. The technique disclosed in the present invention is based on a concept that is the reverse of that of the conventional secondary recrystallization technique, in that precipitates or impurities in grain boundaries are excluded. Unlike the technique using surface energy, secondary recrystallization can be effectively realized even in the presence of oxides on the steel sheet surface, if any.
The results of experiments, which led to successful development of the present invention, will now be described.
The following steel slabs were manufactured by continuous casting: a steel slab A containing 0.070 wt. % C, 3.22 wt. % Si, and 0.070 wt. % Mn, and having an Al content reduced to 10 ppm, an N content reduced to 30 ppm, an O content reduced to 15 ppm and contents of the other impurities limited to up to 50 ppm, respectively; a steel slab B containing 0.065 wt. % C, 3.32 wt. % Si, 0.070 wt. % Mn, 0.025 wt. % Al, and 30 ppm N, and having contents of the other impurities limited to up to 50 ppm, respectively; and a steel slab C containing 0.055 wt. %, 3.25 wt. % Si, and 0.070 wt. % Mn, and having an Al content reduced to 10 ppm, an N content reduced to 30 ppm, an O content reduced to 60 ppm and contents of the other impurities limited to up to 50 ppm, respectively. These slabs were heated to 1,100° C. and hot-rolled and finished into hot-rolled sheets having a thickness of 2.6 mm. Each hot-rolled sheet was soaked in a nitrogen atmosphere at 1,000° C. for a minute and rapidly cooled. Thereafter, the soaked sheet was cold-rolled into a final thickness of 0.34 mm. Then, the cold-rolled sheet was subjected to decarburization annealing in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 65° C. at a temperature of 840° C. for 120 seconds to reduce the C content to 0.0020 wt. %. A chemical analysis carried out for the other constituents before finish annealing showed almost no change in the contents other than that of carbon in any of steels A, B and C. None of the impurity elements exceeded 50 ppm in content.
Thereafter, an annealing separator mainly comprising MgO was coated, and then, final finish annealing was applied. The final finish annealing was accomplished in a nitrogen atmosphere to 1,050° C. at a heating rate of 20° C/h. For comparison purposes, the similar final finish annealing was carried out in an Ar atmosphere.
As a result, while steel A was secondary-recrystallized when subjected to the final finish annealing in the nitrogen atmosphere, not in the Ar atmosphere. In contrast both steel B and steel C were not secondary-recrystallized in any of these atmospheres. The product of secondary-recrystallized steel A showed a magnetic flux density of 1.87 T, which was a sufficiently satisfactory level for magnetic properties of a grain-oriented magnetic steel sheet.
In this experiment, occurrence of secondary recrystallization of a high-purity steel not containing an inhibitor at all and having reduced Al and O contents was clearly demonstrated by carrying out a final finish annealing in a specific annealing atmosphere.
Steel A after finish annealing at 1,050° C. had a nitrogen content of 35 wtppm when the finish annealing was applied in the nitrogen atmosphere, and 3 wtppm when it was applied in the Ar atmosphere. That is, a correlation was observed between the annealing atmosphere and the nitrogen content.
As a result of further experimental efforts based on the aforementioned findings, it was revealed that the nitrogen content in steel during annealing at a temperature of at least 850° C. up to the end of secondary recrystallization in the finish annealing exerts an effect on the occurrence of secondary recrystallization. In an additional experiment, the nitrogen content was adjusted by acting on the nitrogen content in the slab material and on the nitrogen partial pressure in the finish annealing atmosphere. The nitrogen content in the steel was measured by taking out a sample in the middle of the final finish annealing conducted at a heating rate of 20° C./h and analyzing the same. The magnetic flux density was measured by discontinuing the final finish annealing at 1,050° C. The results obtained are shown in FIG. 2.
As shown in
A further additional experiment was carried out with a view to obtaining further findings about the effect of trace constituents (Al, B, V, Nb, Se, S, Ni, O, N, Sn, Sb, Cu, Mo and Cr) contained in the material before the final finish annealing. The basic composition of molten steel was fixed to 0.06 wt % C, 0.06 wt % Mn and 3.3 wt % Si, and similar steps as in the aforementioned experiment were followed to investigate magnetic properties. The final finish annealing was carried out in a nitrogen atmosphere.
Upon setting forth the grain size, grains having a grain size smaller than 1 mm are excluded because the number of such fine grains is larger than that of usual secondary recrystallized grains having a grain size larger than 1 mm, and inclusion of these fine grains would result in a large fluctuation of the value of average grains size.
In a thickness-direction cross section, there should preferably be present extra-fine grains having a grain size of from at least 0.03 mm to up to 0.30 mm in a number within a range of from at least 3/mm2 to up to 200/mm2.
A grain size of fine grains of under 0.03 mm leads to a poor generating effect of magnetic poles, thus permitting no improvement of iron loss. A grain size of over 0.03 mm results in a lower magnetic flux density. The grain size of fine grains should therefore be within a range of from at least about 0.03 mm to up to about 0.30 mm. Further, as shown in
Further, in order to obtain a high magnetic flux density, in the final finish annealing, the steel sheet should preferably be heated by imparting a temperature gradient within a range of from at least about 1.0° C./cm to up to about 10° C./cm in a temperature region of from at least about 850° C. to the completion of secondary recrystallization.
An experiment carried out to investigate finish annealing conditions favorable for improving iron loss of a product based on the manufacturing method of a grain-oriented magnetic steel sheet not using an inhibitor will now be described.
Using a steel composition comprising 0.070 wt % C, 3.22 wt % Si, 0.070 wt % Mn and 0.0030 wt % Al as a basic composition, a slab containing 5 wtppm Se, 6 wtppm S, 5 wtppm N and 15 wtppm O in addition to the basic composition was manufactured by the continuous casting process. Then, after heating to 1,100° C., the slab was hot-rolled to a finished steel sheet thickness of 2.6 mm. The resultant hot-rolled steel sheet was soaked at 1,000° C. in a nitrogen atmosphere for a minute, and then rapidly cooled. The sheet was then cold-rolled into a final thickness of 0.34 mm. The resultant sheet was soaked at 840° C. in an atmosphere comprising 75% hydrogen and 25% nitrogen and having a dew point of 65° C. to carry out a decarburization annealing for 120 seconds, to reduce the C content to 0.0020 wt %. Thereafter, after coating MgO as an annealing separator, a final finish annealing was conducted in a hydrogen atmosphere to study the effect of the final finish annealing on magnetic flux density.
First, during the final finish annealing, an experiment of heating at a rate of 20° C./h was carried out without imparting a temperature gradient. Secondary recrystallization was started at 900° C. and completed at 1,030° C. A magnetic flux density of the product of B8=1.883 T was obtained in this experiment.
Thereafter, the final finish annealing of imparting various temperature gradients up to 1,050° C. at a heating rate of 20° C./h was carried out. This annealing was accomplished by the following two processes. One comprised the steps of heating an end of a sample to 900° C., the secondary recrystallization starting temperature region, imparting a temperature gradient to the sample, and starting heating at a rate of 20° C./h while keeping the temperature gradient. The other process comprised the steps of imparting a temperature gradient to the sample by heating an end of the sample to 850° C., a temperature lower than that for the start of secondary recrystallization, and heating the same at a rate of 20° C./h while keeping the temperature gradient.
When the temperature at which imparting a temperature gradient is started is over about 850° C., or when imparting a temperature gradient is discontinued before the completion of secondary recrystallization, magnetic flux density decreases. The temperature gradient should therefore be imparted within a temperature region of from at least about 850° C. to the completion of secondary recrystallization. On the other hand, a temperature gradient from the room temperature may be imparted because the lower limit temperature for starting imparting a temperature gradient exerts no particular effect on magnetic flux density. However, within the temperature region of from at least about 850° C. to the completion of secondary recrystallization, it is necessary to continue imparting the temperature gradient. When the heating rate in the temperature region in which the temperature gradient is imparted is over about 50° C., secondary recrystallization grains of undesired orientations are produced and magnetic flux density decreases. The heating rate should therefore be up to about 50° C./h. The direction of the temperature gradient imparted to the steel sheet may be arbitrarily selected. The temperature gradient suffices to be within a range of from at least about 1.0° C./cm to up to about 10° C./cm. It is not necessary that it is constant. Recommended techniques for imparting a temperature gradient include a technique of moving a coil in an annealing furnace imparted with a furnace temperature gradient, and a technique of heating by controlling the furnace temperature for each zone while keeping the fixed coil.
Japanese Patent Publication No. 58-50925 discloses a technique of causing progress of secondary recrystallization while giving a temperature gradient on the boundary between the primary recrystallization region and the secondary recrystallization region. This technique comprises the steps of imparting a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region, and causing growth of secondary recrystallization grains nucleated at a high temperature by the temperature gradient toward the low temperature side. In this technique, a temperature gradient is imparted even in the state of the primary recrystallization texture before start of secondary recrystallization, and heating is conducted while imparting the temperature gradient until the completion of secondary recrystallization. When applying this technique to a composition not using an inhibitor, magnetic flux density is not always improved, although it is easy to cause growth of the secondary recrystallization grains to coarser grains. In contrast, when applying the method of the invention of imparting a temperature gradient even in the state of primary recrystallized grain texture before start of secondary recrystallization and heating while maintaining the temperature gradient to a composition not using an inhibitor, magnetic flux density was improved. When no inhibitor is present, grain growth tends to proceed easily at temperatures lower than the secondary recrystallization starting temperature, and a considerable change in texture occurs in the stage of up to nucleation of secondary recrystallization grains. In the presence of a temperature gradient at this point, an appropriate change in texture is caused by grain growth, and this is considered to permit improvement of magnetic flux density. Slightly varying with the process conditions, the temperature at which secondary recrystallization is completed should preferably be within a range of from about 900 to about 1,050° C.
Steel slabs having the compositions shown in Table 1 were manufactured by continuous casting. After heating to 1,050° C. for 20 minutes, each slab was hot-rolled into a thickness of 2.5 mm. The resultant hot-rolled sheet was subjected to a hot-rolled sheet annealing at 1,000° C. for 60 seconds, and cold-rolled into a final thickness of 0.34 mm. Then, a decarburization annealing was applied at 830° C. for 120 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 60° C. to reduce the C content in steel to 0.0020 wt %. Then, after coating an annealing separator mainly comprising Mgo, a final finish annealing was carried out. For comparison purposes, borax was partially employed as an annealing separator. In the final finish annealing, the sheet was heated to 1,050° C. at a rate of 15° C./h in an atmosphere shown in Table 2.
In the course of the aforementioned manufacturing steps, the steel sheet with a film before the final finish annealing was analyzed to investigate the contents of Al, B, V, Nb, Se and S. Magnetic flux density B8 and iron loss W17/50 for the steel sheet after the final finish annealing were measured. Further, during the final finish annealing, the sample was taken out from the coil outer winding at temperatures of 850, 900, and 950° C. to analyze the nitrogen content in steel.
The steel sheet with the oxide film after the final finish annealing was analyzed to investigate the contents of Al, B, V, Nb, Se and S. The results are comprehensively shown in Table 2.
As is clear from Table 2, in each of steel samples Nos. 1 to 11 prepared in compliance with the invention, a steel slab not containing an inhibitor constituent and having an O content in steel inhibited to up to 30 wtppm was used, and the Al content in the steel sheet with the oxide film before the final finish annealing was reduced to up to 100 wtppm, and the contents of B, V, Nb, Se, S, and N were reduced to up to 50 wtppm, respectively. During the final finish annealing, the nitrogen content within a temperature range of from 850 to 950° C. was controlled within a range of from 6 to 80 ppm. In any of these cases, a product having satisfactory magnetic properties was obtained.
A thin slab containing 7 wtppm C, 3.4 wt % Si, 0.15 wt % Mn, 29 wtppm N, 10 wtppm O, 19 wtppm Al, 3 wtppm B, 10 wtppm V, 20 wtppm Nb, 10 wtppm Se, and 10 wtppm S, and the balance substantially Fe, and having a thickness of 4.5 mm was manufactured by continuous casting. The slab was cold-rolled into a final thickness of 0.90 mm.
Analysis of the contents of Al, B, V, Nb, Se, S, and N in the cold-rolled steel sheet before the final finish annealing showed that each of these contents was reduced to up to 50 wtppm in all cases.
Then, after coating an annealing separator mainly comprising MgO, the final finish annealing was carried out. The final finish annealing was accomplished by heating to 950° C. at a rate of 15° C./h in an atmosphere shown in Table 3. Magnetic flux density B8 and the maximum magnetic permeability μmax of the thus obtained grain-oriented magnetic steel sheet were measured. During the final finish annealing, samples were taken out from the outer winding of the coil at temperatures of 850, 900, and 950° C. to analyze the nitrogen content in steel. The result is shown in Table 3.
As shown in Table 3, when a thin slab of a high-purity composition not containing an inhibitor constituent with a reduced C content was used as a material as in Nos. 1 to 4, a product of a high magnetic permeability was obtained by reducing the contents of Al, B, V, Nb, Se, S, and N in the steel sheet with an oxide film before the final finish annealing to up to 50 ppm, respectively, and controlling the nitrogen content within a range of from 6 to 80 ppm in a temperature range of from 850 to 950° C. during the final finish annealing, even when omitting the decarburization annealing.
Steel slabs comprising the compositions shown in Table 4 were manufactured. Then, each slab was heated to 1,250° C. for 20 minutes, and hot-rolled into a hot-rolled sheet having a thickness of 2.8 mm. Then, after subjecting the sheet to a hot-rolled sheet annealing at 1,000° C. for 60 seconds, the annealed sheet was finished through cold rolling into a final thickness of 0.29 mm. Thereafter, a decarburization annealing was applied at 850° C. for 120 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 40° C. to reduce the C content in steel to 0.0020 wt %, and after coating an annealing separator mainly comprising a constituent shown in Table 5, a final finish annealing was applied. The final finish annealing was carried out by heating the sheet to 1,100° C. at a rate of 20° C./h in a mixed atmosphere of 50% nitrogen and 50% hydrogen, and holding the sheet at this temperature in a hydrogen atmosphere for five hours.
Magnetic flux density B8 and iron loss w17/50 were measured for each product sheet thus obtained. The sheet with a film after the final finish annealing was composition-analyzed to investigate the contents of Al, B, Se and S. The result is also shown in Table 5.
As is clear from Table 5, a product of a satisfactory iron loss was obtained when the contents of Al, B, Se and S in the magnetic steel sheet after the final finish annealing were reduced to up to 20 wtppm, respectively, in accordance with the present invention.
Steel slabs comprising the compositions shown in Table 6 were manufactured. Then, each slab was heated to 1,100° C. for 20 minutes, and hot-rolled into a hot-rolled sheet having a thickness of 2.4 mm. Then, after subjected the sheet to a cold rolling into an intermediate thickness of 1.8 mm, and applying an intermediate annealing at 1,100° C. for 30 seconds, the sheet was finished through a warm rolling at 200° C. into a final thickness of 0.22 mm. Thereafter, a decarburization annealing was applied at 880° C. for 100 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 60° C. to reduce the C content in steel to 0.0020 wt %, and after coating an annealing separator mainly comprising MgO, a final finish annealing was applied. The final finish annealing was carried out by heating the sheet to 1,100° C. at a rate of 20° C./h in a mixed atmosphere of 50% nitrogen and 50% hydrogen. After the final finish annealing, magnesium phosphate containing 50% colloidal silica was coated, and the coating was baked at 800° C. for two minutes also for flattening annealing. Then, after baking, a magnetic domain dividing treatment was applied by irradiating a pulse laser at intervals of 15 mm in the rolling direction and in the transverse direction.
Magnetic flux density B8 and iron loss W17/50 were measured for each product sheet thus obtained. The sheet with a film after the final finish annealing was composition-analyzed to investigate the contents of Al, B, Se and S. The result is also shown in Table 6.
As shown in Table 6, a product of a satisfactory iron loss was obtained when the contents of Al, B, Se and S in the magnetic steel sheet after the final finish annealing were reduced to up to 20 ppm, respectively.
A steel slab containing 0.005 wt % C, 3.45 wt % Si, 0.15 wt % Mn, 0.30 wt % Ni, 50 wtppm Al, 15 wtppm N, and 10 wtppm O and the balance substantially Fe was manufactured by continuous casting. Then, after heating at 1,050° C. for 20 minutes, the slab was hot-rolled into a hot-rolled sheet having a thickness of 2.5 mm. Then, after a hot-rolled sheet annealing at 1,000° C. for 60 seconds, the sheet was finished through a cold rolling into a final thickness of 0.34 mm. Then, the resultant sheet was subjected to a decarburization annealing at 900° C. for 10 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 40° C. to reduce the C content in steel to 0.0020 wt %. After coating an annealing separator mainly comprising MgO, a final finish annealing was applied. The final finish annealing was carried out under conditions shown in Table 7.
Magnetic flux density B8 and iron loss W17/50 were measured for each product sheet thus obtained. Also investigated was the average grain size of secondary recrystallized grains as calculated by excluding grains having a grain size smaller than 1 nm, and the frequency of presence of extra-fine grains having a grain size of from at least 0.03 mm to up to 0.30 mm existing on a thickness direction cross-section. The result is also shown in Table 7.
As is clear from Table 7, a satisfactory iron loss property was available with an average grain size of secondary recrystallized grains of at least 3 mm as converted into a diameter of a corresponding circle, and within a range of frequency of presence of from at least 5/mm2 to up to 100/mm2 of extra-fine grains having a grain size of from at least 0.03 mm to up to 0.30 mm on a thickness direction cross-section.
A slab containing 40 wtppm C, 3.23 wt % Si, 0.20 wt % Mn, 0.0030 wt % Al, 5 wtppm Se, 6 wtppm S, 13 wtppm N, 12 wtppm O and the balance substantially Fe by continuous casting. The slab was heated at 1,050° C. for 20 seconds, and finished through a hot rolling into a thickness of 2.5 mm. Thereafter, a hot-rolled sheet annealing was applied at 1,000° C. for 60 seconds, and then, finished through a cold rolling into a final thickness of 0.34 mm. Then, soaking was applied at 830° C. and a decarburization annealing was applied for 20 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 60° C. to reduce the C content to 10 wtppm. Subsequently, after coating MgO as an annealing separator, a final finish annealing was carried out. The final finish annealing was carried out by imparting a temperature gradient under conditions shown in Table 8 in up and down directions of the coil and heating to 1,050° C. Magnetic flux density B8 and iron loss W17/50 were measured for the sheet thus obtained. The results are shown in Table 8.
The results shown in Table 8 suggest that a product of a high magnetic flux density is available by using a slab having a composition in which the contents of Se, S, N and O are reduced to 30 wtppm or less, respectively, not using an inhibitor, and by imparting a temperature gradient of from 1.0 to 10° C./cm within a temperature range of from 850 to 1,050° C. during the final finish annealing.
A slab comprising the composition shown in Table 9 was finished through a direct hot rolling without reheating, into a thickness of 4.0 mm. After carrying out a hot-rolled sheet annealing under conditions shown in Table 9, the sheet was finished through a cold rolling into a thickness of 1.8 mm, and the sheet was soaked at 950° C. and subjected to an intermediate annealing for 60 seconds. Thereafter, the sheet was finished through a cold rolling into a final thickness of 0.22 mm, and a decarburization annealing was applied comprising soaking at 830° C. for 120 seconds in an atmosphere comprising 75% hydrogen and 25% nitrogen with a dew point of 60° C. to reduce the C content to 0.0020 wt %. After coating an annealing parting agent mainly comprising MgO onto the surface of the sheet, a final finish annealing was carried out. In the final finish annealing, a temperature gradient of 2.5° C./cm was imparted in up and down directions of the coil within the temperature range of at least 800° C., and the annealing was completed by heating to 1,000° C. in a mixed atmosphere comprising 25% nitrogen and 75% hydrogen at a rate of 15° C./h. Magnetic flux density B8 and iron loss W17/50 were measured for the steel sheet thus obtained. The result is also shown in Table 9.
Table 9 reveals that, even when an intermediate annealing is conducted, a product of a high magnetic flux density is available by using a slab of a high-purity composition not using an inhibitor, in which the contents of Se, S, N and O are reduced to up to 30 ppm, respectively and carrying out a final finish annealing by imparting a temperature gradient within a temperature range of from 800 to 1,000° C.
According to the present invention, as described above, a product having satisfactory magnetic properties was created by using a steel slab having a high-purity composition not containing an inhibitor constituent, reducing the Al content to down to 100 wtppm, and the contents of B, V, Nb, Se, S, and N to down to 50 wtppm, respectively, in the steel sheet with an oxide film before final finish annealing, and controlling the nitrogen content within a range of from about 6 to 80 wtppm in a temperature range of from about 850 to 950° C. during final finish annealing. In order to obtain a further excellent iron loss property, it is desirable to achieve a crystal texture in which the average grain size as calculated by excluding grains smaller than 1 mm is down to about 3 mm as converted into a diameter of a corresponding circle, and the frequency of presence of extra-fine grains having a grain size of from at least about 0.03 mm to about 0.30 mm on the thickness direction cross section is at least about 3/mm2 to about 200/mm2, or impart a temperature gradient to the sheet in the finish annealing.
According to the present invention, high-temperature heating of slab or high-temperature purification annealing for removing impurities is not necessary, providing a remarkable economic benefit. Further, in the present invention, for a use not requiring a forsterite film, it is possible to use a material not containing C and omit the decarburization annealing step.
TABLE 1
Slab chemical composition (wt %)
N
O
Al
B
V
Nb
Se
S
No.
C
Si
Mn
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
Ni
Sn
Sb
Cu
Mo
Cr
1
0.076
3.2
0.17
5
24
10
3
20
20
20
15
0.01
0.01
tr
0.01
tr
tr
2
0.071
3.3
0.13
11
8
8
2
20
10
20
4
0.01
0.01
tr
0.01
tr
tr
3
0.042
3.2
0.01
40
35
32
4
30
20
10
11
0.01
0.01
tr
0.01
tr
tr
4
0.003
3.3
0.15
21
21
75
16
20
10
20
15
0.01
0.01
tr
0.01
tr
tr
5
0.040
3.9
1.41
32
20
45
4
10
20
10
9
0.01
0.01
tr
0.01
tr
tr
6
0.095
3.5
0.17
35
11
31
9
40
20
10
11
0.01
0.01
tr
0.01
tr
tr
7
0.065
3.6
0.15
9
14
21
2
10
10
10
5
0.05
0.01
tr
0.01
tr
tr
8
0.055
3.1
0.14
21
19
11
2
10
10
10
8
0.01
0.05
tr
0.01
tr
tr
9
0.069
3.6
0.16
21
14
11
2
10
10
10
9
0.01
0.01
0.05
0.01
tr
tr
10
0.075
3.5
0.12
31
14
11
2
10
10
10
5
0.01
0.01
0.01
0.05
tr
tr
11
0.045
3.3
0.18
30
10
9
3
10
10
10
5
0.01
0.01
tr
0.01
0.05
tr
12
0.052
3.3
0.18
15
34
9
5
20
20
20
6
0.01
0.01
tr
0.01
tr
tr
13
0.031
3.4
0.16
80
22
22
8
20
30
10
7
0.01
0.01
tr
0.01
tr
tr
14
0.074
3.3
0.15
39
24
130
5
30
40
10
16
0.01
0.01
tr
0.01
tr
tr
15
0.068
3.3
0.15
33
23
170
2
20
10
10
6
0.01
0.01
tr
0.01
tr
tr
16
0.083
3.4
0.24
33
16
10
60
30
10
10
8
0.01
0.01
tr
0.01
tr
tr
17
0.043
3.3
0.17
24
38
18
7
100
10
10
6
0.01
0.01
tr
0.01
tr
tr
18
0.033
3.2
0.16
39
22
8
11
30
70
10
14
0.01
0.01
tr
0.01
tr
tr
19
0.065
3.1
0.07
43
16
22
3
20
20
140
15
0.01
0.01
tr
0.01
tr
tr
20
0.054
3.3
0.07
9
13
14
5
20
30
20
11
0.01
0.01
tr
0.01
tr
tr
21
0.055
3.4
0.19
17
25
43
7
30
20
10
4
0.01
0.01
tr
0.01
tr
tr
22
0.040
3.3
0.17
21
64
15
7
30
30
10
6
0.01
0.01
tr
0.01
tr
tr
23
0.028
3.1
0.25
24
11
95
3
10
20
10
13
0.01
0.01
tr
0.01
tr
0.30
TABLE 2
Nitrogen con-
Al, B, V, Nb, Se, S and contents
centration in at-
Nitrogen content in steel
Magnetic
(over 50 ppm only) before and
mosphere dur-
during final finish
flux
Iron loss
after final finish annealing
Annealing
ing final finish
annealing (wtppm)
density
W17/50
No.
before (wtppm)
after (wtppm)
separator
annealing (%)
850° C.
900° C.
950° C.
B8 (T)
(W/kg)
Remarks
1
All < 50
All < 50
Magnesia
100
25
48
49
1.88
1.21
Example of Invention
2
All < 50
All < 50
Magnesia
40
16
25
13
1.86
1.23
Example of Invention
4
All < 50
All < 50
Magnesia
100
35
45
43
1.84
1.30
Example of Invention
5
All < 50
All < 50
Magnesia
100
46
62
70
1.79
1.27
Example of Invention
6
All < 50
All < 50
Magnesia
0
65
53
32
1.87
1.20
Example of Invention
7
All < 50
All < 50
Magnesia
25
23
28
31
1.90
1.18
Example of Invention
8
All < 50
All < 50
Magnesia
25
33
48
43
1.84
1.15
Example of Invention
9
All < 50
All < 50
Magnesia
25
23
38
45
1.87
1.15
Example of Invention
10
All < 50
All < 50
Magnesia
25
43
39
42
1.85
1.16
Example of Invention
11
All < 50
All < 50
Magnesia
25
33
38
41
1.85
1.15
Example of Invention
12
All < 50
All < 50
Magnesia
0
13
3
3
1.51
3.89
Comparative Example
13
All < 50
All < 50
Magnesia
100
91
105
110
1.71
1.59
Comparative Example
14
Al: 120
Al: 100
Magnesia
100
53
74
85
1.49
4.03
Comparative Example
15
Al: 140
Al: 120
Magnesia
100
93
109
139
1.43
4.23
Comparative Example
16
B: 60
B: 60
Magnesia
100
64
78
103
1.45
4.09
Comparative Example
17
V: 100
V: 90
Magnesia
100
75
88
109
1.51
3.89
Comparative Example
18
Nb: 70
Nb: 70
Magnesia
100
45
56
76
1.56
3.56
Comparative Example
19
Se: 140
Se: 130
Magnesia
100
13
45
55
1.65
3.02
Comparative Example
20
S: 120
S: 120
Magnesia
100
9
54
65
1.63
3.13
Comparative Example
21
All < 50
B: 80
Borax
100
39
44
56
1.75
1.93
Comparative Example
22
All < 50
All < 50
Magnesia
100
44
50
45
1.69
2.35
Comparative Example
23
All < 50
All < 50
Magnesia
100
29
41
38
1.84
1.18
Example of Invention
TABLE 3
Nitrogen concentration in
Nitrogen content in steel during
atmosphere during final
final finish annealing (wtppm)
Magnetic flux
Maximum magnetic
No.
finish annealing (%)
850° C.
900° C.
950° C.
density B8 (T)
permeability μmax
Remarks
1
100
43
48
49
1.83
56000
Example of Invention
2
75
37
38
33
1.84
58000
Example of Invention
3
50
33
29
22
1.84
56000
Example of Invention
4
25
29
23
9
1.83
53000
Example of Invention
5
5
19
6
3
1.60
13000
Comparative Example
TABLE 4
Steel Slab Chemical Composition
C
Si
Mn
Ni
O
N
Al
B
Se
S
No.
(wtppm)
(wt %)
(wt %)
(wt %)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
1
300
3.23
0.12
0.01
17
11
23
tr
tr
13
2
30
3.47
0.15
0.01
15
9
50
tr
tr
8
3
730
3.77
0.25
0.01
10
11
15
tr
tr
7
4
330
3.01
0.92
0.01
19
10
13
tr
tr
10
5
190
3.31
0.13
0.01
13
25
33
tr
tr
10
6
310
3.33
0.10
0.01
19
80
230
tr
tr
230
7
370
3.25
0.07
0.01
19
30
12
tr
180
21
8
410
3.41
0.12
0.01
19
70
10
73
tr
11
9
510
3.34
0.10
0.01
19
66
190
tr
tr
13
10
10
3.44
0.14
0.01
29
10
21
tr
tr
10
11
30
3.21
0.12
0.01
19
21
30
tr
tr
5
TABLE 5
Main constituents
Analysis after final
of annealing
finish annealing (wtppm)
Iron loss
Magnetic flux
No.
parting agent
Al
B
Se
S
W17/50 (W/kg)
density B8 (T)
Remarks
1
MgO
8
tr
tr
11
1.11
1.89
Example of Invention
2
MgO
15
tr
tr
6
1.13
1.87
Example of Invention
3
MgO
6
tr
tr
5
1.13
1.87
Example of Invention
4
MgO
8
tr
tr
9
1.15
1.87
Example of Invention
5
MgO
4
tr
tr
10
1.08
1.90
Example of Invention
6
MgO
11
tr
tr
86
1.28
1.89
Comparative Example
0
7
MgO
10
tr
70
9
1.35
1.87
Comparative Example
8
MgO
5
55
tr
8
1.27
1.86
Comparative Example
9
MgO
10
tr
tr
7
1.38
1.86
Comparative Example
10
Al2O3
70
tr
tr
10
1.48
1.87
Comparative Example
11
MgO.Al2O4
80
tr
tr
5
1.41
1.87
Comparative Example
TABLE 6
Steel slab chemical composition
C
Si
Mn
Ni
O
N
Al
B
Se
S
No.
(wtppm)
(wt %)
(wt %)
(wt %)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
(wtppm)
1
580
3.33
0.12
0.40
13
9
23
tr
tr
19
2
30
3.57
0.15
0.33
15
9
63
tr
tr
9
3
730
3.47
0.25
0.21
9
11
15
tr
tr
13
4
330
3.31
0.92
0.01
19
10
13
tr
tr
16
5
390
3.31
0.13
0.01
13
85
220
tr
tr
11
Analysis after final
finish annealing (wtppm)
Iron loss
Magnetic flux
No.
Al
B
Se
S
W17/50 (W/kg)
density B8 (T)
Remarks
1
7
tr
tr
10
0.70
1.93
Example of Invention
2
5
tr
tr
5
0.71
1.92
Example of Invention
3
3
tr
tr
5
0.71
1.92
Example of Invention
4
4
tr
tr
6
0.73
1.91
Example of Invention
5
105
tr
tr
5
0.79
1.91
Comparative Example
TABLE 7
Product plate secondary
Product plate
Final finish annealing conditions
recrystallization grains
magnetic properties
Heating
Annealing
Maximum reachable
Average grain
Number of fine
Magnetic flux
Iron loss
No.
rate (° C./h)
atmosphere
temperature (° C.)
size (mm)
grains (number/mm2)
density (T)
(W/kg)
Remarks
1
10
N2 = 100%
1025
26
12.3
1.90
1.15
Example of Invention
2
20
N2 = 100%
1030
33
32.4
1.89
1.19
Example of Invention
3
5
N2 = 100%
1000
9
62.5
1.88
1.19
Example of Invention
4
10
N2 = 50%
1100
11
22.3
1.90
1.16
Example of Invention
H2 = 50%
5
10
N2 = 50%
1020
33
5.3
1.90
1.18
Example of Invention
Ar = 50%
6
10
H2 = 100%
1020
19
72.3
1.88
1.19
Example of Invention
7
10
Ar = 100%
1040
20
12.9
1.89
1.18
Example of Invention
8
10
N2 = 100%
1050
23
29.5
1.90
1.14
Example of Invention
9
10
N2 = 100%
1020
25
82.3
1.88
1.19
Example of Invention
10
3
H2 = 100%
1130
2
232.0
1.82
1.65
Comparative Example
11
50
N2 = 100%
1150
53
1.5
1.85
1.55
Comparative Example
TABLE 8
Temperature
gradient
Tem-
imparting
perature
Magnetic
Iron
start
gradient
Heating
flux
loss
temperature
(° C./
rate
density
W17/50
(° C.)
cm)
(° C./h)
B8 (T)
(W/kg)
Remarks
1
850
1.0
15
1.955
1.16
Example of
Invention
2
850
2.0
25
1.974
1.10
Example of
Invention
3
850
5.0
15
1.965
1.15
Example of
Invention
4
850
8.0
25
1.954
1.19
Example of
Invention
5
850
2.0
20
1.970
1.13
Example of
Invention
6
850
2.0
5
1.985
1.06
Example of
Invention
7
850
2.0
3.5
1.975
1.13
Example of
Invention
9
850
0.5
15
1.880
1.38
Comparative
Example
10
900
12.0
5
1.875
1.39
Comparative
Example
TABLE 9
Magnetic
flux
Iron loss
Steel
Molten steel chemical composition (wt %) (wtppm for O, N, Al and Se)
density
W17/50
symbol
C
Si
Mn
Ni
Sn
Sb
Cu
Mo
Cr
O
N
Al
Se
S
B8 (T)
(W/kg)
Remarks
1
520
3.35
0.13
0.30
tr
tr
tr
tr
tr
12
10
35
tr
18
1.99
0.79
Example of Invention
2
340
3.52
0.13
0.13
tr
tr
tr
tr
tr
13
15
43
tr
8
1.98
0.80
Example of Invention
3
630
3.57
0.25
tr
tr
tr
tr
tr
tr
13
13
21
tr
15
1.97
0.82
Example of Invention
4
30
3.42
0.25
tr
0.30
tr
tr
tr
tr
12
12
39
tr
17
1.56
0.77
Example of Invention
5
30
2.17
0.20
tr
tr
0.03
tr
tr
tr
11
11
13
tr
10
1.96
0.78
Example of Invention
6
520
3.22
0.02
tr
tr
tr
0.03
tr
tr
19
8
79
tr
13
1.97
0.80
Example of Invention
7
430
3.59
0.35
tr
tr
tr
tr
0.03
tr
10
10
15
tr
11
1.96
0.80
Example of Invention
8
330
3.35
0.05
tr
tr
tr
tr
tr
0.32
9
11
15
tr
13
1.96
3.80
Example of Invention
9
430
3.33
0.90
tr
tr
tr
tr
tr
tr
19
20
153
tr
16
1.70
1.59
Comparative Example
10
520
3.23
0.13
tr
tr
tr
tr
tr
tr
13
15
20
85
11
1.61
1.80
Comparative Example
11
420
3.36
0.13
tr
tr
tr
tr
tr
tr
10
19
24
tr
81
1.61
1.85
Comparative Example
12
530
3.30
0.30
tr
tr
tr
tr
tr
tr
15
64
21
tr
15
1.73
1.78
Comparative Example
13
350
3.20
0.08
tr
tr
tr
tr
tr
tr
54
14
23
tr
18
1.70
1.83
Comparative Example
Kurosawa, Mitsumasa, Hayakawa, Yasuyuki, Komatsubara, Michiro
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Oct 03 2002 | JFE Steel Corporation | (assignment on the face of the patent) | / | |||
| Apr 01 2003 | Kawasaki Steel Corporation | JFE Steel Corporation | CHANGE OF NAME SEE DOCUMENT FOR DETAILS | 014488 | /0117 |
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