JP2533987B2 - Hot rolling method for continuous cast slab for unidirectional electrical steel sheet. - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling method for a slab manufactured by continuous casting in a process for manufacturing a grain-oriented electrical steel sheet, which is particularly excellent in productivity and has few surface defects in the product and magnetic properties. It relates to a hot rolling method for improvement.

[0002]

2. Description of the Related Art Grain-oriented electrical steel sheets have been widely used as iron core materials for transformers and the like due to their excellent magnetic properties such as high magnetic flux density and low iron loss. As a characteristic of the manufacturing method, an inhibitor element such as MnS or AlN that controls the grain orientation is used in order to obtain secondary recrystallized grains in which the so-called Goss orientation of [110] <001> is highly integrated. In order for this inhibitor element to appropriately bring about the intended action, after sufficiently dissociating and forming a solid solution (hereinafter referred to as solution heat treatment) during slab heating prior to hot rolling, hot rolling is performed under appropriate conditions. It is very important to finely and uniformly disperse and precipitate by cooling, and high temperature heating of the slab at, for example, 1300 ° C. or higher is performed for solution treatment of the inhibitor.

[0003] When a gas-fired heating furnace is used as a method of heating the slab at a high temperature, a large amount of molten scale is generated in the heating furnace, which causes not only a decrease in yield but also an increase in yield.
Not only does this impair the stable operability of the heating furnace, but it also causes defects in the surface of the product and so-called edge cracking defects at the ends in the width direction.
In addition, since it takes an extremely long time to reach the target temperature in this type of heating furnace, coarsening of the slab structure occurs during that time, so that giant elongated grains are developed in the subsequent rolling, As a result, the magnetic properties of the product become unstable.

With respect to the above problems, Japanese Patent Laid-Open No. 63-10
In Japanese Patent Laid-Open No. 9115 and Japanese Patent Laid-Open No. Hei 1-162725, a slab is preheated to about 1250 ° C. in a gas combustion type heating furnace, and then high temperature heating is performed for a short time in an induction heating furnace controlled to an inert atmosphere. Is proposed. However, according to these methods, the yield in the hot rolling stage,
Although it is possible to improve operability and magnetic characteristics, a new problem with high temperature heat generation inside the slab is the characteristic of the continuous cast slab, and in the case of unidirectional electrical steel sheet, it is more remarkable than other materials. The center segregation of a certain slab and the slab internal opening due to the grain boundary melting of the internal crack portion occur. These internal openings remain until the product stage, and the internal gas may expand during high temperature annealing to cause secondary recrystallization, and may appear as blistering defects on the product surface.

Further, in recent years, as proposed in Japanese Examined Patent Publication No. 59-42561, in order to improve the productivity of continuous casting, wide-width reduction rolling is performed at the slab stage using a vertical large-diameter roll or the like. We are implementing the desired product width. However, in this wide slab reduction rolling method, tension acts in the thickness direction of the slab to leave a stress that tends to cause internal holes, which further increases the probability of occurrence of blistering defects and becomes a factor inhibiting productivity.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art in the process of manufacturing a grain-oriented electrical steel sheet. That is, in order to improve the productivity of the continuous casting process, wide reduction rolling is performed as necessary, and in order to obtain a grain-oriented electrical steel sheet having good magnetic properties and surface properties, an atmosphere-controlled type and internal heat generation are used. Type electric heating furnace, hot rolling method that does not generate blistering defects on the surface of the product due to a large amount of melt scale and slab structure coarsening caused by newly created internal slab holes The purpose is to provide.

[0007]

To achieve the above object, the present invention has the following arrangement. That is, C:
60 mm after preheating a slab for unidirectional electrical steel sheets produced by continuous casting containing 0.02 to 0.085% and Si: 2 to 4.5% to 900 to 1250 ° C. in a gas combustion type heating furnace The above wide reduction rolling is performed, and then 1300 to 1450 ° C. in an electric heating furnace in a non-oxidizing gas atmosphere.
In a series of steps of hot rolling after heating to the range of 10 ° C. and holding for a predetermined time, (1) the temperature difference in the longitudinal direction in the slab after preheating is 50 ° C.
The following shall be performed (2) The wide reduction rolling performed after the preheating of the slab is performed in a plurality of passes in which the width direction reduction amount in one pass is 50 mm or less, (3) After the wide reduction rolling, When the slab is electrically heated, the heating rate in the range of 1000 to 1300 ° C. is 2.
The gist is a hot rolling method for a continuously cast slab for unidirectional electrical steel sheets, which is characterized in that the temperature is 5 to 20.0 ° C./min.
The contents of the present invention will be described in detail below. A concentrated segregation zone of impurities such as S and P occurs at the center of the continuously cast slab for unidirectional electrical steel sheets. This is an inevitable phenomenon that occurs because the heat transfer coefficient of the slab for a grain-oriented electrical steel sheet is small.

1 inside the slab having such a dense segregation zone
When heated to a high temperature of 300 to 1450 ° C, S
Grain boundary melting occurs due to the low melting point of the thickened portion, and internal holes are generated in the subsequent rough rolling stage. Fig. 1 shows S in a continuously cast slab for unidirectional electrical steel sheets manufactured by a normal method.
The S segregation situation is schematically shown, and S is concentrated in the central portion of the slab as shown in FIGS. According to the investigation by the present inventors, the molten steel analysis showed that the S content of 0.24% was concentrated in the central portion at the slab stage by 1.60%.
Further, the positions of occurrence of blistering defects in the plate thickness direction, which is a problem to be solved by the present invention, have a distribution as shown in FIG. 7C, which corresponds well to the S segregation position at the slab stage.

Generally, in order to develop good magnetic properties of the grain-oriented electrical steel sheet, high temperature heating of 1300 ° C. or higher in the slab stage is essential. At this time, when an electric heating furnace of a slab internal heat generation type is used, the internal opening as described above occurs in the subsequent processing, which causes blistering of the steel plate. In particular,
When the slab width is integrated to improve productivity and large width reduction rolling is performed, this internal opening is promoted.

As a result of diligent studies on the occurrence of such internal openings, the present inventors have found that blister-shaped surface defects due to the internal openings are different from the portion that was in contact with the supporting member such as the skid of the slab after preheating. There is a deviation in the baking temperature from the part (non-contact part), and such a deviation is maintained during the subsequent high-temperature heating in the induction heating furnace, and becomes lower at the skid contact position, so this part is kept at a predetermined high solution temperature. If the liquefaction temperature is set, the non-contact part becomes higher in temperature at the same time, and this occurs because the melting point part, which is the central segregation zone of this part, is melted, and the reduction amount of one pass in horizontal width reduction rolling is It was found that when the value is large, a pushing force is exerted on the central segregation zone, so that blistering defects tend to occur frequently. Then, it has been found that this can be prevented by specifying the low temperature preheating condition of the continuous casting slab and the subsequent high temperature solution heat treatment condition, or by selectively implementing the wide reduction rolling condition.

FIG. 2 shows an investigation of the occurrence of blisters based on the relationship between the temperature deviation in the longitudinal direction of the slab after the preheating and the heating rate in the solution treatment by the subsequent induction heating. That is, the sample to be investigated has a composition of C: 0.
07%, Si: 3.25%, Mn: 0.07%, P:
0.01%, S: 0.024%, Al: 0.024%,
N: 0.0090%, Cu: 0.05%, Sn: 0.1
A slab for unidirectional electrical steel sheets containing 0% and consisting essentially of Fe, and having a size of 250 mm thickness x 1200 mm width (2
(50 mm thickness x 1200 mm width), the slab is preheated at 1150 ° C in a gas heating furnace, and baked in such a manner that the temperature difference in the longitudinal direction is in the range of 10 to 100 ° C, and then N ₂ atmosphere Introduced into the electric induction heating furnace
Upon heating to 0 ° C., high temperature heating was performed from 1000 ° C. to 1300 ° C. at various heating rates. FIG. 2 shows the result when a unidirectional electrical steel sheet was manufactured by adopting the process of hot rolling and hot coiling with a thickness of 2.3 mm.
As is clear from the figure, the occurrence of blistering defects disappears as the temperature deviation in the slab longitudinal direction of preheating becomes smaller, that is, at 50 ° C. or less, and as the temperature rising rate of induction heating becomes slower, that is, at 20 ° C. / If it is not more than a minute, it can be almost suppressed even in the case of the temperature deviation of 50 ° C. If the heating rate is too slow, the productivity will drop, so 2.5
C / min or more. In addition, the temperature rising range is 1000 to 13
The temperature of 00 ° C. is 1000 ° C. is the average charging temperature at which the preheated slab is charged into the electric heating furnace after extraction, and 1300 ° C. is sufficient for the inhibitor forming elements such as Mn and S. This is because it is the minimum temperature required for solution treatment.

FIG. 3 shows the relationship between the width reduction amount per pass and the blistering occurrence rate during wide reduction rolling. That is, a slab having the same composition as in the case of FIG. 2 is preheated to have a temperature deviation of 40 ° C. in the longitudinal direction of the slab, and width reduction rolling is performed with a reduction amount of each pass so that the total width reduction amount is 120 mm. After slab shaping by rolling down the dog bone with a horizontal roll, it is heated in an induction heating furnace in a non-oxidizing gas atmosphere at a heating rate of 15 ° C / min for 1000
The results are obtained when a unidirectional electrical steel sheet was manufactured by adopting a process of heating from ℃ to 1300 ℃, soaking for 20 minutes and hot rolling into a hot coil of 2.3 mm thickness. In FIG. 3, when the amount of reduction per pass is 70 mm or more, the maximum amount of reduction in each pass that reduces 120 mm is shown. As is clear from the figure, in horizontal width reduction rolling, as the amount of reduction in one pass exceeds 50 mm, blister defects occur more frequently. This is because, as described above, due to the large horizontal reduction, the spreading force acts more on the center segregation. Therefore, if one pass is subjected to light pressure reduction and multiple passes are applied, the stress is dispersed and blistering defects can be suppressed. For this reason, the present invention limits the amount of reduction in one pass in the wide reduction rolling following preheating to 50 mm or less. In the present invention, the continuous casting slab for unidirectional electrical steel sheets is preheated in a gas combustion type heating furnace. Since this heating is performed at a relatively low temperature of 900 to 1250 ° C., the generation of molten slag is small, and the existing heating furnace can be used for efficient operation. The slab with little temperature deviation in the longitudinal direction after preheating is transferred to an atmosphere-controlled electric heating (induction heating) furnace. If necessary, before the induction heating, the wide reduction rolling is performed under the condition that the blistering defects as described above do not occur. That is, the casting size of this type of slab is set to a constant maximum width that is constrained from casting stability, and by performing width reduction so as to obtain a desired product width, the productivity of the continuous casting process is increased. To improve. The width reduction means is not particularly limited, but for example, a vertical large-diameter roll proposed in Japanese Patent Publication No. 59-42561 can be used.

In the induction heating furnace, the slab is subjected to high temperature solution treatment in a furnace in a non-oxidizing atmosphere such as N ₂ or Ar gas. This is a means necessary for achieving a solid solution of MnS, AIN, etc. contained in the slab, and imparting excellent magnetic properties to the final product that has been cold rolled, finish annealed, and the like. Therefore, the heating rate up to a predetermined temperature (1300 ° C.) is limited to 20 ° C./minute or less and heating is performed within a temperature range of 1300 to 1450 ° C. As a heating means, for example, it is preferable to install a furnace in which an slab is vertically inserted in an induction coil and to make the atmosphere non-oxidizing, online or near the online, and in proximity to the rough rolling mill.

The slab that has been heated to a high temperature is quickly bitten into a rough rolling machine and thereafter rough finish-rolled according to a conventional method and wound into a coil to form a hot-rolled steel sheet for unidirectional electrical steel sheets. Then, this hot rolled steel sheet is treated by a usual method for producing a unidirectional electrical steel sheet such as cold rolling or annealing, and a final product having excellent surface properties and magnetic properties can be stably obtained.

[0015]

Example 1 C: 0.08% by weight, Si: 3.22
%, Al: 0.026%, N: 0.0080%, Sn:
0.12%, Cu: 0.05%, Mn: 0.08%,
Each sample of the continuous casting slab containing S: 0.025% and the balance substantially Fe was heated to 1150 ° C. in a gas heating furnace. The temperature difference in the longitudinal direction of the slab at this time is 30
It was a standard. After that, width reduction rolling of 150 mm is performed by a vertical rolling mill in two passes and five passes, and subsequently, it is placed in an induction heating furnace controlled to a non-oxidizing atmosphere and heated to 1000 ° C.
To 1300 ° C from 15 ° C / min and 25 ° C / min 2
After heating at a standard heating rate, soaking is performed at 1350 ° C. for 20 minutes, extraction from the induction heating furnace, and immediate rough rolling,
Finish rolling was performed to obtain a 2.3 mm Tot coil.

[0016] These hot coils under different preheating conditions are pickled, pre-cold rolled to a plate thickness of 1.60 mm, hot coil annealed, and finally cold rolled to a 0.23 mm thick steel strip. did. This cold rolled steel strip is further
After decarburization annealing at 5 ° C and applying the annealing separator, 120
Finish annealing was performed at 0 ° C., and tension coating was applied to obtain a high magnetic flux density grain-oriented electrical steel sheet. The blistering defects of these final products are shown in Table 1. As is clear from the table, the products manufactured in the region of the present invention are good with no occurrence of blistering defects.

[0017]

[Table 1]

[0018]

As described above, according to the method of the present invention, a slab for grain-oriented electrical steel sheets is produced in an induction heating furnace of an internal high temperature exothermic type, which is subjected to wide reduction rolling and controlled atmosphere without generating blistering defects. Can be heated, which makes it possible to obtain a product having excellent surface properties and magnetic properties.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a center segregation of a slab and a crack starting point distribution of blistering defects.

FIG. 2 is a diagram showing a relationship between a temperature deviation in a longitudinal direction of a slab after preheating for blistering defects and a heating rate door in a solution treatment by subsequent induction heating.

FIG. 3 is a diagram showing a relationship between a width reduction amount per pass and a blistering occurrence rate at the time of wide reduction rolling for blistering defects.

Claims (1)

(57) [Claims] 1. C: 0.02 to 0.085%, Si: 2
A slab for a grain-oriented electrical steel sheet manufactured by continuous casting containing 4.5% was prepared in a gas-fired heating furnace at 900-1250.
After preheating to ℃, rolling with a large width of 60 mm or more, and then using an electric heating furnace in a non-oxidizing gas atmosphere
In a series of steps of performing hot rolling after heating in the range of 300 to 1450 ° C and soaking and holding for a predetermined time, (1) the temperature difference in the longitudinal direction within the slab after preheating is 50 ° C.
The following shall be performed (2) The wide reduction rolling performed after the preheating of the slab is performed in a plurality of passes in which the width direction reduction amount in one pass is 50 mm or less, (3) After the wide reduction rolling, When the slab is electrically heated, the heating rate in the range of 1000 to 1300 ° C. is 2.
The hot rolling method of the continuous casting slab for unidirectional electrical steel sheets, which is 5 to 20.0 ° C./min.