JP4608514B2 - Method for producing grain-oriented electrical steel sheet with extremely high magnetic flux density - Google Patents

Description

  The present invention relates to a method for stably producing a grain-oriented electrical steel sheet having a remarkably high magnetic flux density on an industrial scale.

  The grain-oriented electrical steel sheet is a steel sheet containing about 2 to 4% of Si and highly accumulating the crystal grain orientation of the product in the {110} <001> orientation. It is mainly used as a core material for static inductors such as transformers, and is required to have low energy loss, that is, iron loss when excited by alternating current. As demand for energy saving in recent years increases, Further reduction of iron loss is demanded.

  Many developments have been made to produce grain-oriented electrical steel sheets that satisfy these various characteristics. The magnetic flux density of the steel sheets (magnetic flux density when a magnetic field of 800 A / m is applied: represented by the B8 value) It is clear that the effect is particularly high when the height is increased (for example, see Patent Document 1).

  There are various methods for improving the magnetic flux density of grain-oriented electrical steel sheets. For example, Patent Document 2 discloses a technique for improving the magnetic flux density by adding Bi. Further, Patent Document 3 and Patent Document 4 disclose methods for improving the magnetic flux density by adding Te.

  However, although these methods improve the magnetic flux density and improve the properties of grain-oriented electrical steel sheets, in order to meet the demand for high-quality grain-oriented electrical steel sheets as the amount of power generation increases worldwide, production stability Sex is not enough. Therefore, it is necessary to establish a technology that achieves both high magnetic flux density and manufacturing stability.

Japanese Patent Publication No.40-15644 JP 06-89805 A JP 06-184640 A JP 06-207220 A

  An object of the present invention is to provide a method for stably producing a grain-oriented electrical steel sheet having a remarkably high magnetic flux density on an industrial scale.

  The gist of the present invention for solving the above problems is as follows.

(1) By mass%, C: 0.02 to 0.10%, Si: 2.5 to 4.5%, Mn: 0.01 to 0.15%, S and Se alone or in combination: 0 0.001 to 0.05%, acid-soluble Al: 0.01 to 0.05%, N: 0.002 to 0.015%, Te: 0.0005 to 0.10%, balance Fe and inevitable After heating the slab composed of impurities to 1280 ° C or higher, hot-rolling, hot-rolled sheet annealing, cold-rolling to make a cold-rolled steel sheet with the final product thickness, decarburization and primary In the method for producing a grain-oriented electrical steel sheet comprising a series of steps in which recrystallization annealing is performed and an annealing separator containing MgO as a main component is applied to the steel sheet surface and then finish annealing is performed, 950 ° C. or more and 1150 ° C. or less in finish annealing The coil heating average speed is 3 ° C./h or more and 10 ° C./h or less. Method of manufacturing oriented electrical steel sheet towards you.

(2) By mass%, C: 0.02 to 0.10%, Si: 2.5 to 4.5%, Mn: 0.01 to 0.15%, S and Se alone or in combination: 0 0.001 to 0.05%, acid-soluble Al: 0.01 to 0.05%, N: 0.002 to 0.015%, Te: 0.0005 to 0.10%, balance Fe and inevitable A slab made of a typical impurity is heated at 1280 ° C. or higher, hot-rolled, then subjected to hot-rolled sheet annealing, and subjected to cold rolling at least twice with intermediate annealing in between to produce a cold-rolled steel sheet having a final product thickness In the manufacturing method of grain-oriented electrical steel sheets, which consists of a series of steps in which decarburization and primary recrystallization annealing are performed, and an annealing separator containing MgO as a main component is applied to the steel sheet surface and then finish annealing is performed. The average coil heating rate of 950 ° C. or higher and 1150 ° C. or lower during annealing is 3 ° C./h or higher and 10 ° C. / method for producing oriented electrical steel sheets towards you, characterized in that the h or less.

(3) tropism towards according to (1) or (2) the coil temperature increase average speed of 850 to 950 ° C. in the final annealing, characterized by the following 10 ° C. / h or higher 50 ° C. / h A method for producing electrical steel sheets.

  According to the present invention, a grain-oriented electrical steel sheet having a remarkably high magnetic flux density can be stably produced on an industrial scale. Therefore, the present invention meets the recent demand for energy saving, and the effect is enormous.

  The production method of the present invention will be described in detail.

  The present inventors conducted the following experiments in order to invent a stable manufacturing technology for grain-oriented electrical steel sheets with extremely high magnetic flux density. In a laboratory vacuum melting furnace, by mass, C: 0.08%, Si: 3.25%, Mn: 0.08%, S: 0.027%, acid-soluble Al: 0.03%, N : Slab A containing 0.009%, balance Fe and unavoidable impurities, and slab B to which Te: 0.01% was added, and at 1300 ° C. and 1350 ° C. for 1 hour After annealing, hot rolling was performed.

  The hot-rolled sheet was annealed at 1100 ° C. for 120 seconds, pickled, and then cold-rolled to a sheet thickness of 0.23 mm. Furthermore, this cold-rolled sheet was decarburized and annealed at 850 ° C. in wet hydrogen for 150 seconds, and an annealing separator mainly composed of MgO was applied in a water slurry. Finish annealing was performed at an ultimate temperature of 1150 ° C. for 20 hours.

  The steel sheet was washed with water, sheared to a single-plate magnetic measurement size, coated with an insulating film mainly composed of aluminum phosphate and colloidal silica, and baked to evaluate the magnetic flux density B8 value.

  Finish annealing is in an atmosphere containing nitrogen: 25%, hydrogen: 75%, and the rate of temperature rise is less than 850 ° C., 50 ° C./h, from 850 ° C. to less than 950 ° C., 30 ° C./h, from 950 ° C. to 1150 ° C. It changed in the range of 53-50 degreeC / h shown in Table 1. After reaching 1150 ° C., the atmosphere was switched to a hydrogen atmosphere and a 20-hour holding annealing was performed.

  Here, B8 is a magnetic flux density value when a magnetic field of 800 A / m is applied at 50 Hz, and a higher value is preferable. Evaluation was determined to be good when B8> 1.940T at both slab heating temperatures of 1300 ° C and 1350 ° C.

  The reason why the two conditions of slab heating are essential is that the slab of the actual machine plate always has a temperature deviation in the longitudinal and width directions, so in order to obtain good characteristics over the entire length of the product coil, This is because it is considered essential that the heating temperature test materials at 1300 ° C. and 1350 ° C. are good in laboratory-scale experiments.

  Table 1 shows the results. When the slab heating temperature is 1350 ° C. and the rate of temperature increase from 950 to 1150 ° C. in the finish annealing is in the range of 3 to 25 ° C./h, B8> 1.940T is obtained by adding Te (0.01 mass%). The magnetic flux density is good, but the magnetic flux density does not reach 1.940 T at a heating rate of 50 ° C./h. When the slab heating temperature is 1300 ° C., the magnetic flux density B 8> 1.940 T is satisfied under the condition that the temperature increase rate of Te addition and finish annealing is 3 ° C./h or more and 20 ° C./h or less. When the speed was 25 ° C./h or more, the magnetic flux density B8 did not reach 1.940T.

  From these facts, B8> 1.940T at both the slab heating temperatures of 1300 ° C. and 1350 ° C. under the conditions of Te addition and the rate of temperature increase from 950 to 1150 ° C. in the finish annealing at 3 to 20 ° C./h. Thus, it can be seen that good characteristics can be obtained.

  From the above, the present inventors have newly found a method for realizing a remarkably high magnetic flux density over the entire length and width by controlling the temperature increase rate up to 950 to 1150 ° C. in Te addition and finish annealing. The present invention has been completed.

  Subsequently, embodiments of the present invention will be described below. The present invention is applied to a production method using AlN and MnS as main inhibitors by Taguchi, Sakakura et al. (See, for example, Japanese Patent Publication No. 40-15644) as a basic production method. The reason for this is that the present technology eliminates the deviation of the full width characteristics of the product plate due to the steel plate temperature deviation from the slab heating to the hot rolling process. This is because a solution method of AlN and MnS in solution is then finely precipitated and used as an inhibitor for improving characteristics.

  C has various roles, but if it is less than 0.02% by mass, the crystal grain size at the time of slab heating becomes too large and the iron loss of the product deteriorates. On the other hand, if it exceeds 0.10% by mass, decarburization annealing after cold rolling requires not only a long time, but is not economical, and decarburization tends to be incomplete. This is not preferable because it causes a magnetic defect called magnetic aging. In the following description, “%” may be simply referred to as “%”, but “%” means “% by mass”.

  For this reason, the lower limit of the C content is 0.02%, and the upper limit is 0.10%. A more appropriate range within this range is 0.05 to 0.09%.

  Si is an extremely effective element for increasing the electrical resistance of steel and reducing the eddy current loss that constitutes a part of the iron loss. The mass% ranges from 2.5% to 4.5%. Must be controlled. If it is less than 2.5%, eddy current loss of the product cannot be suppressed, and if it exceeds 4.5%, workability deteriorates, which is not preferable.

  Mn is an important element for forming MnS and / or MnSe, which is an inhibitor that influences secondary recrystallization, and it is necessary to control Mn in a range of 0.01% to 0.15%. If it is less than 0.01%, the absolute amount of MnS and MnSe necessary for causing secondary recrystallization is insufficient, which is not preferable. On the other hand, if it exceeds 0.15%, not only the solid solution during slab heating becomes difficult, but also the precipitation size tends to become coarse, and the optimum size distribution as an inhibitor is impaired.

  S and / or Se is an important element that forms an inhibitor with Mn described above, and must be controlled within a range of 0.001% or more and 0.05% or less alone or in total mass%. If it deviates from the above range, a sufficient inhibitor effect cannot be obtained.

  Acid-soluble Al is a main inhibitor constituting element for producing a high magnetic flux density grain-oriented electrical steel sheet and needs to be controlled in a range of 0.01% to 0.05% by mass. If it is less than 0.01%, the quantity is insufficient, and the inhibitor strength is insufficient. On the other hand, if it exceeds 0.05%, AlN precipitated as an inhibitor becomes coarse, and as a result, the inhibitor strength is lowered, which is not preferable.

  N is an important element that forms the above-described acid-soluble Al and AlN, and is required to be controlled in a range of 0.002% to 0.015% by mass. If it deviates from the above range, a sufficient inhibitor effect cannot be obtained.

  Te is an element effective for enhancing the inhibitor and improving the magnetic flux density of the steel sheet. For example, Te is disclosed in JP-A-6-184640 and JP-A-6-207220. It is necessary to control in the range of 0.0005 to 0.10% by mass%, and if it is less than 0.0005%, a sufficient effect cannot be obtained, and if it exceeds 0.10%, the rollability deteriorates, It is not preferable.

  In addition, Sn, Sb, Cu, Ag, As, Mo, Cr, P, Ni, B, Pb, V, Ge, Ti, and Bi are used as elements for stabilizing secondary recrystallization. In addition, it is useful to contain 0.0005 to 1.0% by mass in total. If the amount of these elements is less than 0.0005%, the effect of stabilizing the secondary recrystallization is not sufficient, and if it exceeds 1.0%, the effect is saturated. Is limited to 1.0%.

  The molten steel for producing grain-oriented electrical steel sheets with the components adjusted as described above is cast by a normal method. There is no particular limitation on the casting method. Next, slab heat treatment is performed, and the lower limit of the heating temperature is 1280 ° C. If it is less than 1280 degreeC, inhibitor components, such as MnS, MnSe, and AlN, cannot fully be made into solution. The upper limit is not particularly defined, but is preferably 1450 ° C. or less from the viewpoint of equipment protection.

  The above-mentioned slab piece becomes a hot-rolled sheet by subsequent hot rolling. The thickness of the hot-rolled sheet is not particularly specified, but is usually 1.8 to 3.5 mm because it is related to the cold rolling rate described later. The hot-rolled sheet is cold-rolled immediately or after being annealed for a short time. The annealing is performed in a temperature range of 750 to 1200 ° C. for 30 seconds to 10 minutes, and is effective for enhancing the magnetic properties of the product.

  Cold rolling is performed once or divided into two or more times with intermediate annealing. If the cold rolling is performed once, the full width characteristics of the product are likely to be unstable. If the cold rolling is divided into two or more times, the product characteristics are stabilized but the ultimate magnetic flux density tends to be low.

  In any case, the final cold rolling reduction ratio is preferably in the range of 80 to 95%. In the case where the cold rolling is divided into two or more times, the intermediate annealing is preferably performed in a temperature range of 750 to 1200 ° C. for 30 seconds to 10 minutes.

  Regarding decarburization annealing, it is essential in terms of product characteristics to be carried out in a wet atmosphere containing hydrogen nitrogen and to reduce C to 20 ppm or less. Thereafter, a powder mainly composed of MgO is applied and coiled. Then, batch-type finish annealing is performed on this coil, and then unwinding, removing powder, applying a slurry liquid mainly composed of aluminum phosphate and colloidal silica, and baking are performed to obtain a product of grain-oriented electrical steel sheet. Finalize.

  The finish annealing is a step of secondary recrystallization of {110} <001> oriented grains, and is extremely important for improving the magnetic flux density of the steel sheet. Usually, it implements in nitrogen-hydrogen mixed atmosphere. The temperature rising rate up to 850 ° C. is desirable from the viewpoint of productivity, and is preferably in the range of 15 to 100 ° C./h. However, the moisture content in the MgO powder is reduced to reduce the glass coating in the product. For the purpose of improving the adhesion to the steel plate, the holding annealing may be performed during the temperature increase.

  In the subsequent temperature range from 850 ° C. to 1150 ° C., secondary recrystallization is developed, and then annealing is performed at a temperature of 1150 to 1200 ° C. for about 20 hours, and N, S, Se, etc. are diffused outside the steel sheet. As a result, the magnetic properties of the product plate can be improved.

Subsequently, coil heating conditions in the temperature range from 850 ° C. to 1150 ° C. in the finish annealing temperature raising process will be described. First, the reason why the coil heating average speed in the temperature range of 950 to 1150 ° C., which is most important for secondary recrystallization control, is set to 3 ° C./h or more and 10 ° C./h or less will be described.

  The steel sheet to which Te is added exhibits secondary recrystallization in such a temperature range, but from the viewpoint of secondary recrystallization stability, the coil heating rate is preferably relatively slow.

The lower limit is 3 ° C./h, and if it is slower than this, the productivity is hindered. The upper limit is set to 20 ° C./h, and if it is faster than this, secondary recrystallization failure is likely to occur in the coil, particularly at a site where slab heating is not sufficient, which is not preferable from the viewpoint of manufacturing stability. In the present invention, the upper limit was set to 10 ° C./h based on the examples.

The magnetic characteristics are particularly good when the coil heating average speed is 15 ° C./h or less, but even if the temperature rising speed is less than 5 ° C./h, the effect of improving magnetic characteristics is saturated and does not change. proper coil temperature increase average speed range than in-band is the 5 ° C. / h or less.

  The reason why the average coil heating rate in the temperature range from 850 ° C. to 950 ° C. in the first half is set to 10 ° C./h or more and 50 ° C./less will be described.

  This temperature range is a region where the inhibitors AlN, MnS, MnSe, etc. start to dissociate, dissolve, and diffuse. In order to control the secondary recrystallization and improve the magnetic flux density of the product, it is an important temperature range, and it is preferable to heat the coil relatively quickly.

  The lower limit is set to 10 ° C./h from the viewpoint of further improving the magnetic flux density of the steel sheet. The upper limit is 50 ° C./h, and if it is faster than this, the load on the actual equipment increases, which is not preferable from the viewpoint of equipment protection. A more appropriate coil temperature increase average speed range in this temperature range is 15 to 35 ° C./h.

  In comparison with the prior art, Japanese Patent Application Laid-Open No. 6-184640 or Japanese Patent Application Laid-Open No. 6-207220 discloses a method for manufacturing a high magnetic flux density unidirectional electrical steel sheet using a slab containing Te. These manufacturing methods are limited to a method of subjecting a hot-rolled sheet or a hot-rolled sheet annealed sheet to preliminary cold rolling (one that performs cold rolling twice), and coil heating conditions for finish annealing are defined. Absent.

  However, the present invention combines the coil heating conditions in Te addition and finish annealing, and even when the cold rolling is performed once or twice, the grain-oriented electrical steel sheet having a remarkably high magnetic flux density over the entire length of the coil. It presents a method for stable production on an industrial scale, and the difference in technology is clear.

  Japanese Patent Application Laid-Open No. 5-78743 discloses a method for producing a grain-oriented electrical steel sheet that is excellent in both magnetic properties and film properties using a slab containing Te. Although a technique for improving the magnetic flux density of the steel sheet by adding a very small amount of Te or the like is disclosed, the slab heating temperature is limited to less than 1280 ° C., which is clearly different from the present invention.

Example 1
A steel slab containing the components shown in Table 2 and the balance consisting of inevitable impurities and Fe was produced in a laboratory vacuum melting furnace. The slab was subjected to hot rolling after annealing at 1300 ° C. and 1350 ° C. for 1 hour. The hot-rolled sheet was annealed at 1100 ° C. for 120 seconds, pickled, and then cold-rolled to a thickness of 0.23 mm.

  Further, this cold-rolled sheet was subjected to decarburization annealing at 850 ° C. in wet hydrogen for 150 seconds, and an annealing separator mainly composed of MgO was applied as a water slurry. Under various heating conditions, the maximum temperature reached 1150 Finish annealing was carried out at 20 ° C. for 20 hours. After washing this steel plate with water, it was sheared to a size for single-plate magnetic measurement, and an insulating coating mainly composed of aluminum phosphate and colloidal silica was applied and baked, and the magnetic flux density B8 was measured.

  Note that the finish annealing temperature is nitrogen: 25%, hydrogen: 75% atmosphere, and the rate of temperature increase is 50 ° C./h to less than 850 ° C., 30 ° C./h to 850 ° C. to less than 950 ° C., 950 ° C. to 1150 ° C. It changed in the range of 5-50 degrees C / h shown in Table 3 until below.

  After reaching 1150 ° C., the atmosphere was switched to a hydrogen atmosphere and a 20-hour holding annealing was performed. Here, B8 is the magnetic flux density when a magnetic field of 800 A / m is applied at 50 Hz, and the higher one is preferable. Evaluation was determined to be good when B8> 1.940T at both slab heating temperatures of 1300 ° C and 1350 ° C.

The results are shown in Table 3. It is slab B containing Te that satisfies B8> 1.940T at both slab heating temperatures of 1300 ° C. and 1350 ° C., and the coil heating rate from 950 ° C. to 1150 ° C. in the finish annealing is It was in the range of 3 ° C./h or more and 20 ° C./h or less. At 3 ° C./h to 10 ° C./h , B8 ≧ 1.948T, which was even better.

(Example 2)
A steel slab containing the components shown in Table 4 and the balance being inevitable impurities and Fe was produced in a laboratory vacuum melting furnace. The slab was hot-rolled after annealing at 1350 ° C. and 1400 ° C. for 1 hour. The hot-rolled sheet was annealed at 1000 ° C. for 100 seconds, pickled, and then cold-rolled to obtain a steel sheet having a thickness of 1.7 mm. The steel sheet was subjected to an intermediate annealing at 1050 ° C. for 100 seconds, and then cold-rolled to obtain a cold-rolled sheet having a thickness of 0.23 mm.

  Further, this cold-rolled sheet is subjected to decarburization annealing in wet hydrogen at 850 ° C. for 150 seconds, and an annealing separator mainly composed of MgO is applied as a water slurry, and the maximum temperature reached 1150 ° C. under various heating conditions. And finish annealing for 20 hours. After washing this steel plate with water, it was sheared to a size for single-plate magnetic measurement, and an insulating coating mainly composed of aluminum phosphate and colloidal silica was applied and baked, and the magnetic flux density B8 was measured.

  Note that the finish annealing temperature increase is an atmosphere containing nitrogen: 25% and hydrogen: 75%, and the temperature increase rate is 50 ° C./h to less than 850 ° C., 1030 ° C./h to 850 ° C. to less than 950 ° C., and 950 to 1150 The temperature was changed within the range of 5 to 50 ° C./h shown in Table 5 until the temperature was equal to or below. After reaching 1150 ° C., the atmosphere was switched to a hydrogen atmosphere and a 20-hour holding annealing was performed. Here, B8 is the magnetic flux density when a magnetic field of 800 A / m is applied at 50 Hz, and the higher one is preferable. Evaluation was determined to be good when B8> 1.940T at both slab heating temperatures of 1350 ° C. and 1400 ° C.

The results are shown in Table 5. It is slab D containing Te that satisfies B8> 1.940T when the slab heating temperature is 1350 ° C. and 1400 ° C., and the coil heating rate from 950 ° C. to 1150 ° C. in the finish annealing is 3 It was in the range of not less than 20 ° C / h. At 3 ° C./h to 10 ° C./h , B8 ≧ 1.948T, which was even better.

(Example 3)
In mass%, C: 0.08%, Si: 3.26%, Mn: 0.08%, S: 0.025%, acid-soluble Al: 0.03%, N: 0.008%, Te: A steel slab containing 0.005% and the balance of inevitable impurities and Fe was produced in a laboratory vacuum melting furnace. The slab was annealed at 1300 ° C. and 1350 ° C. for 1 hour, and then hot-rolled. The hot-rolled sheet was annealed at 1100 ° C. for 120 seconds, pickled, and then cold-rolled to a thickness of 0.23 mm.

  Further, this cold-rolled sheet was subjected to decarburization annealing at 850 ° C. in wet hydrogen for 150 seconds, and an annealing separator mainly composed of MgO was applied as a water slurry. Under various heating conditions, the maximum temperature reached 1150 Finish annealing was carried out at 20 ° C. for 20 hours. The steel sheet was washed with water, sheared to a single-plate magnetic measurement size, coated with an insulating film mainly composed of aluminum phosphate and colloidal silica, and baked, and the magnetic flux density B8 was measured.

  Note that the finish annealing temperature increase is nitrogen: 25%, hydrogen: 75% atmosphere, the temperature increase rate is 50 ° C./h to less than 850 ° C., 850 ° C. to less than 950 ° C. It changed in the range of h, and set it as 10 degreeC / h to 950 degreeC or more and 1150 degrees C or less. After reaching 1150 ° C., the atmosphere was switched to a hydrogen atmosphere and a 20-hour holding annealing was performed.

  Here, B8 is the magnetic flux density when a magnetic field of 800 A / m is applied at 50 Hz, and the higher one is preferable. Evaluation was determined to be good when B8> 1.945T at both slab heating temperatures of 1300 ° C. and 1350 ° C.

  The results are shown in Table 6. Whether the slab heating temperature is 1300 ° C. or 1350 ° C., B8> 1.945T is satisfied because the coil heating rate of 850 ° C. or more and less than 950 ° C. in finish annealing is 10 ° C./h or more and 50 ° C./h or less. It was in range.

Claims (3)

By mass%, C: 0.02 to 0.10%, Si: 2.5 to 4.5%, Mn: 0.01 to 0.15%, S and Se alone or in combination: 0.001 0.05%, acid-soluble Al: 0.01 to 0.05%, N: 0.002 to 0.015%, Te: 0.0005 to 0.10%, from the remainder Fe and inevitable impurities The resulting slab is heated to 1280 ° C or higher, hot-rolled, then subjected to hot-rolled sheet annealing, cold-rolled to obtain a cold-rolled steel sheet with the final product thickness, and then decarburized and primary recrystallization annealed. In the method for producing a grain-oriented electrical steel sheet comprising a series of steps in which an annealing separator containing MgO as a main component is applied to the steel sheet surface and then finish annealing is performed, a coil rise of 950 ° C. to 1150 ° C. in the finish annealing is performed. the temperature average speed, towards you, characterized in that the following 3 ° C. / h or higher 10 ° C. / h A method for producing a grain-oriented electrical steel sheet. By mass%, C: 0.02 to 0.10%, Si: 2.5 to 4.5%, Mn: 0.01 to 0.15%, S and Se alone or in combination: 0.001 0.05%, acid-soluble Al: 0.01 to 0.05%, N: 0.002 to 0.015%, Te: 0.0005 to 0.10%, from the remainder Fe and inevitable impurities After the slab to be heated to 1280 ° C. or higher, hot rolled, then subjected to hot rolled sheet annealing, and subjected to cold rolling at least twice sandwiching the intermediate annealing to obtain a cold rolled steel sheet having a final product thickness In the method for producing a grain-oriented electrical steel sheet comprising a series of steps in which decarburization and primary recrystallization annealing are performed, and an annealing separator containing MgO as a main component is applied to the steel sheet surface and then finish annealing is performed, The average temperature rising rate of the coil from 3 ° C to 1150 ° C is 3 ° C / h to 10 ° C / h Method for producing oriented electrical steel sheets towards you, characterized in that a. Method for producing oriented electrical steel sheet towards the claim 1 or 2, characterized in that the coil temperature increase average speed of 850 to 950 ° C. in the final annealing, or less 10 ° C. / h or higher 50 ° C. / h.