EP0589418A1

EP0589418A1 - Process for producing oriented electrical steel sheet having minimized primary film, excellent magnetic properties and good workability

Info

Publication number: EP0589418A1
Application number: EP93115198A
Authority: EP
Inventors: Hotaka c/o NIPPON STEEL CORPORATION Honma; Osamu c/o Nippon Steel Corporation Tanaka; Hiroaki c/o Nippon Steel Corporation Masui; Katsuro C/O Nippon Steel Corporation Kuroki; Tsutomu c/o NIPPON STEEL CORPORATION Haratani; Masao C/O Nippon Steel Corporation Ono; Isao c/o Nippon Steel Corporation Iwanaga
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1992-09-21
Filing date: 1993-09-21
Publication date: 1994-03-30
Also published as: KR940007196A; KR960010595B1

Abstract

A steel sheet is coated with an annealing separator composed mainly of MgO and, added thereto, a sulfur compound and optionally at least one of a Cl compound, a carbonate, a nitrate and a sulfate and subjected to finish annealing in an atmosphere having a limited N₂ content to provide an oriented electrical steel sheet having a minimized primary film (a solid substance composed mainly of forsterite), a high magnetic flux density and good workability. Further, the provision of grooves in an intermediate step enables the iron loss to be reduced to a very low value.

Description

The present invention relates to a process for producing an oriented electrical steel sheet having excellent magnetic properties.
In the production of electrical steel sheets for transformers and other applications, which contain 2.5 to 4.5% of Si and have excellent magnetic properties, the formation of a primary film capable of ensuring an insulating property and imparting tension to the surface of a steel sheet to improve magnetic properties necessary for improving the performance of the transformer and, at the same time, having a good adhesion to the steel sheet has been one important task of the conventional oriented electrical steel sheets.
Specifically, in the prior art, after primary crystallization annealing involving decarburization, the steel sheet is coated with a slurry of a fine powder of magnesia (magnesium oxide MgO) in water, optionally dried and then baked in the step of high-temperature finish annealing which also serves as secondary recrystallization annealing wherein MgO is reacted with SiO₂ or Si to form a ceramic insulating primary film, i.e., forsterite (Mg₂SiO₄). The forsterite imparts tension to the steel sheet, which is useful for improving magnetic properties, especially iron loss (transformer efficiency).
Further, it is also well known that the conditions for the formation of the forsterite play an important role in growing somewhat coarse secondary recrystallized grains having a Goss orientation ({110}[001] crystal orientation) in the longitudinal direction (rolling direction) of the steel sheet, which crystal orientation is indispensable to an improvement in the magnetic permeability and magnetic flux density.
An attempt to effect secondary recrystallization without forming the sufficiently dense primary film during the stage of raising the temperature in the secondary recrystallization unfavorably causes fine nitride, sulfide or the like (an inhibitor) within the steel sheet to be escaped in an intact condition or after decomposition, in an early stage.
For this reason, the effect of the inhibitor, which serves to preferentially grow grains having a Goss orientation with the formation of grains having other orientations being inhibited, cannot be exhibited during the temperature rise, which provides a steel sheet having very poor magnetic properties and comprising fine grains because the growth of secondary recrystallized grains having a Goss orientation has been effected partially or not at all.
In some cases, titanium oxides (TiO₂ etc.) or other compounds are added to the MgO to form a denser primary film.
Since, however, the primary film composed mainly of forsterite is a solid substance, there occurs a problem of working, such as shearing of products, which leads to a lowering in the service life of tools.
For this reason, in many cases, oriented electrical steel sheets having no primary film are required. Therefore, the development of techniques capable of satisfying a requirement that the regulation role of the inhibitor in the secondary recrystallization is not inhibited and the resultant product has no primary film is desired.
Meanwhile, from the viewpoint of energy saving, as can be seen from the appearance of amorphous materials in recent years, there is an ever-increasing demand for a reduction in the iron loss in electrical steel sheets which has a great influence on the energy conversion efficiency of transformers, and it has become difficult for the above-described prior art to cope with this demand. In the prior art, besides the above-described method, use is made of a method wherein a groove or some damage is intentionally provided on the surface of the product steel, after the completion of the secondary recrystallization by mechanical methods or energy irradiation methods, such as a laser beam, to divide the magnetic domain and thereby reduce the iron loss.
Even this method, however, cannot produce an iron loss comparable to that of the amorphous material.
The present invention clarifies the above-described problems and minimizes the formation of a solid substance composed mainly of forsterite, i.e., a primary film, based on the following technical finding, to thus provide a process for producing a grain oriented electrical steel sheet having a high magnetic flux density, good workability and a very low iron loss value.
The present inventors have studied on the relationship between the average thickness of the primary film, composed mainly of forsterite, on the surface of the steel sheet and the magnetic properties and, as a result, have found the following facts.
At the outset, it was found that limiting of the thickness of the primary film to 0.3 µm or less enables the workability (punchability) to be improved and, at the same time, the magnetic flux density to be enhanced.
This effect can be confirmed by the following experiment.
A hot-rolled steel sheet for an oriented electrical steel sheet having a chemical composition specified in Table 1 was quenched after annealing, pickled, cold-rolled to a thickness of 0.23 mm, subjected to primary recrystallization annealing and coated with various annealing separators comprising an MgO powder and, added thereto, (S) sulfur compounds with the S content being varied. The coated steel sheets were subjected to finish annealing to form primary films having various average thicknesses and coated with an insulating coating having tension to provide oriented electrical steel sheet samples which were then subjected to measurement for magnetic flux density.
The results are shown in Fig. 1. As is apparent from Fig. 1, when the S content (the proportion of weight of S per 100 of weight of MgO) is less than 0.5%, the primary film is not reduced and the thickness thereof does not become 0.3 µm or less. On the other hand, when the S content exceeds 10%, the surface of the steel sheet is roughened, which not only has an adverse effect on the magnetic properties but also gives rise to a lowering in the integration density in the orientation of the secondary recrystallized grains, so that no satisfactory effect of lowering the iron loss can be attained due to a lowering in the magnetic flux density.
Thus, the S compound can minimize the thickness of the primary film and further has an important effect that nitriding during finish annealing can be inhibited.
In the present invention, MnS and AlN are used as the inhibitor in the secondary recrystallization. MnS disappears at a relatively low temperature to optimize the primary recrystallization structure. Thereafter, the disappearance of AlN leads to development of secondary recrystallization to provide a good secondary recrystallized structure. However, when nitriding occurs during finish annealing, since the inhibitor is excessively strengthened and the optimization of the primary recrystallized structure by the disappearance of MnS cannot be attained, the sharpness of Goss texture in the orientation of the secondary recrystallized grains deteriorates.
The addition of the S compound to the annealing separator inhibits the penetration of nitrogen into the steel sheet. In the conventional oriented electrical steel sheet, the formation of the primary film inhibits the penetration of nitrogen to some extent. However, when the primary film disappears during secondary recrystallization as in the present invention, the effect of inhibiting the nitriding by the primary film is lost. Therefore, in the present invention, the S compound is added to inhibit the formation of the primary film and, at the same time, to inhibit nitriding, thereby providing a good orientation sharpness of Goss texture.
Besides the S compounds, Cl compounds, carbonates, nitrates, sulfates, etc. have the same effect as the additive for reducing the primary film. Therefore, at least one of these additives may further be added in an amount of 1 to 15 parts by weight in terms of the total amount of Cl, (CO₃)⁻², (NO₃)⁻² and (SO₄)⁻² based on 100 parts by weight of MgO.
Examples of the above-described additives contemplated in the present invention include a sulfide, or a chloride, a carbonate, a nitrate and a sulfate of Li, K, Bi, Na, Ba, Ca, Mg, Zn, Fe, Zr, Sn, Sr, Al, etc. When they are used as the additive, at least one of sulfides of the above-described elements is added in an amount of 0.5 to 20 parts by weight based on 100 parts by weight of MgO, or at least one of compounds, such as chlorides, carbonates, etc. of the above-described elements, is added in an amount of 2 to 20 parts by weight based on 100 parts by weight of MgO.
Although the above-described compounds, such as chlorides, have the effect of reducing the primary film, they cannot inhibit the nitriding during finish annealing. For this reason, the chlorides and the like should be added together with the S compound.
It is noted that the chlorides and the like are very useful for accelerating the action of reducing the primary film on the surface of the steel sheet. Especially, Cl ions react directly with a matrix at the interface of the primary film and the matrix to form a chloride at the interface of the primary film and the matrix, which strongly promotes the peeling of the primary film. Therefore, it is necessary to add the chlorides etc. when no satisfactory effect is attained by using the S compound alone or the yield is deteriorated due to the presence of sites where the secondary recrystallization could not be sufficiently developed because of uneven decarburization, finish annealing, etc.
When the S content of the steel is high, the desulfurization reaction during the finish annealing slows. The addition of the S compound in a large amount to this steel further delays the desulfurization reaction. In such a case, a slight reduction in the amount of addition of the S compound and, instead, the addition of the chlorides and the like to make up for the effect of reducing the primary film on the surface of the steel sheet is effective.
In the combined use of the S compound and the chlorides and the like, the primary film is more effectively inhibited by virtue of the combined effect, which enables the presence of the primary film on the steel sheet to be brought to substantially zero.
In the conventional method, besides MgO, TiO₂, an antimony compound (Sb₂(SO₄)₃), a boron compound (Na₂(BO₄)₃), strontium/barium compounds, carbide/nitride, etc. are added to annealing separators to facilitate the reaction. In the present invention, these additives can exhibit their effect and do not change the subject matter of the present invention.
It has been found that the formation of grooves satisfying particular requirements on the steel sheet having the above-described very thin primary film contributes to a remarkable lowering in the iron loss of the steel sheet.
This effect can be confirmed by the following experiment.
A grain oriented electrical steel sheet having a chemical composition specified in Table 2 was hot-rolled, and the hot-rolled sheet was quenched after annealing and cold-rolled to a thickness of 0.23 mm. Grooves having a depth of 15 µm were provided, at a pitch of 5 mm by using a roll, in the direction of the width of the steel sheet, and the steel sheet was then subjected to primary recrystallization annealing, coated with various annealing separators comprising a MgO powder and, added thereto, various compounds, subjected to the coated steel sheets to finish annealing to form primary films having various average thicknesses and coated with an insulating coating having tension to provide oriented electrical steel sheet samples which were then subjected to measurements for iron loss.
The results are shown in Fig. 2. As is apparent from this drawing, the iron loss improves with a reduction in the thickness of the primary film, and this tendency is particularly significant when the thickness of the primary film is 0.3 µm or less. This shows that, even when grooves are provided before the primary recrystallization annealing, i.e., an intermediate step, and filled with forsterite or the like in the subsequent step to reduce the magnetic domain regulation effect, if the primary film on the surface of the steel sheet has a small average thickness or is absent, the division of the magnetic domain can be satisfactorily aeffected.
The present inventors have made further studies and, as a result, have confirmed that grooving during the primary recrystallization annealing or the intermediate step before or after the primary recrystallization annealing in addition to the above-described step of cold rolling can provide a satisfactory magnetic domain division effect.
In the prior art, use is made of a method wherein grooves are provided on a steel sheet close to a product after the formation of the primary film, that is, in the final step, to divide the magnetic domain. In this method, the magnetic domain regulation effect is better than that attained by the provision of grooves in the intermediate step. On the other hand, in the present invention, a satisfactory magnetic domain division effect can be attained even by the grooving in the intermediate step, which can be effected at a low cost, which is very valuable from the viewpoint of industry.
Further, it has been confirmed that a further improvement in the magnetic domain division effect ( a further lowering in the iron loss) can be attained by grooving the steel sheet at a temperature in the range of from 300 to 950°C.
This effect can be confirmed by the following experiment.
A hot-rolled steel sheet for a grain oriented electrical steel sheet having a chemical composition specified in Table 3 was quenched after annealing, pickled, cold-rolled to a thickness of 0.23 mm and subjected to primary recrystallization annealing. Grooves having a depth of 15 µm were provided at a pitch of 5 mm, by using a roll, in the direction of width of the steel sheet, at the temperature immediately after the primary recrystallization annealing, i.e., a temperature in the range of from 700 to 600°C. The steel sheet was then cooled, coated with various annealing separators comprising a MgO powder and, various compounds, subjected to finish annealing to form primary films having various average thicknesses and coated with an insulating coating having tension to provide grain oriented electrical steel sheet samples which were then subjected to measurements for iron loss. The results are shown in Fig. 3. As is apparent from this figure, the iron loss improves with a reduction in the thickness of the primary film, and this tendency is particularly significant when the thickness of the primary film is 0.3 µm or less.
As is apparent from Fig. 3, the degree of lowering in the iron loss is larger than that in Fig. 2.
With respect to conditions for grooves, it is important to provide grooves having an average depth of 1 to 50 µm and an average width of 500 µm or less in a regular arrangement in a direction at an angle of 45 to 90° to the longitudinal direction of rolling of the steel sheet by a mechanical, chemical, optical, thermal, electrical or other energy irradiation method. This is because the grooves enable the magnetic domain of the product to be more finely divided, which contributes to a reduction of the iron loss.
Basically, the grooves may be provided by any means, for example, a mechanical method using a grooved roll, a grooved or sprocket press, etc., an energy irradiation method using a laser beam, a plasma, etc., a method wherein water, an oil or the like is blown against the steel sheet at a high pressure, chemical etching with an acid or like, electrical etching or a method comprising a combination of the above methods so far as the formed grooves satisfy the above-described requirements.
Thus, when the thickness of the primary film is 0.3 µm or less, a combination of the primary film with the above-described grooves contributes to a remarkable improvement in the magnetic properties. The reason for this has not been elucidated yet.
However, it can be easily estimated that, if the primary film is thick, the flow of the magnetic flux of the steel sheet is inhibited and this tendency is significant when a large amount of an oxide, such as spinal (MgAl₂O₄), is present just under the forsterite. Therefore, when the primary film present on the surface of the steel sheet is minimized or completely eliminated and, instead, regular grooves are formed, the magnetic flux flows in a smooth manner. As a result, the iron loss can be sufficiently lowered. For this reason, the above-described limitation is imposed on the depth and pitch of grooves.
The present invention has been completed based on the above-described studies and observations.

Fig. 1 is a diagram showing the relationship between the S content of the sulfide in the annealing separator and the magnetic flux density;
Fig. 2 is a diagram showing the relationship between the average thickness of the primary film and the iron loss when grooves are provided at room temperature on the steel of the present invention; and
Fig. 3 is a diagram showing the relationship between the average thickness of the primary film and the iron loss when grooves are provided at a high temperature on the steel of the present invention.

At the outset, the chemical ingredients of the electrical steel sheet produced by the process of the present invention will be described.
As described above, the electrical steel sheet, to which the present invention is applicable, is limited to a steel comprising Al as a main ingredient besides Si and a main inhibitor comprising Si₃N₄ or AlN or MnS in the case of a steel having a high S content. It is a matter of course that the addition of other additive elements, such as Sn, Se, Sb, Cu, Bi, Nb, Ti, V and Ni as auxiliary elements, besides Si and Al, for the purpose of improving the magnetic properties does not change the subject matter of the present invention.
Steels containing AlN or Si₃N₄ or MnS are known in the art, and the technique according to the present invention can be applied to all of these cases. However, the following production process is optimal for the purpose of efficiently exhibiting the features of the present invention.
Specifically, this production process is characterized in that, in a steel having a Si content of 2.5 to 4.5%, Al is contained in an amount of 0.010 to 0.050% in terms of the amount of an acid solved Al during the production of the steel by the melt process and N is added in an amount of 0.0030 to 0.0120% during the production of the steel by the melt process. S and Mn are incorporated in respective amounts of 0.008 to 0.06% and 0.03 to 0.20%.
As described above, in the present invention, Si is necessary in an amount of at least 2.5% for the purpose of forming the forsterite and lowering the iron loss. On the other hand, when the Si content exceeds 4.5%, it is difficult to ensure a sufficient amount of recrystallized grains having a Goss orientation in the secondary recrystallization.
Al is useful for forming an AlN inhibitor and should be present in an amount of at least 0.010% in terms of the amount of acid solved Al during the production of the steel by the melt process. In the present invention, however, when the amount of Al exceeds 0.050% in terms of the acid solved Al content, not only no suitable amount of AlN is formed but also the amount of formation of Al₂O₃ is increased, which is detrimental to the purity of the steel and has an adverse effect of the magnetic properties.
N is indispensable to the formation of Si₃N₄ and AlN inhibitors and, in the present invention, should be present in an amount of at least 0.0030% after the completion of the primary annealing, that is, during the production of the steel by the melt process. On the other hand, when the N content exceeds 0.0120%, an excessive amount of Al or Si is consumed, which is unfavorable for secondary recrystallization.
S is necessary in an amount of at least 0.008% during the production of the steel by the melt process for the purpose of effectively using MnS as the inhibitor when a positive utilization of S is intended. On the other hand, when the S content exceeds 0.06%, MnS unfavorably aggregates. The same effect can be attained when sulfurizing is effected by any method before the secondary recrystallization.
Mn is necessary for the formation of MnS and should be present in an amount of at least 0.03% during the production of the steel by the melt process. When the Mn content exceeds 0.20%, it becomes difficult to form MnS.
C is necessary for ensuring the amount of γ phase in the hot rolling and should be present in an amount of at least 0.03% for ensuring the magnetic properties contemplated in the present invention. On the other hand, if the C content exceeds 0.120%, it becomes difficult to provide a favorable aggregate structure in the primary recrystallization annealing.
In the present invention, although other elements are not particularly characteristic as compared with elements used in the conventional steel, elements such as Sn, Se, Sb, Cu, B, Nb, Ti, V and Ni can improving the magnetic properties and the use thereof does not change the subject matter of the present invention.
The steps constituting the process of the present invention will now be described.
A molten steel, which has been tapped in a converter or an electric furnace and optionally subjected to refining to regulate the composition, is subjected to continuous casting, ingot making/slabbing, thin slab continuous casting to omit the hot rolling or the like and to provide a slab having a thickness of 30 to 400 mm (50 mm or less in the case of thin slab continuous casting). In this case, the lower limit of the thickness is 30 mm from the viewpoint of the productivity, while the upper limit of the thickness is 400 mm from the viewpoint of preventing abnormal distribution of Al₂O₃ due to center segregation. In the case of thin slab continuous casting, the upper limit of the thickness is 50 mm from the viewpoint of suppressing the formation of coarse grains caused by a reduction in the cooling rate.
The slab thus obtained is reheated to 1,200°C or above by gas heating, electric heating or the like and subsequently hot-rolled into a hot coil having a thickness of 10 mm or less. In this case, the lower limit of the reheating temperature is 1,200°C from the viewpoint of melting MnS and AlN. When the temperature exceeds 1,400°C, surface roughening is likely to occur. On the other hand, the upper limit of the thickness of the hot coil is 10 mm from the viewpoint of attaining a cooling rate capable of forming proper precipitates. In the thin slab continuous casting, it is possible to directly provide a coil, and in this case, the thickness is preferably 10 mm or less.
In some cases, the hot coil thus provided is again annealed at 800 to 1250°C and then subjected to water cooling, air cooling or other treatment or a combination thereof to suitably improve the magnetic properties. In this case, the lower limit of the annealing temperature is 800°C from the viewpoint of dissolving AlN, while the upper limit, for preventing the coarsening of AlN grains is 1250°C.
After the completion of the above-described treatment, the hot coil is pickled in a direct or batch manner and then subjected to cold rolling. The cold rolling is effected with a reduction ratio of 60 to 95%. The lower limit of the reduction ratio is 60% from the viewpoint of recrystallization, preferably 70% from the viewpoint of increasing grains having a {111}〈112〉 orientation in the primary annealing to accelerate the formation of grains having a Goss orientation in the secondary recrystallization annealing. On the other hand, when the reduction ratio exceeds 95%, grains unfavorable for the magnetic properties called "deviated Goss grains", which are grains having a Goss orientation rotated within the plane, occur during the secondary recrystallization annealing.
The above-described process is the so-called "single cold rolling process". When use is made of the so-called "double cold rolling process" wherein cold rolling is effected twice with annealing between the cold rollings (i.e., cold rolling-annealing-cold rolling is effected), the reduction ratio of the first cold rolling and the reduction ratio of the second cold rolling are 10 to 80% and 50 to 95%, respectively. In this case, 10% is the minimum reduction ratio of the first cold rolling necessary for the recrystallization, 80% and 95% are respectively the upper reduction ratios for the first cold rolling and the second cold rolling limited from the viewpoint of forming proper grains having a Goss orientation during the secondary recrystallization, and 50% is the lower reduction ratio of the second cold rolling necessary for properly forming grains having a {111}〈112〉 orientation formed in the primary annealing in the double cold rolling process.
The so-called "inter-pass aging" wherein the steel sheet is heated to a temperature in the range of from 100 to 400°C is also useful for improving the magnetic properties. When the aging temperature is below 100°C, no aging effect can be attained, while when it exceeds 400°C, the dislocation is unfavorably recovered.
The steel sheet is then subjected to primary recrystallization annealing. In this connection, the formation of grooves at a temperature in the range from 300 to 950°C in the course of or before or after the primary recrystallization annealing is important to the present invention. When the temperature of the steel sheet is below 300°C, a strain occurs in the steel sheet, which causes fine grains to be formed around grooves after the secondary recrystallization, so that the iron loss is significantly increased. The temperature of the steel sheet is preferably 600°C or above. On the other hand, when it exceeds 950°C, since the primary crystallized grains are coarsened, a Goss orientation favorable to iron loss cannot be provided during the secondary recrystallization. The formation of grooves in a high temperature region in a period between the initiation of the temperature rise and the completion of cooling in the primary recrystallization annealing is preferred also from the viewpoint of energy saving. However, the effect of the present invention can be similarly attained also by heating the steel sheet after the completion of cooling in the primary recrystallization annealing and forming grooves at a temperature in the range from 300 to 950°C.
In the double cold rolling process, the grooves may be formed during any of the first annealing and the second annealing. Alternatively, the formation of the grooves may be divided between the first annealing and the second annealing.
The grooves thus provided remain after the completion of the finish annealing, and the combined effect of the grooves and the minimized primary film composed mainly of forsterite and having an average thickness as small as 0.3 µm provides a low iron loss value unattainable by the prior art. The reason why the thickness of the primary film is preferably 0.3 µm or less is as described above, and when the thickness exceeds 0.3 µm, no sufficient iron loss value is provided by grooving in the intermediate step according to the present invention.
Although the method of forming grooves is as described above, when the average depth of the grooves is less than 1 µm, no magnetic domain division effect can be attained. On the other hand, when the average depth of the grooves exceeds 50 µm, the depth is excessively large, which prevents smooth flow of the magnetic flux, so that the iron loss becomes poor. It is preferably 5 to 30 µm. The grooves are preferably arranged in a regular manner. This is because the magnetic domain can be regularly divided. In general, it is preferred for the grooves to be provided at a substantially constant pitch having an angle of 45 to 90° (right angle) to the longitudinal direction of the steel sheet. When the angle is less than 45°, the direction of the division of the magnetic domain does not coincide with a crystallographic orientation favorable for the magnetism. The pitch of the grooves is preferably 2 to 20 mm. When the pitch is less than 2 mm, the magnetic domain is excessively divided, which leads to an increase in 90° domain, so that the magnetic strain as well as the iron loss becomes poor. On the other hand, when the pitch of the grooves exceeds 20 mm, the effect of dividing the magnetic domain cannot be attained.
The primary recrystallization annealing is effected in both the single cold rolling process and the second cold rolling process. It is useful to effect decarburization in the annealing. As described above, C is not only unfavorable to the growth of the secondary recrystallized grains but also leads to a deterioration in the iron loss when it remains as an impurity. It is preferred for the C content to be lowered in the stage of production of the steel by the melt process because the step of decarburization can be shortened and, further, grains having a {111}〈112〉 orientation are increased. Setting of a proper dew point in the primary crystallization serving also as decarburization annealing enables an oxide layer necessary for the subsequent formation of the primary film to be ensured.
The primary recrystallization annealing temperature is preferably 700 to 950°C. The lower limit of the primary recrystallization annealing temperature is 700°C from the viewpoint of causing the recrystallization, while the upper limit is 950°C from the viewpoint of preventing the occurrence of coarse grains.
In the present invention, it is important that the amount of oxidation in the primary recrystallization annealing be 1000 ppm or less in terms of the [O] content and the FeO/SiO₂ is 0.25 or less. When the [O] content exceeds 100 ppm, the SiO₂ content and FeO content of the oxide film inevitably become high and the thickness of the oxide film increases, which is disadvantageous in the glass film decomposition reaction during the high temperature finish annealing. The amount of oxidation is preferably 400 to 800 ppm in terms of the [O] content. On the other hand, the FeO/SiO₂ ratio is preferably 0.25 or less. This is because, when the FeO/SiO₂ ratio exceeds 0.25, the reactivity for the formation of a glass film in the first half of the high-temperature finish annealing is extremely increased, which gives rise to an increase in the amount of formation of the forsterite in the first half of the high-temperature finish annealing, so that the subsequent step of the decomposition reaction of the forsterite does not sufficiently proceed.
After the completion of primary annealing, a magnesium oxide (composed mainly of MgO; hereinafter referred to as "MgO") powder is dissolved in water or an aqueous solution composed mainly of water and coated in a slurry form on the steel sheet. In this case, a very small amount of a suitable compound is added for the purpose of accelerating a forsterite formation reaction that facilitates the melting of the MgO powder during the subsequent secondary recrystallization annealing.
When TiO₂ is added, the amount thereof added is preferably 1 to 15%. In this case, the lower limit of 1% is from the viewpoint of attaining the effect of accelerating the forsterite reaction. When the amount of TiO₂ exceeds 15%, the proportion of MgO becomes so small that the forsterite reaction does not proceed.
Antimony compounds, such as Sb₂(SO₄)₃, have the effect of melting MgO at a relatively low temperature. When they are added, the amount of addition thereof is preferably 0.05 to 5%. The lower limit of the amount of addition thereof is 0.05% from the viewpoint of causing the above-described melting at a low temperature. On the other hand, when the amount of addition exceeds 5%, a proper reaction for the formation of forsterite from MgO is inactivated.
Boron compounds, such as Na₂B₄O₇, and strontium/barium and carbide/nitride compounds having the same function as the boron compounds, have the effect of melting MgO at somewhat higher temperature than that for the antimony compounds. When these compounds are added, the amount of addition thereof is preferably 0.05 to 5%. The lower limit of the amount of addition thereof is 0.05% from the viewpoint of attaining the above-described effect. On the other hand, when the amount of addition thereof exceeds 5%, a proper reaction for the formation of forsterite from MgO is inactivated.
These compounds may be added in the form of a combination thereof. In this case, the amount of addition of the compounds is expressed as a percentage of the weight proportion of the compound per 100 parts by weight of MgO.
In the present invention, in order to bring the average thickness of the primary film after the completion of the high-temperature finish annealing to 0.3 µm or less, use is made of an annealing separator comprising a MgO powder and, added and mixed therewith, a S compound having a S content of 0.5 to 10%.
When the S content is less than 0.5% by weight, the thickness of the primary film exceeds 0.3 µm, while when it exceeds 10%, the magnetic flux density lowers.
The above-described annealing separator may further comprise at least one member selected from the group consisting of Cl compounds, carbonates, nitrates and sulfates in an amount of 1 to 15% in terms of the total amount of Cl, (CO₃)⁻², (NO₃)⁻² and (SO₄)⁻². This can more effectively inhibit the formation of the primary film, so that the thickness of the primary film can be brought to substantially zero.
When they are metallic compounds, the annealing separators may comprise a MgO powder and, added and mixed therewith, 0.5 to 20% of at least one of sulfides of Li, K, Bi, Na, Ba, Ca, Mg, Zn, Fe, Zr, Sn, Sr, Al, etc. and/or 2 to 20% of at least one of carbonates, nitrates and chlorides of these elements. When the amount of the sulfide is less than 0.5%, the primary film cannot be effectively reduced. On the other hand, when it exceeds 20% by weight, the film formation process becomes so unstable that it is difficult to attain the high magnetic flux density and low iron loss contemplated in the present invention. The object of the present invention cannot be attained also when the amount of the carbonates, nitrates and chlorides of the above elements is less than 2% or exceeds 20%.
In the present invention, water of hydration of MgO is also important and limited to 0.5 to 5.0%. When the content of water of hydration is less than 0.5%, the reactivity of MgO is deteriorated. On the other hand, when it exceeds 5.0%, the dew point in an atmosphere between the steel sheets becomes so high that an uneven oxide film occurs on the surface of the steel sheet, which makes it impossible to have an even, very thin primary film or a non-primary film.
In the high-temperature finish annealing serving also as the secondary recrystallization, the maximum arrival temperature is preferably 1100 to 1300°C. The lower limit temperature is 1100°C from the viewpoint of ensuring the secondary recrystallization. On the other hand, when the temperature exceeds 1300°C, the grains are excessively coarsened, which leads to an increase in the iron loss.
In the secondary recrystallization annealing, the following points are important. In the present invention, the primary film composed mainly of forsterite becomes remarkably reduced or absent by virtue of the effect of the addition of special additives to the MgO powder, so that the nitrogen compound inhibitors (such as AlN and Si₃N₄) necessary for the secondary recrystallization in the annealing are likely to escape during finish annealing. On the other hand, the function of MnS as an inhibitor is also important. For this reason, it is important to prevent the penetration of N into the steel by regulating the partial pressure of nitrogen (P_N2) in the atmosphere gas during the finish annealing to 25% or less, which enables secondary recrystallization to be stably effected. If N penetrates, in a large amount, in a temperature range of from 700°C to the maximum arrival temperature, the amount of AlN becomes so large that the growth of grains having a sound Goss orientation during the secondary recrystallization annealing cannot be expected as opposed to an inhibitor having a suitable strength, such as MnS. In the present invention, the addition of the S compound to the annealing separator inhibits nitriding during finish annealing to enable grains having a more sound Goss orientation to be stably grown. When the second crystallization temperature is below 700°C, N does not penetrate into the steel, while when the temperature exceeds the maximum arrival temperature, the secondary recrystallization or the like is unfavorably completed. Still preferably, this annealing is effected in a hydrogen atmosphere because it becomes possible to attain a further effect of the present invention that secondary recrystallized grains having an excellent Goss orientation can be provided. In the high-temperature finish annealing, when the heating rate is excessively high, the inhibitor is likely to escape before the satisfactory recrystallization occurs. Especially, in the present invention that aims at a very thin primary film or no primary film, stable magnetic properties can be provided by limiting the heating rate to 30 °C/hr or less.
Important features of the process for producing a grain oriented electrical steel sheet according to the present invention are as described above. Further, when the present invention is practiced on a commercial scale, the coating of the steel sheet after the secondary recrystallization with a high tension film (coating) having an organic or inorganic insulating film in combination with a heat treatment or the like or the coating of such a film by a sol-gel process is especially important for the purpose of improving insulating properties and magnetic properties. The reason for this is that, in the present invention, since the primary film having a high tension property, such as forsterite, is minimized or absent, the coating of an insulating film having a high tension property is effective to compensate for the minimized primary film or the absence of the primary film.

EXAMPLES

Example 1

A material comprising, in terms of weight, 0.056% of C, 3.35% of Si, 0.10% of Mn, 0.27% of acid solved Al, 0.0070% of N and 0.0165% of S with the balance consisting of Fe and unavoidable impurities was hot-rolled to a thickness of 2.0 mm, annealed at 1120°C for 2 min, pickled, cold-rolled to a final steel sheet thickness of 0.225 mm. Then, the steel sheet was subjected to decarburization annealing at 850°C for 3 min in an atmosphere comprising 25% of N₂ and 75% of H₂ and having a dew point of 55°C. In this case, the oxygen content of the steel sheet was 600 ppm.
The steel sheet was coated with an annealing separator having a composition specified in Table 4 and then subjected to final finish annealing under atmosphere conditions (A) and (B) specified in Table 5. Subsequently, the annealed steel sheet was coated with a solution comprising 100 ml of 20% colloidal silica, 50 ml of 50% Al(H₂PO₄) and 5g of CrO₃ at a coverage after drying of 6 g/m² and subjected to heat flattening and baking at 880°C for 45 sec. The film properties of surface of the steel sheet and the workability of the steel sheet are given in Table 5.
As is apparent from the results shown in Table 5, all the steel sheets according to the present invention had substantially no glass film, a metallic gloss and a good punchability. The magnetic flux density was very good for the samples using annealing conditions (A) falling within the scope of the present invention wherein the partial pressure of N₂ in the temperature rise atmosphere was low. On the other hand, all the samples using the annealing conditions (B) had a low magnetic flux density and poor magnetism even when use was made of an annealing separator falling within the scope of the present invention.

Example 2

Steels comprising chemical ingredients specified in Tables 6 and 9 were prepared by a melt process in a converter, and steel sheets were prepared under conditions specified in Tables 7, 8, 10 and 11. Some of the hot-rolled sheets were annealed under conditions of 1120°C for 30 sec and cooled with water after the completion of the annealing. All the samples except for sample F were subjected to inter pass aging at 250°C. The formation of grooves was effected after or during cold rolling. Thereafter, the steel sheets were subjected to primary annealing.
Further, the steel sheets were coated with a powder. In this case, the powder was dissolved in water to form a slurry that was then coated on the steel sheets and dried at 350°C. In this connection, the amount of addition of a compound is expressed as a percentage of the weight proportion of the compound per 100 parts by weight of MgO. Thereafter, the coated sheet sheets were subjected to secondary recrystallization annealing at various average heating rates in a temperature range of from 700°C to the maximum arrival temperature. In this case, the maximum arrival temperature was 1,200°C.
Further, the steel sheets were heat-coated with a high-tension insulating film (a secondary film) comprising a phosphoric acid compound, subjected to blanking and stress relieving annealing at 850°C for 4 hr in a dry atmosphere comprising 90% of N₂ and 10% of H₂ and subjected to a magnetic measurement test.

The results are given in Tables 8 and 11.

Table 4

Conditions	Conditions for MgO			Conditions for Annealing Separator
	CAA value (Sec.)	Particle Size (10 µm)	Water of Hydration (%)
Invention 1	100	50	3.0	MgO 100g + SnS 10g
Invention 2	100	50	2.3	MgO 100g + K₂S 10g
Invention 3	100	50	3.1	MgO 100g + CaS 5g + NaNO₃ 5g
Invention 4	100	50	2.8	MgO 100g + K₂S 5g + MgCl₂ 5g
Invention 5	100	50	2.6	MgO 100g + SrS 5g + Na₂SO₄ 5g
Comp. Ex. 1	100	50	3.0	MgO 100g + TiO₂ 5g + H₃BO₃ 0.4

Table 5

Conditions for Final Annealing	Conditions for Annealing Separator	Surface Appearance of Steel Surface after Finish Annealing	Magnetic Flux Density [B₈(T)]	*Number of Times of Punching (50 µm Burr)
(A) Invention; Partial Pressure of N₂ during Heating: 15%	Invention 1	Substantially wholly metallic gloss	1.91	4.8
	Invention 2	Wholly metallic gloss	1.91	5.5
	Invention 3	Wholly metallic gloss	1.92	7.6
	Invention 4	Substantially wholly metallic gloss	1.92	9.5
	Invention 5	Substantially wholly metallic gloss	1.92	7.2
	Comp. Ex. 1	Uniform primary film formed	1.90	0.9
(B) Comp. Ex.; Partial Pressure of N₂ during Heating: 50%	Invention 1	Substantially wholly metallic gloss	1.85	4.9
	Invention 2	Wholly metallic gloss	1.86	5.9
	Invention 3	Wholly metallic gloss	1.88	10.4
	Invention 4	Substantially wholly metallic gloss	1.87	9.6
	Invention 5	Substantially wholly metallic gloss	1.85	7.2
	Comp. Ex. 1	Uniform primary film formed	1.93	1.0

Note)
*Number of times of punching: × 10⁴ (number of operation punching necessary to cause a burr having height of 50 µm)

All the maximum depth, pitch and angle to the rolling direction of grooves are measurements for products after the completion of the secondary recrystallization annealing.
The magnetic measurement was effected by the SST testing method for a single sheet having a size of 60 × 300 mm. In this test, the B₈ value [magnetic flux density (Tesla) at 800 A/m] and W_17/50 (iron loss value (w/kg) in 1.7 Tesla at 50 Hz) and W_13/50 (iron loss value (w/kg) in 1.3 Tesla at 50 Hz) were measured.
As is apparent from Tables 8 and 11, the materials falling within the scope of the present invention had a sufficiently low iron loss and can attain the object of the present invention.

Example 3

Steels comprising chemical ingredients specified in Table 12 were produced in a converter, and grain oriented electrical steel sheets were produced under conditions specified in Tables 13 and 14. Some of the hot-rolled sheets were annealed under conditions of 1,120°C at 30 sec and cooled with water after the completion of the annealing. All the samples except for sample F were subjected to between-pass aging at 250°C. Thereafter, the steel sheets were subjected to primary annealing. The formation of groove was effected during the primary annealing. Further, the steel sheets were coated with a powder. In this case, the powder was dissolved in water to form a slurry that was then coated on the steel sheets and dried at 350°C. In this connection, the amount of addition of a compound is expressed as a percentage of the weight proportion of the compound per 100 parts by weight of MgO.
Thereafter, the coated sheet sheets were subjected to secondary recrystallization annealing at various average temperature rise rates in a temperature range of from 700°C to the maximum arrival temperature. In this case, the maximum arrival temperature was 1,200°C. Further, the steel sheets were heat-coated with a high-tension insulating film (a secondary film) comprising a phosphoric acid compound, subjected to blanking and stress relieving annealing at 850°C for 4 hr in a dry atmosphere comprising 90% of N₂ and 10% of H₂ and subjected to a magnetic measurement test. The results are given in Table 14.
All the maximum depth, pitch and angle to the rolling direction of grooves are measurements for products after the completion of the secondary recrystallization annealing. The magnetic measurement was effected by the SST testing method for a single sheet having a size of 60 × 300 mm. In this test, the B₈ value [magnetic flux density (Tesla) at 800 A/m] and W_17/50 (iron loss value (w/kg) in 1.7 Tesla at 50 Hz) and W_13/50 (iron loss value (w/kg) in 1.3 Tesla at 50 Hz) were measured.
As is apparent from Table 14, the materials falling within the scope of the present invention had a sufficiently low iron loss and can attain the object of the present invention.

Claims

A process for producing a grain oriented electrical steel sheet having a minimized primary film, excellent magnetic properties and good workability, comprising the steps of: heating a slab comprising, in terms of % by weight, 0.03 to 0.12% of C, 2.5 to 4.5% of Si, 0.010 to 0.050% of acid solved Al, 0.0030 to 0.0120% of N, 0.01 to 0.06% of S and 0.03 to 0.20% of Mn with the balance consisting of Fe and unavoidable impurities at a temperature of 1200°C or above; hot-rolling the heated slab to provide a hot-rolled sheet; subjecting the hot-rolled steel sheet to cold rolling once or at least twice with intermediate annealing between cold rollings and effecting annealing and quenching before final cold rolling; subjecting the cold-rolled sheet to primary recrystallization annealing including decarburization and then coating the steel sheet with an annealing separator comprising 100 parts by weight of MgO and, added thereto, a S (sulfur) compound in an amount of 0.5 to 10 parts by weight in terms of S; subjecting the coated steel sheet to finish annealing serving also as secondary recrystallization annealing; and then coating the annealed steel sheet with an insulating film.
The process according to claim 1, wherein the annealing separator coated on the cold-rolled steel sheet further comprises at least one member selected from the group consisting of Cl compounds, carbonates, nitrates and sulfates in an amount of 1 to 15 parts by weight in terms of the total amount of Cl, (CO₃)⁻², (NO₃)⁻² and (SO₄)⁻² based on 100 parts by weight of MgO.
A process for producing a grain oriented electrical steel sheet having minimized primary film, excellent magnetic properties and good workability, comprising the steps of: heating a slab comprising, in terms of % by weight, 0.03 to 0.12% of C, 2.5 to 4.5% of Si, 0.010 to 0.050% of acid solved Al, 0.0030 to 0.0120% of N, 0.01 to 0.06% of S and 0.03 to 0.20% of Mn with the balance consisting of Fe and unavoidable impurities to a temperature of 1,200°C or above; hot-rolling the heated slab to provide a hot-rolled sheet; subjecting the hot-rolled steel sheet to cold rolling once or at least twice with intermediate annealing between cold rollings and effecting annealing and quenching before final cold rolling; subjecting the cold-rolled sheet to primary recrystallization annealing including decarburization and then coating the steel sheet with an annealing separator comprising 100 parts by weight of MgO and, added thereto, 2 to 30 parts by weight in total of at least one member selected from the group consisting of sulfides of Li, K, Bi, Na, Ba, Ca, Mg, Zn, Fe, Zr, Sn, Sr and Al; subjecting the coated steel sheet to finish annealing serving also as secondary recrystallization annealing; and then coating the annealed steel sheet with an insulating film.
The process according to claim 3, wherein the annealing separator coated on the cold-rolled steel sheet further comprises 2 to 30% by weight in total of at least one member selected from the group consisting of chlorides, carbonates, nitrates and sulfates of Li, K, Bi, Na, Ba, Ca, Mg, Zn, Fe, Zr, Sn, Sr and Al based on 100 parts by weight of MgO.
The process according to any one of claims 1 to 4, wherein MgO used in the annealing separator has such a particle size distribution that particles having a diameter of 10 µm or less occupy 30% or more of the MgO, a citric acid activity (CAA value) of 50 to 300 sec (as measured at 30°C) and a water of hydration content of 5% or less.
The process according to any of claims 1 to 5, wherein the oxygen content of the steel sheet in the decarburization annealing is 900 ppm or less and the FeO to SiO₂ ratio in the oxide film is 0.20 or less.
The process according to any of claims 1 to 6, wherein a steel comprising 1 to 7% of Si and more than 0.045 to 0.20% of P is used as a starting material.
The process according to any of claims 1 to 7, wherein the temperature rise in the finish annealing is effected in an atmosphere comprising N₂ and H₂ with the nitrogen content being 25% or more at a temperature rise rate of 20 °C/hr or less.
The process according to any of claims 1 to 8, wherein grooves having an average depth of 1 to 50 µm and an average width of 500 µm or less are formed at an angle of 45 to 90° to the rolling direction of the steel sheet at intervals of 2 to 20 mm by means of at least one of a press, a sprocket roll, marking, a laser beam and local etching method in at least one stage of during cold rolling, after cold rolling during, after decarburization annealing, after finish annealing or after the insulating film treatment.
The process according to claim 9, wherein the grooves are formed on the steel sheet at a steel sheet temperature of 300 to 950°C in a step before the finish annealing.
The process according to any of claims 1 to 10, wherein in the coating of the insulating film, a baking treatment is effected once or at least twice so that the thickness of insulating film after baking is 2 to 6 µm.
The process according to claim 10 or 11, wherein the groove are formed on the steel sheet at a steel sheet temperature of 300 to 950°C during cold rolling, after cold rolling or any one stage from the initiation of the temperature rise in the primary recrystallization annealing to the completion of cooling.
The process according to any of claims 1 to 12, wherein the steel sheet as hot-rolled is annealed and quenched from the annealing temperature.