GB2130241A

GB2130241A - Method for producing a grain- oriented electrical steel sheet having a high magnetic flux density

Info

Publication number: GB2130241A
Application number: GB08325076A
Authority: GB
Inventors: Yozo Suga; Tadashi Nakayama; Toyohiko Konno; Fumio Matsumoto; Keiji Honma
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1982-09-24
Filing date: 1983-09-20
Publication date: 1984-05-31
Also published as: SE8305095L; IT8322974A0; KR840006019A; SE8305095D0; IT1167387B; FR2533586B1; DE3334519C2; FR2533586A1; CA1210670A; GB8325076D0; KR890000882B1; DE3334519A1; GB2130241B; SE460482B; IT8322974A1

Abstract

A feature of the present invention is to set S</=0.007%, Mn=0.08 SIMILAR 0.45%, P=0.015 SIMILAR 0.45% in a slab. The present inventive idea does away with the conventional concept of using MnS as an inhibitor and prevents incomplete secondary recrystallization by the S content, which is decreased to a level as low as possible. In addition appropriate amounts of Mn and P are added. Due to these advantages, a high Si content, which leads to a watt loss reduction, can be employed in the present invention. In addition, the temperature of slab heating can be drastically decreased as compared with the prior art. The watt loss of a product produced by a low- temperature slab-heating is considerably lower than a product produced by a high-temperature slab- heating. After hot-rolling, the material is annealed at 850-1200 DEG C and then cold-rolled with a reduction of 180%. The invention may particularly be applied to continuously cast slab.

Description

SPECIFICATION Method for producing a grain-oriented electrical steel sheet having a high magnetic flux density Background of the invention Field of the invention The present invention relates to a method for producing a grain-oriented electrical steel sheet having a high magnetic flux density.

Description of the prior art Grain-oriented electrical steel sheet is a soft magnetic material composed of crystal grains having a so called Goss texture, expressed by 11001 < 001 > by the Miller index in which the crystal orientation of the sheet plane is the 1110) plane and the crystal orientation of the rolling direction is parallel to the (001 > axis. Grainoriented electrical steel sheet is used for cores of transformers, generators, and other electrical machinery and devices.

Grain-oriented electrical steel sheet must have excellent magnetization and watt loss characteristics. The magnetization characteristic is defined by the magnitude of the magnetic flux density induced in the grain-oriented electrical steel sheet by a predetermined magnetic field.

Here, B10 is used. Soft magnetic material having a high magnetic flux density, i.e., a good magnetization characteristic, can advantageously reduce the size of the electrical machinery and devices.

Watt loss is defined as power lost due to consumption as thermal energy in a core when it is energized by an alternating magnetic field having a predetermined intensity. Here, W17150 is used. As is known, the watt loss characteristic is influenced by the magnetic flux density, sheet thickness, the impurities, resistivity, and grain size of the grain-oriented electrical steel sheet.

Increased demand has arisen for grain-oriented electrical steel sheet having a low watt loss along with the trend toward energy conservation.

Grain-oriented electrical steel sheet is produced by hot-and-cold rolling a slab to the desired final sheet thickness and then finally annealing the resultant steel strip to realize selective growth of the (1101 < 001 > oriented primary-recrystallized grains, i.e., to realize socalled secondary recrystallization.

To realize secondary recrystallization, fine precipitates, such as MnS and AIN, must be finely and uniformly dispersed in phases in the steel, while the steel is subjected to processes prior to the final high temperature annealing, so as to suppress growth of primary recrystallized grains having orientations other than the 11101 < 001 > orientation during the final high temperature annealing (inhibitor effect) controlling the secondary recrystallization, it is possible to increase the proportion of the accurately 11 10)(001 > oriented grains in the crystal grains, thereby increasing the magnetic flux density of the grain-oriented electrical steel sheet and, thus, reducing the watt loss. It is important to develop production techniques allowing control of the secondary recrystallization.

Japanese Examined Patent Publication (Kokoku) No. 40-15644 (Taguchi et al) and Japanese Examined Patent Publication (Kokoku) No. 51-13469 (Imanaka et al) disclose basic techniques for producing a grain-oriented electrical steel sheet having a high magnetic flux density and decreased watt loss.

The basic techniques disclosed in the above two Japanese examined patent publications however suffer from some fundamental problems.

In the method disclosed in Japanese Examined Patent Publication No. 40-15644, it is difficult to achieve overall optimum production condition and to stably produce grain-oriented electrical steel sheets having high magnetic flux density. As a result, the method is not appropriate for the stable production of products having the best magnetic properties.

The method disclosed in Japanese Examined Patent Publication No. 51-13469 involves double cold rolling and use of an expensive element, such as Sb or Se. This method therefore involves high production costs.

Also, both the prior art methods require high slab heating temperatures, disadvantageous from the viewpoint of the energy used for heating the slab, decreased yield due to slag generation and increased repair costs of slab-heating furnaces.

When heating a slab to make it rollable, one must raise the siab heating temperature high enough to solid-dissolve MnS, and other inhibitor elements. These later precipitate as MnS, AIN, and the like, when the steel is hot-rolled or subjected to hot-strip annealing. The greater the degree of orientation desired, the larger the amount of MnS, AIN, and other fine precipitates that must be present in the steel and, therefore, the higher the necessary slab heating temperature. Japanese Unexamined Patent Publication No.48-51852 discloses an improvement of the method of Japanese Examined Patent Publication No. 40-15644. In this method, the Si content of the starting material is increased. A high silicon content however, narrowly restricts the conditions under which AIN can be ensured in the hot-rolled strip.

Also, since the silicon content is high, the temperature range at which AIN precipitates during hot-rolling in an appropriate manner for the secondary recrystallization shifts higher, requiring a higher slab heating temperature.

The adoption of continuous casting has created additional problems in the production of grainoriented electrical steel sheet. In continuous casting linear, secondary-recrystallization- incomplete portions, referred to as streaks, are occasionally generated in the steel. This impairs the magnetic properties of the steel. The problem of streaks is greatly aggravated by a high Si content. When the Si content exceeds 3.0%, stable production of grain-oriented electrical steel sheet becomes extremely difficult. Japanese Unexamined Patent Publication No.48-53919 (M. F. Littman) discloses to remove the problem of streaks by subjecting a continuously cast steel strand at double hot-rolling steps when producing a hot rolled strip.Japanese Unexamined Patent Publication No. 50-37009 (Akira Sakakura et al) discloses a method for producing grain-oriented electrical steel sheet wherein a hot-rolled steel strip is produced by double hot-rolling steps.

These two prior art methods, however, do not fully utilize the advantages of continuous casting, i.e., omission of rough rolling. Two later publications, Japanese Unexamined Patent Publication No. 53-19913 (Morio Shiozaki et al) and Japanese Unexamined Patent Publication No.

54-1202124 (Fumio Matsumoto et al), disclose how to employ single hot-rolling to produce grainoriented electrical steel sheet using a continuously cast strand. These proposals, however, necessitate reconstruction of a casting or rolling installation and still do not completely solve the problem of streak generation.

Summary of the invention It is an object of the present invention to provide a method for producing a grain-oriented electrical steel sheet having a magnetic flux density B10 of 1.89 Tesla or more using a single cold-rolling step, wherein a stable secondary recrystallization can be obtained under less strict condition than in prior art, even at a low slab heating temperature, at a high Si content than in the prior art, and/or using a continuously cast slab.

The essence of the method according to the present invention resides in the steps of: preparing a slab which has a temperature of 14300C or less and which consists of from 0.025% to 0.075% of C, from 3.0% to 4.5% of Si, from 0.010% to 0.060% of acid soluble aluminum, from 0.0030% to 0.0130% of N, not more than 0.007% of S, from 0.08% to 0.45% of Mn, and from 0.015% to 0.045% of P, the balance being Fe; hot rolling the slab to form a hot-rolled strip annealing the hot-rolled strip at a temperature in the range of from 8500C to 1 2000C for a short period of time; heavily coldrolling the annealed strip at a reduction of not less than 80%, thereby obtaining the final sheet thickness; continuously decarburization-annealing the obtained cold-rolled strip in a wet hydrogen atmosphere and then applying an annealing separator on the strip; and, carrying out a final high temperature annealing.

One of the features according to the present invention is the sulfur content of 0.007% or less.

In the prior art, as disclosed in Japanese Examined Patent Publications Nos. 30-3651,4015644, and 47-25250, sulfur is believed useful for producing grain-oriented electrical steel sheet since sulfur forms MnS, one of the indispensible precipitates for generating secondary recrystallization. According to these publications, the effect of sulfur is most prominent in a certain range of content which is determined by the amount of solute MnS brought into solid solution during the slab-heating process. AIN also forms precipitates believed useful for producing a grainoriented electrical steel sheet. Conventionally, both MnS and AIN precipitates were used as inhibitors.

The present inventors investigated in detail the precipitation behaviour of MnS and AIN. They discovered that when a slab having the composition of an electrical steel sheet is heated and then hot-rolled and when a hot-rolled strip is annealed, MnS first precipitates at a high temperature and AIN then precipitates at a low temperature. Since MnS is already present in the steel when AIN precipitates, AIN tends to precipitate around the MnS, resulting in complex precipitation. Thus, the size and dispersion state or AIN are influenced by the precipitation states of MnS. That is, when the amount of MnS precipitated is great, the AIN is large sized and is dispersed non-uniformly.

As known from Japanese Unexamined Patent Publication No.48-51852, a fundamenal metallurgical concept for producing a grainoriented electrical steel sheet having a high magnetic flux density with a single cold-rolling process is to create an appropriate dispersion station of AIN by utilizing the aer transformation which occurs during hot-rolling or annealing.

When the Si content is high, the y transformation is disadvantageously changed, so that dispersion of AIN is impaired. In the case of continuous casting, this is believed to result in generation of streaks.

Based on the above-described discoveries and consideration of the ct-ty transformation, the present inventors descreased the precipitation amount of MnS. They then discovered that, even with a high content of Si in the steel, the dispersion state of AIN can be kept uniform and the AIN precipitates be kept small in size.

One of the features according to the present invention therefore resides in the point that the sulfur content is lower than in the prior art. Even with this, the precipitation of AIN can be controlled appropriately and the generation of streaks in continuous casting which may occur when the Si content is high, can be prevented.

Since the sulfur content is low, the precipitation amount of MnS according to the present invention is less than in the prior art. The decrease in the precipitation amount of MnS means the total amount of the inhibitors is decreased, which tends to a decrease in the magnetic flux density. To compensate for the decrease in the magnetic flux density, Mn and P are added into steel in appropriate amount.

Another feature according to the present invention resides in the fact that the Mn and P added to the steel do not change the inhibitors but render the primary recrystaliization texture appropriate before the secondary recrystallization.

That is, they compensate for the abovementioned decrease in magnetic flux density and even increase in the magnetic flux density by texture control. The crystal grains are refined and have a uniform size, with the result that the second recrystailization is stabilized.

Another feature according to the present invention is that Si content in the starting material is at least 3.0%, while stabilizing the secondary recrystallization and thus preventing the generation of streaks. This results in one of the lowest watt losses and highest magnetic flux density available in the high grade grain-oriented electrical steel sheet.

Brief description of the drawings The present invention will be described in further detail with reference to the drawings, wherein: Fig. 1 A through 1 D show photographs of the crystal gra in-macrostructu res of the products produced using steels containing 0.004%, 0.007%, 0.015%, and 0.025% of S; Fig. 2A through 2D show photographs of the crystal grain-macrostructures of the products produced using continuously cast strands containing 0.004%, 0.007%, 0.012%, and, 0.030% of S; Fig. 3 is a graph of the influence of Mn and P upon magnetic flux density B,0; Fig. 4 is a graph of the influence of Mn and P upon B10 regarding a product produced by using a continuously cast slab containing 0.0090% of N;; Fig. 5 is a graph of the magnetic properties of the products produced under the same conditions as in Fig. 3 but at slab heating temperatures of both 11500Cand 13500C.

Fig. 6 is a graph of the influence of a slabheating temperature upon the magnetic flux density of products; Fig. 7 is a graph of the relationship between the magnetic flux density (B,0) of products and the heating rate in a temperature range of from 7000C to 1100 C, such heating being carried out during a final high temperature annealing; Fig. 8 is a graph of the relationship between the magnetic flux density, the watt loss, and Cr content; and Fig. 9 is a graph similar to Fig. 5 regarding Crcontaining steels.

Description of preferred embodiments Four steels, in which the S contents were 0.004%, 0.007%, 0.015%, and 0.025%, respectively, and which contained 0.030% of C, 3.45% of Si, 0.030% of acid-soluble aluminium, and 0.0085% of nitrogen, were prepared in the form of 40 mm thick small samples. They were heated to 1 2000C in a furnace and then withdrawn from the furnace, allowing them to cool in an ambient air down to the temperature of 1000 C. The four steels were then held in a furnace for 30 seconds at 1000 C. The four steels having the temperature of 1 0000C were hot-rolled by three passes to form 2.3 mm thick hot-rolled sheets.Then, the following processes were successively carried out: continuous annealing at 1 000C for 2 minutes; cold-rolling to form a 0.30 mm cold-rolled sheet; decarburization annealing in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.

As is apparent from the crystal-grain macrostructures of the products shown in Figs.

1 A through 1 D no incomplete secondary recrystallization occurs when the S content is 0.007% or less. Also, according to experiments of the present inventors, no incomplete secondary recrystallization occurs when the Si content was 4.5% or less and when the S content was 0.007% or less. Accordingly, the S content is limited to 0.007% or less in the present invention. The S content is desirably decreased in the molten stage of melting steel because the desulfurization treatment during the final high temperature annealing can be facilitated. According to the present melting techniques for decreasing sulfur, the S content which can be easily attained without incurring cost increases is usually 0.001% or more.

Four continuously cast slabs, in which the S contents were 0.004%, 0.007%, 0.012%, and 0.030%, respectively, and which contained 0.055% of C, 3.30% of Si, 0.25% of Mn, 0.030% of acid-soluble aluminum, and 0.0080% of N, were heated to 14100C in a furnace and were hot-rolled to form 2.3 mm thick hot-rolled sheets.

Then, the following process were successively carried out: continuous annealing at 11 500C for 2 minutes; cold-rolling to form a 0.30 mm cold rolled strip; decarburization-annealing in a wet hydrogen atmosphere; application of MgO as annealing separator; and final high temperature annealing at 1 200 C for 20 hours.

Also as is apparent from the crystal grain macrostructures shown in Figs. 2A through 2D, streaks are less likely to generate when the content is lower, and streaks do not generate at all when the S content is 0.007% or less.

Continuous cast slabs in which the Mn and P contents were varied, and which contained 0.050% of C, 3.40% of Si, 0.002% of S, 0.030% of acid-soluble aluminum, and 0.0080% of nitrogen, were prepared in the form of 40 mm thick small samples. They were heated to 11 500C in a furnace and were hot-rolled by three passes to form a 2.3 mm thick hot-rolled sheets. The finishing temperature of hot rolling was approximately 8200C.

Then, the following processes were successively carried out: continuous annealing at 11 000C for 2 minutes; cold-rolling to form a 0.30 mm coid-rolled sheet; decarburization-annealing in a wet hydrogen atmosphere; application of MgO: and final high temperature annealing at 1 2000C for 20 hours.

The magnetic flux density B10 of the products is shown in Fig. 3. In Fig. 3 x corresponds to B,0 < 1.80 Tesla, A corresponds to 1.80 < B10 < 1.89 Tesla, o corresponds to 1.89 < Bo~1.91 Tesla, and corresponds to 1.91 Tesla < Br0. As is apparent from Fig. 3, when the Mn content is low, the secondary recrystallization becomes unstable, and when the Mn content is high, the magnetic flux density B10 is high. When Mn is added in more than a certain content, however, it is ineffective for enhancing the magnetic flux density B10 and is uneconomical since the amount of additive alloy becomes disadvantageously great.

When the P content is too low, the magnetic flux density B10 is low and the generation of incomplete secondary recrystallization is increased. When the P content is too high, the frequency of cracking during cold rolling is increased.

Thus, the Mn content is limited to the range of from 0.08% to 0.45%, and the P content is limited to the range of from 0.015% to 0.045% according to the present invention. In these ranges the magnetic flux density B10 is 1.89 Tesla or more, the secondary recrystallization is stable, and the problem of cracking is not significant.

Continuously cast slabs, in which the Mn and P contents were varied, and which contained 0.060% of C, 3.45% of Si, 0.004% of S, 0.033% of acid-soluble aluminum, and 0.0090% of nitrogen, were heated to 141 00C and were then hot-rolled to form a 2.3 mm thick hot-rolled strips. Then, the following processes were successively carried out: continuous annealing at 8500C for 2 minutes; cold-rolling to form 0.30 mm cold-rolled strips; decarburization-annealing in a wet hydrogen atmosphere; application of MgO as annealing separator; and final high temperature annealing at 1200 Cfor20 hours.

The magnetic flux density B10 of the product is shown in Fig. 4, wherein x corresponds to ~B,0 < 1.89 Tesla, o corresponds to 1.89 < B io < 1 .92 Tesla, corresponds to 1.92 Tesla ~B o~1.93 Tesla, and (i) corresponds to 1 .93 < B0. As is apparent from Fig. 4, when the Mn content is too low, the secondary recrystallization becomes unstable, and when the Mn content is high, the magnetic flux density B10 is high. When Mn is added in more than a certain content, however, it is ineffective for enhancing the magnetic flux density B10 and is uneconomical since the amounts of additive alloy becomes disadvantageously great.

When the P content is low, the magnetic flux density B10 is too low and the generation of incomplete secondary recrystallization is increased. When the P content is too high, the frequency of cracking during cold rolling is increased.

Thus, the Mn content is limited to the range of from 0.08% to 0.45%, and the P content is limited to the range of from 0.015% to 0.045% according to the present invention. In these ranges, the magnetixflux density B10 is 1.89 Tesla or more, the secondary recrystallization is stable, and the problem of cracking is not significant.

Regarding the other components, steel which is subjected to the processes according to the present invention may be melted in a converter, electric furnace, or open-hearth furnace, provided that the composition of steel falls within the ranges described hereinafter.

The C content is thus at least 0.025%. At a C content of less than 0.025%, secondary recrystallization is instable. Even if secondary recrystallization occurs, the magnetic flux density is low (B,, is 1.80 Tesla at the highest). On the other hand, the C content is 0.075% at the highest, since the decarburization annealing time is long and thus unecomonical when the C content exceeds 0.075%.

The Si content is 4.5% at the highest. At an Si content exceeding 4.5%, numerous cracks occur during the cold-rolling. The Si content is at least 3.0%, preferably at least 3.2%. At an Si content less than 3.0%, the highest grade watt loss, i.e., W17150 of 1.05 w/kg at the sheet thickness of 0.30 mm, cannot be obtained.

Since in the present invention AIN is employed for the precipitates indispensable for the secondary recrystallization, the minimum amount of AIN must be ensured by providing an acid-soluble Al content and N content of at least 0.010% and 0.0030%, respectively. The acid-soluble Al content is 0.060% at the highest. At an acidsoluble Al content exceeding 0.060%, the AIN does not disperse uniformly in the hot-rolled strip, thereby resulting in poor secondary recrystallization. The N content is 0.0130% at the highest. At an N content exceeding 0.0130%, the blisters forth on the surface of the steel sheet.

When the steel has the composition as described above, a high slab heating temperature exceeding 1 3000C, accepted as conventional practice, is not necessary. More surprisingly, when the present inventors heated two slabs to a high temperature and a low temperature, respectively, and then subjected them to the processes for producing grain oriented electrical steel sheets, they found that two obtained products having an identical magnetic flux density will have a considerably lower watt loss when obtained by low-temperature slab heating than that when obtained by high-temperature slab heating. Thus, low-temperature slab heating not only enables production-cost reductions and easy used of a continuously cast strand as the starting material, but also a watt loss reduction.

Fig. 5 illustrates the magnetic properties of products produced under the same conditions as those of Fig. 3 but at slab heating temperatures of both 11 500C and 1350"C. From the comparison of the two products (11 500C and 1 3500C), it is apparent that a lower slab-heating temperature can drastically decrease the watt loss for the same magnetic flux density.

When the slab-heating temperature is 1 2800C or less, slag does not form at all during the slab heating. In addition, when the slab-heating temperature is 1 2800C or less and when the Si content is 3.0% or less, the highest grade product, i.e., a product which exhibits a watt loss W11150 of 1.50 w/kg or less at a sheet thickness of 0.30 mm, can be obtained.

The lowest slab heating temperature is not specifically limited, but is desirably 1 0500C, since at a temperature lower than 10500 C, a great driving force is required for hot-rolling and the shape quality of steel strip is impaired. The lowest slab temperature of 1 0500C is therefore preferred from the viewpoint of the industrial production of the steel.

The slab used may be any slab produced by rough rolling or continuous casting. A continuously cast slab is preferable due to the inherent labor saving and yield-enhancement features of continuous casting. Furthermore, continuous casting ensures a uniform chemical composition in a slab, resulting in uniform magnetic properties in the longitudinal direction of the product.

As is described in Japanese Unexamined Patent Publication No. 53-19913, if a continuously cast slab is heated to a high temperature, such as approximately 1 3200C, streaks generate and thus stable production becomes impossible. However, since the slabheating temperature can be 1 2800C or less according to the present invention, no incomplete secondary recrystallization occurs at all. The present invention therefore makes it possible to provide the highest-grade watt loss while employing low-temperature slab heating comparable to that of carbon steels.

Recent advances in continuous casting techniques have raised the productivity of continuous casting machines to equal the capacity of continuous hot-rolling mills.

Continuous casting machines can therefore now be directly combined with continuous hot-rolling mills. When steels are supplied from a continuous casting machine directly to a continuous hotrolling mill, the continuous hot-rolling mill can carry out rolling without a waiting period.

Therefore, according to one advantageous hot rolling method which can be used in the present invention, a slab is not cooled after continuous casting and is directly hot-rolled while utilizing the sensible heat of the slab. Alternatively, according another advantageous hot-rolling method, a slab is loaded in a recuperator furnace when the temperature of the slab, especially the surface temperature, declines slightly. The slab is subsequently heated in a very compact heating furnace for carbon steels for a short period of time and then hot-rolled.

These hot-rolling methods are in active use for producing carbon steels. By using these methods for producing grain-oriented electrical steel sheet, a high hot-rolling efficiency comparable to that of carbon steels can be obtained.

When a continuous casting machine is directly combined with a continuous hot rolling mill, formation of internal cracks can be advantageously prevented. A slab which contains a large amount of silicon has low heat conductivity and, therefore, a great temperature difference. Thus, thermal stress is created between the surface and inner portions of the slab. If it is cooled after continuous casting, internal cracks are formed in the slab and thus yield is lowered. However, since a slab is not cooled according to the advantageous hot-rolling method, formation of internal cracks can be advantageously prevented, which is an advantage specifically realized for hot-rolling silicon steels.

According to a conventional high-temperature slab heating method, a slab usually has a thickness of from 1 50 mm to 300 mm and is hotrolled by a rough-rolling mill to form a 30 to 70 mm thick intermediate product. The intermediate product is then hot-rolled by a plurality of continuous finishing mills, to form a hot rolled strip having a predetermined thickness.

According to such a conventional method, a slab having a small thickness cannot be used, because the slab is deformed in slab-heating furnace due to high temperature, with the result that the slab cannot be withdrawn from the furnace, or because a slab-heating furnace must be extremely long.

According to the low-temperature slab heating method, a thin cast slab can be used, because a cast slab can be directly hot-rolled. In addition, a thin cast slab can be directly finishing rolled while omitting the rough hot-rolling, thereby carrying out the hot rolling very effectively. If a slab is too thin, however, the production efficiency is low in continuous casting. On the other hand, if a slab is too thick, the load applied to a finishing hotrolling mill is extremely great. A slab thickness is thus preferably from 30 mm to 70 mm.

The magnetic flux density is strongly influenced by a slab-heating temperature.

Continuously cast slabs which contained 0.057% of C, 3.50% of Si, 0.25% of Mn, 0.039% of P, 0.033% of acid-soluble Al, and 0.0093% of N were heated and hot-rolled by the single hot rolling method to form 2.5 mm thick hot-rolled strips. Then, the following processes were successively carried out: continuous annealing at 11 200C for 2 minutes; cold-rolling to form 0.30 mm cold-rolled sheets; decarburization-annealing at 8500C for 2 minutes in a wet hydrogen atmosphere; application of MgO as annealing separator; and final high temperature annealing at 1 2000C for 20 hours.

The magnetic flux density B,o of the products are shown in Fig. 6. As is apparent from Fig. 6 a higher magnetic qux density B10 may be obtained with a slab heating temperature exceeding 12800 C. In many cases, such a higher magnetic flux density is specifically desired. For example, a high magnetic flux density is desirable especially when a laser-beam irradiation technique for reducing the watt loss is utilized for the grainoriented electrical steel sheet produced by the method according to the present invention, because the watt loss reduction is greater at a higher magnetic flux density. Such a technique is effective for providing especially low watt loss.

If a slab heating temperature is extremely high, a heating installation cannot withstand a high temperature from an industrial point of view. The highest slab-heating temperature should be 14300 C.

In the method according to the present invention, the hot-rolled strip is annealed at a temperature of from 8500C to 1 2000C for a short period of time and then rapidly cooled to control the precipitation state of AIN. If the annealing temperature is lower than 8500C, a high magnetic flux density cannot be obtained. On the other hand, if the annealing temperature is higher than 12000 C, the secondary recrystallization becomes incomplete. An annealing time of 30 seconds or longer is sufficient for attaining the object of annealing, and an annealing time longer than 30 minutes is economically disadvantageous.

The annealing time is usually from 1 to 30 minutes.

The annealing hot-rolled strip, which may be referred to as a hot-coil, is then cold-rolled. Heavy cold-rolling with a reduction a degree or draft of at least 80% is necessary in the cold-rolling for producing a grain-oriented electrical steel sheet having a high magnetic flux density.

The cold-rolled strip is then decarburizationannealed. The aims of the decarburization annealing are to decarburize and primaryrecrystallize a cold-rolled strip and simultaneously to form on it an oxide layer which is necessary as an insulating film.

An annealing separator, which is necessary for forming an insulating film on the product, is applied on the surface of decarburization-annealed cold-rolled strip. The annealing separator is mainly composed of MgO and may additionally comprise, if necessary, one or more of TiO2,Al2O3, CaO, B-compound, S-compound, and Ncompound.

Subsequently, final high-temperature annealing is carried out. The aims of the final high-temperature annealing are to secondaryrecrystallization and purify a decarburizationannealed strip and form an insulating film mainly composed of forsterite. Final high-temperature annealing is usually carried out at a temperature of 11 000C or more in a hydrogen atmosphere or a mixture atmosphere containing hydrogen. The temperature is then usually elevated to approximately 1 2000C and purification annealing is carried out so as to reduce N and S in steel to a level as small as possible.

After the final high temperature annealing, a coating liquid mainly composed of, for example, phosphoric acid, chromic acid anhydride, and aluminum phosphate is applied on the steel strip, and annealing for flattening is carried out. Due to the coating film, the insulating film is further strengthened and can generate a high tension. An insulating film which essentially consists of MgO.SiO2 is finally formed.

Regarding the conditions of the final hightemperature annealing, the heating rate at a temperature range where the secondary recrystallization occurs is preferably slow, because this is effective for attaining a stable high magnetic flux density. In metallurgical terms, in slow heating in a secondary recrystallization temperature range, the secondary recrystallized grains having a smaller inclination from the (1101 < 001 > orientation generate at a lower temperature. Therefore, the slow heating can enhance the volume proportion of the secondary recrystallization grains which are generated at a low temperature and thus are close to the 11 < 001 > orientation, with the result that the magnetic flux density is enhanced.In addition, since the growth of crystal grains is less liable to be suppressed due to fine MnS particles in the present invention, in which the S content is low and thus the inhibiting effect due to fine MnS is small, as compared with the conventional methods, in which the amount of MnS is great, the grain growth occurs relatively actively at a low temperature. Thus, slow heating is particularly effective in the case of low S steel for increasing the volume proportion of the secondary recrystallized grains which are generated at a low temperature and are thus close to the (1101 < 001 > orientation, and thus enhancing the magnetic flux density.

From Fig. 7, it will be understood how the magnetic flux density B10 is influenced by the heating rate in a temperature range of from 7000Cto 1 1000C.

Molten steel which contained 0.060% of C, 3.35% of Si, 0.25% of Mn, 0.033% of acidsoluble Al, 0.030% of P, 0.005% of S, and 0.0085% of N, was continuouly cast to form a strand. Slabs cut form a strand were heated to 1 4000C and then hot-rolled to form a 2.3 mm thick hot-rolled strips. Then, the following processes were successively carried out: continuous annealing at 1 2000C for 2 minutes; cold-rolling to form 0.30 mm cold-rolled sheets; decarburization-annealing at 8500C for 2 minutes in a wet hydrogen atmosphere; application of annealing separator; and final high temperature annealing at 1 2000C for 20 hours. The heating rate was varied in the final high temperature annealing.

As is apparent from Fig. 7, the magnetic flux density B10 is higher when the heating rate is lower. The magnetic flux density B10 is particularly high when the heating rate is 1 50C/hour or less.

During slow heating is at a temperature range of from 7000Cto 1 1000C, the secondary recrystallization is completed. At a heating rate lower than 1 50C/hour, the magnetic flux density does not greatly vary depending upon temperature, but the value-dispersion of the magnetic flux density decreases at a low heating rate. The minimum heating rate is desirably 7 OC/hour in the light of economic efficiency. The temperature is then usually elevated to approximately 1 2000C and purification annealing is carried out so as to reduce N and S in steel to a level as small as possible.

The grain-oriented electrical steel sheet may contain, in addition to the above described elements, a minor amount of one or more additive elements, for example, Cr.

Continuous casting slabs which contained 0.06% of C, 3.33% of Si, 0.30% of Mn, 0.035% of P, 0.030% of acid-soluble Al, 0.0085% of N, 0.004% S, and varying contents of Cr were heated to 1 3500C and hot-rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: continuous annealing at 11 200C for 2 minutes; cold-rolling to form 0.30 mm cold-rolled strips; decarburization-annealing in a wet hydrogen atmosphere; application of MgO as annealing separator; final high temperature annealing at 1 2000C for 20 hours. Cr can advantageously broadened the range of the acid-soluble Al at which a high magnetic flux density is obtained.

From Fig. 8, it will be understood that Cr can also decrease the watt loss for indentical magnetic flux densities. A Cr content exceeding 0.25%, however, is inappropriate because the effects of Cr are not enhanced and the decarburization rate is lowered in the decarburization annealing.

Continuously cast slabs which contained 0.06% of C, 3.33% of Si, 0.004% of S, 0.033% of P, 0.032% of acid-soluble Al, 0.0090% of N, and 0.15% of Cr, were heated to 11 500C and 1 3500C and then hot-rolled to form 2.3 mm thick hot-rolled strips. Then, the following processes were successively carried out: continuous annealing at 11 500C for 2 minutes; cold-rolling to form 0.30 mm cold-rolled strips; decarburization-annealing at 8500C for 2 minutes in a wet hydrogen atmosphere; application of MgO as annealing separator; and final high temperature annealing at 1 2000C for 20 hours.A tension coating which was mainly composed of colloidal silica was applied on the products. The magnetic properties, shown in Fig. 9, are those measured after forming the tension coating.

As is apparent from Fig. 9, the slab-heating temperature exert an influence upon the magnetic properties, i.e., lower siab-heating temperature allows a lower loss with the same magnetic flux density.

The present invention is now further described by way of examples.

Example 1 Molten steel which contained 0.053% of C, 3.30% of Si, 0.25% of Mn,0.030% of P,0.006% of 5, 0.027% of acid-soluble Al, and 0.0090% of N, was cast into an ingot. The ingot was rough hot rolled to form a 250 mm thick slab. The slab was heated to 11 500C and then hot rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: continuous annealing at 1 0800C for 2 minutes; cold-rolling to form a 0.30 mm cold-rolled sheet; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.

The magnetic properties of the product in the rolling direction were as follows: B1o=1.91 Tesla W17/so=1 01 w/kg.

No incomplete secondary recrystallization occurred.

Example 2 Molten steel which contained 0.058% of C, 3.45% of Si, 0.20% of Mn, 0.035% of P, 0.005% of S, 0.026% of acid-soluble Al, and 0.0090% of N, was cast into a 250 mm thick strand by continuous casting followed by cooling down to 2500C. The cut slab was heated to 1 2000C and then hot-rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: annealing at 1 0800C for 2 minutes; cold-rolling to form a 0.30 mm coldrolled sheet; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.

The magnetic properties of the product in the rolling direction were as follows: B1o=1.91 Telsa W,7/50=0.97 w/kg.

No incomplete secondary recrystallization occurred.

Example 3 Molten steel which contained 0.055% of C, 3.35% of Si, 0.20% of Mn, 0.035% of P, 0.006% of S, 0.027% of acid-soluble Al, 0.009% of N, was cast by continuous casting using a mold having a 250 mm thick mold cavity. After solidification of molten steel, cut slabs were loaded quickly without cooling in a car bottom type heatreserving furnace. When the temperature of the slab was homogenized so that the average temperature of the slab was approximately 1 300C, the hot-rolling was carried out, to form 2.3 mm thick hot-rolled sheets.Then the following processes were successively carried out: annealing at 1 0800C for 2 minutes; coldrolling to form a 0.30 mm cold-rolled sheet; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.

The magnetic properties of the product in the rolling direction were as follows: B1o=1 .90 Tesla W7,50=1 .04 w/kg.

No incomplete secondary recrystallization occurred.

Example 4 Molten steel which contained 0.060% of C, 3.35% of Si, 0.15% of Mn, 0.030% of P, 0.002% of S, 0.028% of acid-soluble Al, 0.0090% of N, was cast by continuous casting using a mold with a 250 mm thick mold cavity. During the continuous casting, heat-insulation was carried out in a continuous casting machine. And one end surface of the strand, which was liable to cool, was gas-heated for a short period of time, so as to decrease the cooling to a level as small as possible, such cooling occurring after solidification of the molten steel. The strands, i.e., slabs, were quickly transferred to the inlet side of a hot-rolling mill, and the hot-rolling was initiated when the cross-sectional central part and surface part of slabs had a temperature of approximately 12000C, and approximately 10500C.

The slabs were hot-rolled to form a 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: annealing at 1 0800C for 2 minutes; cold-rolling to form a 0.30 mm cold-rolled sheet; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.

The magnetic properties of the product in the rolling direction were as follows: B1o=1.89 Tesla W17/50=1.05 w/kg.

Example 5 Molten steel which contained 0.060% of C, 3.30% of Si, 0.20% of Mn, 0.035% of P, 0.006% of S, 0.030% of acid-soluble Al, and 0.0080% of N, was by continuous casting to form slabs. Slabs were heated to 1 3800C and then hot rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: annealing at 11 300C for 2 minutes; coldrolling to form a 0.30 mm cold-rolled sheet; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours.The heating rate at a temperature of from 7000C to 11 000C was 1 00C/hour in the final high-temperature annealing. Flattening annealing was carried out and then a tension film mainly composed of chromic oxide an hydride was applied on the sheet surface.

The magnetic properties of the product in the rolling direction were as follows: B1o=1.93 Tesla W 17/50=1.02 w/kg.

The product was then irradiated with a laser beam to form spot-like irradiation regions in the C direction (perpendicularto the rolling direction).

The magnetic properties of the laser-irradiated product were excellent as follows.

B10=1.93 Tesla W17,50=0.91 Tesla Example 6 Molten steel which contained 0.057% of C, 3.45% of Si, 0.29% of Mn, 0.039% of P, 0.003% of S, 0.032% of acid-soluble Al, and 0.0090% of N, was cast by continuous casting to form slabs.

Slabs were heated to 1 3800C and then hot rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: annealing at 11 300C for 2 minutes; coldrolling to form 0.30 mm cold-rolled strips; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and final high temperature annealing at 1 2000C for 20 hours. The heating rate at a temperature of from 700 to 1 1000C was 200C/hour in the final high-temperature annealing. Flattening annealing was carried out and then a tension film mainly composed of chromic oxide anhydride was applied on the sheet surface.

The magnetic properties of the product in the rolling direction were as follows.

B1o=1.92 Telsa W15/50=1.05 w/kg.

Example 7 Molten steel which contained 0.060% of C, 3.38% of Si, 0.20% of Mn, 0.040% of P, 0.005% of S, 0.033% of acid-soluble Al, 0.0085% of N, and 0.1 6% of Cr, was by continuous casting to form slabs. Slabs were heated to 1400 C and then hot rolled to form 2.3 mm thick hot-rolled strips. Then, the following processes were successively carried out: annealing at 112000 for 2 minutes; cold-rolling to form 0.30 mm coldrolled strips; decarburization-annealing at 850 C for 2 minutes in a wet hydrogen atmosphere; application of MgO; and, final high temperature annealing at 1200 C for 20 hours.

The heating rate at a temperature of from 700 to 1 1000C was 1 00C/hour in the final hightemperature annealing. Flattening annealing was carried out and then a tension film mainly composed of chromic oxide anhydride was applied on the sheet surface.

The magnetic properties of the product were as follows in the rolling direction.

B1o=1.93 Tesla W17/so=0.99 w/kg.

The product was then irradiated with a laser beam to form spot-like irradiation regions in the C direction (perpendicular to the rolling direction).

B10=1.93 Tesla W17,5o=0.88 Tesla Example 8 Molten steel which contained 0.053% of C, 3.35% of Si, 0.25% of Mn, 0.035% of P, 0.003% of S, 0.029% of acid-soluble Al, 0.0080% of N, and 0.15% of Cr, was cast by continuous casting to form slabs. Slabs were heated to 11 500C and then hot rolled to form 2.3 mm thick hot-rolled sheets. Then, the following processes were successively carried out: annealing at 1 0800C for 2 minutes; cold-rolling to form 0.30 mm coldrolled strips; decarburization-annealing at 8500C for 2 minutes in a wet hydrogen atmosphere; application of MgO; and, final high temperature annealing at 1 2000C for 20 hours.The heating rate at a temperature of from 700 to 11 000C was 20"C/hour in the final high-temperature annealing. A tension film mainly composed of chromic oxide anhydride was applied on the sheet surface.

B1o=1.91 Tesla W17150=0.97 w/kg.

Example 9 Molten steel which contained 0.053% of C, 3.45% of Si, 0.23% of Mn, 0.037% of P, 0.003% of S, 0.027% of acid-soluble Al, 0.0090% of N, and 0.20% of Cr, was cast by continuous casting to form slabs by using a mold having a 250 mm thick mold cavity. Slabs were heated to 11 300C and then hot rolled to form 2.3 mm thick hotrolled sheets. Then, the following processes were successively carried out: annealing at 1 0800C for 2 minutes; cold-rolling to form 0.30 mm coldrolled sheets; decarburization-annealing at 8500C for 2 minutes in a wet hydrogen atmosphere; application of MgO; and, final high temperature annealing at 1 2000C for 20 hours.

The heating rate at a temperature of from 700 to 11 000C was 1 00C/hour in the final hightemperature annealing. A tension film mainly composed of chromic oxide anhydride was applied on the sheet surface.

B1o=1.90 Tesla W17/so=1.01 w/kg.

Example 10 Molten steel which contained 0.053% of C, 3.45% of Si, 0.23% of Mn, 0.037% of P, 0.003% of 5, 0.027% of acid-soluble Al, 0.0090% of N, and 0.20% of Cr was cast by continuous casting to form slabs. During continuous casting, heatinsulation was carried out in a continuous casting machine, and one end surface of a strand, which was liable to cool down, was gas-heated, for a short period of time, so as to decrease cooling to a level as small as possible, such cooling occurring after solidification of molten steel. Cut strands, i.e., slabs, were quickly transferred to the inlet side of a hot-rolling mili, and the hot rolling was initiated when the cross-sectional central part and surface part of slabs had a temperature of approximately 1 2000C, and approximately 1 0500C, respectively. 2.5 mm thick hot-rolled strips were formed by hot-rolling. Then, the following processes were successively carried out: annealing at 1 0800C for 2 minutes; coldrolling to form 0.30 mm cold-rolled sheets; decarburization-annealing at 8500C in a wet hydrogen atmosphere; application of MgO; and, final high temperature annealing at 12000Cfor 20 hours.

B10=1.90 Tesla W1vso=1 .03 w/kg.

Claims

1. A method for producing a grain-oriented electrical steel sheet having a high magnetic flux density in terms of B10 of 1.89 Tesla or more, comprising the steps of: preparing a slab which has a temperature of 14300C or less, and which consists of from 0.025% to 0.075% of C, from 3.0% to 4.5% of .Si, from 0.010% to 0.060% of acid soluble aluminum, from 0.0030% to . 0.0130% of N, not more than 0.007% of S, from 0.08% to 0.45% of Mn, and from 0.015% to 0.045% of P, the balance being Fe and unavoidable impurities; subsequently, hot rolling said slab to form a hot-rolled strip;; annealing said hot-rolled strip in a temperature in the range of from 8500C to 1 2000C for a short period of time: subsequently, heavily cold-rolling the annealed strip at a reduction of not less than 80%, thereby obtaining the final sheet thickness; continuously decarburization-annealing the obtained cold-rolled strip in a wet hydrogen atmosphere and then applying an annealing separator on the strip; and, subsequently carrying out a final high temperature annealing.

2. A method according to claim 1, wherein said slab is heated, in a furnace, to a temperature of from more than 1 2800C to 1 4300C.

3. A method according to claim 1, wherein said slab is heated, in a furnace, to a temperature not exceeding 12800C.

4. A method according to Claim 1 or 2, wherein said slab is formed by continuous casting.

5. A method according to claim 4, further comprising a step of directly supplying said slab from a continuous casting machine to a hot rolling mill, while avoiding a cooling down to room temperature, and starting hot-rolling at a temperature not exceeding 1 2800C.

6. A method according to one of claims 1 through 5, wherein in the final high temperature annealing step a heating rate in a temperature range of from 700 to 11 000C is not more than 1 50C/hour.

7. A method according to one of claims 1 through 5, wherein said slab further contains from 0.07 to 0.25% of Cr.

8. A grain-oriented electrical steel sheet having high magnetic flux density in terms of B10 of 1.89 Tesla or more essentially consisting of from 3.0 to 4.5% of Si, from 0.08 to 0.45% of Mn, and from 0.015 to 0.045% P, the balance being essentially Fe, characterized by being produced by suppressing, prior to a final high temperature annealing, a secondary recrystallization by means of an inhibitor essentially consisting of AIN.