JP3133868B2 - Heating method of directional silicon steel slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heating directional silicon steel slabs, and more particularly to a method for effectively suppressing the occurrence of blistering defects during slab heating, and consequently improving product quality including magnetic properties. The goal is to improve it.

[0002]

2. Description of the Related Art Grain-oriented silicon steel sheets are mainly used as iron core materials for transformers and other electric equipment, and are basically required to have excellent magnetic properties such as magnetic flux density and iron loss. In producing such oriented silicon steel sheet,
It is particularly important that the primary recrystallized grains are preferentially recrystallized into crystal grains of the {110} <001> orientation in a so-called finish annealing step.

In order to effectively promote such secondary recrystallization, it is important to first disperse a dispersed phase called an inhibitor, which suppresses the normal growth of primary recrystallized grains, into a uniform and appropriate size. is there. Representative examples of such inhibitors include sulfides and nitrides such as MnS, MnSe, AlN, and VN, and substances having extremely low solubility in steel are used. For this reason, conventionally, a method of heating the slab to a high temperature before hot rolling and completely dissolving the inhibitor component has been adopted, and after the hot rolling step, controlling the precipitation state during the secondary recrystallization step. I have. In addition, as the inhibitors, in addition to those described above, Sb, Sn, As, Pb, Ce,
Grain boundary segregation elements such as Cu and Mo have also been utilized.

Conventionally, to produce a grain-oriented silicon steel sheet,
A slab having a thickness of about 100 to 300 mm is heated to a temperature of 1250 ° C or higher for a long time to completely dissolve the inhibitor component, and then hot-rolled. Then, the hot-rolled sheet is subjected to once or intermediate annealing. After the decarburization annealing, apply an annealing separating agent, and then perform the final finish annealing for the purpose of secondary recrystallization and purification after making the final sheet thickness by cold rolling two or more times. It is.

[0005] However, when such slab heating is performed for a long time, the crystal grains after completion of the heating are remarkably coarsened.
Coarse crystal grains in the slab are difficult to recrystallize in the subsequent hot rolling, and subgrain boundaries and dislocations in unrecrystallized grains serve as precipitation sites, so that the inhibitor component once dissolved is coarsely precipitated, This was a cause of deterioration of the magnetic properties of the product.

In recent years, with the progress of technology, electric heating furnaces such as an electromagnetic induction heating furnace and a resistance heating furnace have been used for slab heating. As a result, heating at a higher temperature became possible, and the solution of the inhibitor component was completed in a short time. Also, the shortening of the heating time also suppresses the coarse growth of the slab grains, so that the deterioration of the magnetic properties due to the secondary recrystallization failure caused by the coarse growth has been greatly improved.

However, when the slab is subjected to the high-temperature heating as described above, a new problem arises in that the slab has blistering defects. When the blistering defect is severe, it is of course impossible to perform hot rolling. However, even if the blistering defect is slight, it causes a serious defect such as a double plate, a broken plate or a hole. Particularly, in an electric heating furnace in which the internal temperature is higher than the surface temperature, uniform heating in the thickness direction is difficult, and it is extremely difficult to appropriately cope with this trouble.

[0008] It is known that, when continuously casting a directional silicon steel slab, magnetic properties are improved by performing electromagnetic stirring of molten steel (for example, Japanese Patent Publication No. 57-41526). In the case where a suitable magnetic stirring is employed, the magnetic properties are improved, but on the other hand, the occurrence of blistering defects tends to be remarkable, which has been a serious problem.

Conventionally, blisters have been known as blister-like defects in silicon steel sheets. Here, the blister refers to a blister-like defect on the surface of the thin plate caused by expansion of the gas contained in the steel when the thin plate is heat-treated, and as a measure for preventing such blister,
Various methods have been proposed as described below. For example, Japanese Patent Publication No. 49-42208 discloses that Al in silicon steel,
A condition that blisters do not occur in the final product by controlling the amounts of H and N is disclosed. In addition, Tokiko Sho 49-4
Japanese Patent No. 2211 discloses a condition in which blisters are not generated by controlling the O concentration in addition to the above three components.

Further, Japanese Patent Application Laid-Open No. 2-259016 discloses a method for producing a grain-oriented silicon steel sheet in which the roll diameter during cold rolling is set to 150 mm or more to reduce surface blister defects. Further, Japanese Patent Application Laid-Open No. 5-1324 discloses a technique for suppressing a blister-like defect on a product surface caused by an opening inside a slab by controlling a temperature difference after preheating and a heating rate of an electric heating furnace. Is disclosed.

[0011] All of the above-mentioned improvement techniques are techniques for preventing defects on the product surface which occur when high-temperature annealing is performed on a thin plate. The blisters have a completely different mechanism of occurrence, and therefore, the above-described technique cannot prevent the occurrence of slab blisters.

[0012]

As described above, up to now, there is still no known technology capable of completely preventing blistering when a slab is heated to a high temperature in order to completely form a solution of an inhibitor. The development was desired. The present invention satisfies the above-mentioned demands advantageously, and effectively prevents the occurrence of slab swelling during high-temperature heating, and thus provides a slab which is a product having good magnetic properties and surface appearance, and is a directional silicon steel. The purpose is to propose a method of heating a slab.

[0013]

First, the details of the invention will be described. By the way, the present inventors have conducted a thorough study on the effects of component conditions, casting segregation, slab heating conditions, and the like on the occurrence of blisters in order to achieve the above object. When the (grain boundary) is maintained at or above its solidus temperature, the crystal grain boundary is partially melted, and continuation of heating causes concentration of solute components including nitrogen in the liquid phase portion, After that, during the re-solidification in the cooling process, the solid solution component becomes supersaturated and gasifies, and the internal pressure of the nitrogen gas thus generated breaks the grain boundary,
We have learned that this is the starting point of blistering.

Hereinafter, the cause of the slab swelling will be described in detail. As a result of observations around the induction heating furnace, confirmation tests under similar conditions, and detailed component analysis, it was found that slab blisters occurred mainly in the maximum concentration segregation zone. At this time, the maximum concentrated segregation position is not limited to the commonly known center segregation position.In continuous casting, with the addition of electromagnetic molten steel stirring, the maximum concentrated segregation zone may appear at a position other than the center thickness. We also found out. It was also confirmed that blisters appeared during the cooling process after high-temperature heating.

Subsequently, as a result of closely observing the structure around the slab bulge portion, a portion once melted at the crystal grain boundary was observed, and silicon, carbon, and other component elements were significantly concentrated in the original melted portion. It became clear that.
On the other hand, nitrogen, carbon,
A layer in which other component elements were reduced was observed. The width of the deficient layer tended to increase in proportion to the holding time at a high temperature. From these facts, the cause of the above-mentioned blister has been clarified.

BOF Steel Making Vol.II Iron & Steel S
As shown in oc. AIME (1975) and others, the solubility of nitrogen at atmospheric pressure just below the melting point in iron and silicon iron is around 0.01%, while that in molten iron that is in equilibrium with it is about 4 times. Has solubility. From the above, when the partial melting state of the grain boundary continues for a certain period of time, the component elements move from the solid phase near the grain boundary to the molten phase, and this is controlled by diffusion. To increase. In the subsequent coagulation process, if the concentration of nitrogen, which is a gas component concentrated in the liquid phase, is about 0.01 wt% or more, supersaturation will occur, and as a result, it is considered that slab blistering will occur. On this occasion,
The amount of gas released is the time during which the grain boundary melting state is maintained,
The detailed examination revealed that the temperature changes depending on the grain boundary density and the cooling rate in the final stage of slab heating. From the above results, it is clear that the reduction of solidification segregation in the slab and the reduction of the variation in heating temperature are effective in suppressing the occurrence of grain boundary melting and the suppression of slab swelling. It is important, and more specifically, it has been found that by controlling the solidification conditions in the continuous casting slab, the occurrence of slab bulging can be effectively suppressed.

The present invention has been developed on the basis of the above findings, and effectively prevents slab bulging which may occur during high-temperature heating of a silicon-containing steel slab, and thus has good magnetic properties and surface properties. This is a method for heating a directional silicon steel slab that can obtain a product having an appearance.

That is, the present invention provides a silicon-containing steel slab containing N: 0.0025 wt% (hereinafter simply referred to as%) or more,
Continuous casting under electromagnetic molten steel stirring, and then heating the slab to a surface temperature of 1350 ° C or more in an electric heating furnace, the temperature of the molten steel in the continuous casting process is set at 25 ° C above the liquidus temperature.
This is a method for heating a directional silicon steel slab characterized by having a high temperature range as described above.

According to the present invention, even when a continuous cast slab to which electromagnetic molten steel is agitated for the purpose of improving magnetic properties is used as the material slab, the electric heating furnace can be used without slab bulging. Can be used to perform high-temperature short-time heating. Here, it is considered that the slab heating temperature can be more preferably controlled by measuring the temperature inside the slab. However, it has been confirmed that the predetermined object can be sufficiently achieved by controlling the surface temperature. Then, it was decided that the surface temperature would be easy to measure.

[0020]

The following is a description of the component composition range of the material.
As the silicon-containing steel which is the material of the present invention, any conventionally known components can be used for components other than N. In the present invention, the N content is particularly defined because the cause of the slab blistering defect to be solved by the present invention is partial melting in the segregated portion, and the H, N, etc. in the liquid phase generated thereby. Is a gas component, especially N concentration. Therefore, when the N concentration in the steel is low, even if the segregated portion partially melts, the N concentration does not occur enough to cause blister defects. Therefore, in the present invention, only steels containing 0.0025% or more of N are targeted.

For reference, preferred composition ranges of other components are as follows. C: 0.01 to 0.10% C is an element useful for not only making the structure uniform and fine during hot rolling and cold rolling, but also for developing the Goss orientation, and is preferably contained at least 0.01%. However, if the content exceeds 0.10%, decarburization becomes difficult, and the Goss orientation is rather disturbed. Therefore, the upper limit is 0.10%.
% Is preferable.

Si: 2.5 to 4.5% Si increases the specific resistance of the steel sheet and contributes to a reduction in iron loss. However, when the content exceeds 4.5%, the cold rolling property is impaired.
Not only lowers the specific resistance, but also randomizes the crystal orientation by α-γ transformation during the final annealing performed for secondary recrystallization and purification, and a sufficient iron loss improvement effect is obtained. Therefore, the content of Si is preferably about 2.5 to 4.5%.

Mn: 0.02 to 0.12% Mn requires at least about 0.02% to prevent hot embrittlement, but too much Mn deteriorates magnetic properties. Therefore, the upper limit is about 0.12%. Is preferred.

As inhibitors, so-called MnS, MnSe
System and AlN system. MnS, MnSe system At least one selected from S and Se: 0.005 to 0.
06% S and Se are both effective elements as inhibitors for controlling secondary recrystallization of grain-oriented silicon steel sheets. From the viewpoint of suppressing power, at least about 0.005% is required,
If it exceeds 06%, the effect will be lost. Therefore, the upper and lower limits are preferably set to about 0.005% and 0.06%, respectively. AlN-based Al: 0.005 to 0.10% The range of Al is also set to the above range for the same reason as in the case of the above-mentioned MnS and MnSe-based. In addition, the above-mentioned MnS, Mn
Se-based and AlN-based can be used in combination. further,
As the inhibitor component, in addition to S, Se, and Al described above,
Cu, Sn, Sb, Mo, Te, Bi and the like also act advantageously, so that they can be contained together in small amounts. The preferred addition ranges of these components are respectively Cu, Sn: 0.01 to 0.15%, S
b, Mo, Te, Bi: 0.005 to 0.1%, and each of these inhibitor components can be used alone or in combination.

Although the slab is intended to be manufactured by continuous casting, it is needless to say that a slab which has been subjected to re-bulking after continuous casting is also included. And
In the present invention, by controlling the heating temperature of the molten steel in the continuous casting step, specifically, by setting the temperature in a temperature range higher than the liquidus temperature by 25 ° C. or more, the continuous casting with electromagnetic induction stirring added. The segregation other than the center segregation, which is remarkable in the above, is sufficiently reduced to prevent slab bulging.

Here, it is not clear why the above-mentioned segregation, which causes blistering, disappears by regulating the heating temperature of the molten steel to a predetermined range. However, generation of crystal nuclei moving by electromagnetic stirring is delayed. Alternatively, it is considered that the crystal nuclei are redissolved by heating at a high temperature. If the heating temperature of the molten steel is too high, there is a disadvantage in operation such as a decrease in the casting speed. Therefore, there is no particular upper limit, but it is appropriate to set the temperature to about 50 ° C. or less.

The slab thus obtained is usually charged as it is, or after being temporarily placed, charged into a heating furnace, heated, or gradually cooled, and then subjected to surface treatment and the like, and then charged into the heating furnace and heated. As described above, the slab heating is performed based on the temperature of the slab surface.If the temperature is less than 1350 ° C, the occurrence of blistering is extremely small, and the effect of shortening the solution solution time of the inhibitor component cannot be expected. Above. Here, as the electric heating furnace, electromagnetic induction heating, resistance heating, or the like is preferable.

After heating the slab as described above,
Hot-rolled steel strip with a thickness of 1.4 to 3.5 mm is formed by hot rolling. The pickling step of this hot-rolled steel strip, the subsequent one or two or more cold rolling steps sandwiching intermediate annealing, the subsequent decarburizing annealing, the application of an annealing separator, and the final finishing annealing step are performed by known means, respectively. Can be used.

[0029]

【Example】

Example 1 C: 0.08%, Si: 3.1%, Mn: 0.07%, Se: 0.02%, A
l: 0.028% and N: 0.008%, with the balance being substantially Fe, the molten steel was heated to (liquidus temperature + 32 ° C) and continuously cast to form a slab while stirring with electromagnetic molten steel. . This slab is subjected to a surface temperature in a hot rolling step:
After being heated at 1410 ° C. for 35 minutes, it was subjected to rough rolling, but no swelling occurred in the slab.

For comparison, the same molten steel as above was heated to (liquidus temperature + 15 ° C.) and (liquidus temperature + 22 ° C.), and continuously cast into a slab with electromagnetic molten steel stirring. When this slab was subjected to hot rolling in the same manner as described above, occurrence of swelling was observed in the slab.

Example 2 Molten steel containing 0.09% of C, 3.5% of Si, 0.05% of Mn, 0.02% of S, and 0.003% of N, and the balance substantially consisting of Fe (liquidus line) (Temperature + 27 ° C.), and continuously cast into a slab while stirring magnetic steel. This slab was heated at a surface temperature of 1380 ° C. for 40 minutes in a hot rolling step, and then subjected to rough rolling, but no swelling occurred in the slab.

For comparison, the same molten steel as described above was heated to (liquidus temperature + 18 ° C.) and (liquidus temperature + 24 ° C.), and continuously cast into a slab while being subjected to electromagnetic molten steel stirring. When this slab was subjected to hot rolling in the same manner as described above, occurrence of swelling was observed in the slab.

[0033]

Thus, according to the present invention, even when electromagnetic molten steel is agitated for the purpose of improving magnetic properties, it is possible to completely prevent the occurrence of blistering defects during slab heating and, consequently, to obtain good magnetic properties. It can contribute to stable production of grain-oriented silicon steel sheets having characteristics and surface appearance.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-53-19193 (JP, A) JP-A-63-119949 (JP, A) JP-A-2-263922 (JP, A) JP-A-5-1991 1324 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B22D 11/00, 11/115 C21D 8/12 H01F 1/16

Claims (1)

(57) [Claims] 1. A silicon steel slab containing N: 0.0025 wt% or more is continuously cast under electromagnetic molten steel stirring. Then, when the slab is heated to a surface temperature of 1350 ° C. or more in an electric heating furnace, the slab is continuously cast. A method for heating a directional silicon steel slab, wherein the temperature of molten steel in the casting step is set to a temperature range higher than the liquidus temperature by at least 25 ° C.