JP3538855B2 - Manufacturing method of grain-oriented silicon 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 method for producing a grain-oriented silicon steel sheet, and more particularly to a method for effectively suppressing the occurrence of blistering defects during slab heating and improving the product quality including magnetic properties. I'm trying.

[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,
What is particularly important is 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. Typical 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 to completely dissolve the inhibitor component has been adopted.From the hot rolling step, the precipitation state during the secondary recrystallization step is controlled. ing. In addition, as inhibitors, Sb, Sn, As, Pb,
Grain boundary segregation elements such as Ce, Cu and Mo are also used.

[0004] Conventionally, to manufacture 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. It is common practice to apply the annealing separator after decarburizing annealing, apply the annealing separating agent, and then perform the final finishing annealing for the purpose of secondary recrystallization and purification after cold rolling two or more times with cold rolling It is.

[0005] However, when such slab heating is performed for a long time, the crystal grains after heating are remarkably coarsened.
The coarse crystal grains in the slab are difficult to recrystallize in the subsequent hot rolling, and the sub-grain boundaries and dislocations in the non-recrystallized grains act 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 advance 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 characteristics 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. If the blistering defect is severe, it is of course impossible to perform hot rolling, but even if the blistering defect is slight, it causes serious defects such as double plate, plate breakage, and perforation. 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, the magnetic properties are improved by electromagnetically stirring the 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 blister 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 Al in silicon steel,
It discloses a condition in which blisters do not occur in the final product by controlling the amounts of H and N. In addition, Tokiko 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. Hei 2-259016 discloses a method for producing a grain-oriented silicon steel sheet in which the roll bulge during cold rolling is reduced to 150 mm or more to reduce surface blister defects. Further, JP-A-5-1324 discloses that
There is disclosed a technique for controlling a temperature difference after preheating and a heating rate of an electric heating furnace to suppress a blister-like defect on a product surface caused by an opening in a slab.

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. However, the blister generated in the thin-plate annealing process and the blister at the slab stage which are a problem in the present invention are considered. The blistering has a completely different mechanism of occurrence, so that the above technique could not prevent slab blistering.

[0012]

As described above, up to now, there is still no known technique capable of completely preventing the occurrence of 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 advantageously satisfies the above-mentioned requirements, and effectively prevents the occurrence of slab swelling during high-temperature heating even when continuous casting is performed using electromagnetically molten steel stirring, which is advantageous in terms of magnetic properties. Further, it is an object of the present invention to propose a method for manufacturing a grain-oriented silicon steel sheet that can obtain good magnetic properties and surface appearance.

[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, etc. 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, Then, during re-solidification in the cooling process, the solid solution component becomes supersaturated and gasifies, and the internal pressure of the nitrogen gas and the like 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 observation around the induction heating furnace, confirmation tests under similar conditions, and detailed component analysis, it was found that slab swelling 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, 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), etc., the solubility of nitrogen at atmospheric pressure just below the melting point in iron and silicon iron is around 0.01%, while in molten iron which is in equilibrium with it, 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. 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% or more, supersaturation is caused, which is considered to lead to slab blistering. On this occasion,
The amount of gas released is the time during which the grain boundary melting state is maintained,
The detailed examination also revealed that the temperature changes depending on the grain boundary density and the cooling rate in the final stage of slab heating. Furthermore, it was also found that if the heating temperature variation between the upper and lower surfaces was large, the possibility of melting the maximum segregated portion was increased. From the above results, it is understood that the reduction in the variation in the heating temperature suppresses the generation of the grain boundary melting and is effective in suppressing the slab swelling, and furthermore, it is important to control the temperature at the position of the maximum concentrated segregation zone.

Conventionally, when a slab is heated only by a gas-fired heating furnace, the temperature of the upper surface is usually set to be lower than that of the lower surface in order to improve the threading stability in a finishing mill where the rolling control is most difficult. By setting it somewhat higher, the temperature uniformity at the time of passing through the finishing mill is increased, and the occurrence of warpage is mainly suppressed. An example of the temperature transition in the heating furnace on the upper and lower surfaces of the slab is shown in FIG. 1 (from rolling technology [Nikkan Kogyo Shimbun] 1971). Further, it was thought that the variation in the heating temperature was caused by the variation in the final electric heating furnace. However, according to the study by the inventors regarding this point, the variation in the primary heating gas-fired heating furnace was considered. It has been found that the variation has a greater effect than the temperature variation of the electric heating furnace. The present invention has been developed on the basis of the above knowledge, and effectively prevents slab blistering, which may occur during high-temperature heating of a silicon-containing steel slab, to obtain good magnetic properties and surface appearance. This is an advantageous method for producing a grain-oriented silicon steel sheet.

That is, the present invention relates to a silicon-containing steel slab containing N: 0.0025 wt% (hereinafter simply referred to as%) or more,
Continuous casting while stirring the electromagnetic molten steel, then heated in a gas combustion type heating furnace and an electric heating furnace, then hot rolled,
Next, after performing cold rolling once or twice or more with intermediate annealing to finish the final sheet thickness, decarburizing annealing, then applying an annealing separating agent to the steel sheet surface, and then performing a final finish annealing In the method for producing a grain-oriented silicon steel sheet comprising the steps of:
After preheating to a temperature of 1300 ° C, when heating to a temperature of 1350 ° C or more in an electric furnace with a non-oxidizing atmosphere, a) Adjust the set temperatures of the upper and lower heating zones in a gas-fired furnace. Then, the temperature difference between the upper and lower surfaces of the slab after extraction from the heating furnace is suppressed to within 20 ° C. B) Heating in the electric heating furnace is performed with reference to the surface temperature.
A method for producing a grain-oriented silicon steel sheet, wherein the temperature is limited to 50 ° C. or more and less than 60 minutes.

According to the present invention, even when a continuous cast slab to which magnetic 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 causing slab swelling. 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. Here, the temperature difference between the upper and lower surfaces of the slab in the gas-fired heating furnace is 20
If the temperature exceeds ℃, blistering often occurs.
Limited to ° C. If the heating in the electric furnace is less than 1350 ° C at the surface temperature, the inhibitor cannot be sufficiently dissolved in a short time, so the heating temperature is
1350 ℃ or higher. Further heating time
When the heating time is 60 minutes or more, the grain growth remarkably deteriorates the magnetic properties, and the effect of suppressing slab swelling is small. Therefore, the heating time is set to less than 60 minutes.

[0020]

The following is a description of the component composition range of the material.
As the silicon-containing steel that is the material of the present invention, any conventionally known components can be used for components other than N. It should be noted that the reason why the N content is particularly defined in the present invention is that 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 concentration of N, especially N. Therefore, when the N concentration in the steel is low, even if the segregated portion is partially melted, the N concentration that causes blistering defects does not occur. Therefore, in the present invention, only the steel containing 0.0025% or more of N is targeted.

For reference, preferred composition ranges of other components are as follows. C: 0.01 to 0.10% C is an element useful not only for uniform micronization of the structure during hot rolling and cold rolling, but also for development of the Goss orientation, and is preferably contained at least 0.01%. However, if the content exceeds 0.10%, decarburization becomes difficult, and the Goss direction is 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, while 2.5% to 4.5%.
Not only lowers the specific resistance, but also causes randomization of the crystal orientation due to 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 set to about 2.5 to 4.5%.

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

As inhibitors, so-called MnS, MnSe
System and AlN system. In the case of MnS and MnSe, 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%, its effect will be lost. Therefore, the upper and lower limits are preferably set to about 0.005% and 0.06%, respectively. In the case of the AlN system, the range of Al: 0.005 to 0.10% Al is also set in the above range for the same reason as in the case of the MnS and MnSe systems described above. Note that the above 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 incorporated 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.

The slab is intended for a slab manufactured by continuous casting, but it goes without saying that a slab which has been subjected to lumping and repressing after continuous casting is also included. The slab is usually put into the heating furnace as it is or after temporary placement, heating, or after slow cooling, after performing surface care, etc., then loading into the heating furnace,
Heated. In the present invention, this heat treatment is particularly important. As described above, a) adjusting the set temperatures of the upper and lower heating zones in the gas-fired heating furnace to adjust the upper and lower surfaces of the slab extracted from the heating furnace. B) Reduce the temperature difference to within 20 ° C. B) Heat the electric heating furnace based on the surface temperature.
By limiting the temperature to 50 ° C or more and less than 60 minutes, the occurrence of slab swelling is prevented. 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 the 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. Can be used.

[0027]

【Example】

Example 1 C: 0.08%, Si: 3.4%, Mn: 0.06%, Se: 0.017%,
Molten steel containing 0.029% of Al and 0.007% of N and substantially the balance of Fe was continuously cast into a slab while stirring with electromagnetic molten steel. This slab has a lower surface temperature of 1200
℃ and a constant temperature, while the upper surface temperature from 1200 ℃ to 12
After heating in a gas-fired heating furnace with various changes up to 30 ° C, it is transferred to an induction heating furnace, where the surface temperature is raised to 50 ° C for each temperature in the range of 1390 to 1410 ° C.
After holding for a minute, the mixture was cooled. Then, from a sheet bar having a thickness of 45 mm, a hot-rolled sheet having a thickness of 2.2 mm was formed, and then subjected to primary cold rolling and then to secondary cold rolling with intermediate annealing.
Finished to a final thickness of 22mm. Then, after applying an annealing separator mainly composed of MgO, 1200 ° C in a hydrogen atmosphere,
A final finish annealing of 8 hours was performed. Table 1 shows the results obtained by examining the magnetic properties of the products thus obtained and the presence or absence of blister defects at the slab stage. In particular, the presence or absence of blistering of each slab is shown in FIG. 2 in relation to the heating temperature and the temperature difference between the upper and lower surfaces.

[0028]

[Table 1]

As is clear from Table 1 and FIG. 2, when the difference between the lower surface temperature and the upper surface temperature of the slab was adjusted within 20 ° C. according to the present invention, not only did the slab blister defect not occur at all, but also Also, the magnetic properties were good. In addition, each of the conforming examples exhibited a beautiful surface appearance.

Example 2 C: 0.07%, Si: 3.2%, Mn: 0.05%, Se: 0.019%,
Al: 0.031% and N: 0.08%, with the balance being substantially
The molten steel having the composition of Fe was continuously cast while performing electromagnetic molten steel stirring to form a slab. The lower surface temperature of this slab is 1150 ℃
And the upper surface temperature from 1150 ° C to 1180
After heating in a gas-fired heating furnace with various changes up to ℃, transfer to an induction heating furnace, and keep it at a surface temperature of 1400 ℃ for 1 to 90 minutes in this induction heating furnace, then cool did. Then, as a sheet bar of thickness: 45mm,
After forming a hot-rolled sheet having a thickness of 2.2 mm, the sheet was finished to a final sheet thickness of 0.22 mm by primary cold rolling and then secondary cold rolling with intermediate annealing. After that, an annealing separator containing MgO as a main component was applied, and then a final finish annealing was performed at 1200 ° C. for 10 hours in a hydrogen atmosphere. Table 2 shows the results obtained by examining the magnetic properties of the products thus obtained and the presence or absence of blister defects at the slab stage. FIG. 3 shows the occurrence of blistering in each slab, organized according to the relationship between the heating time and the temperature difference between the upper and lower surfaces.

[0031]

[Table 2]

As is apparent from Table 2 and FIG. 3, when the difference between the lower surface temperature and the upper surface temperature of the slab is adjusted within 20 ° C. and the high temperature holding time is limited to 60 minutes or less according to the present invention, No slab blister defects were generated, and the magnetic properties were good. Furthermore, all of the conforming examples exhibited a beautiful surface appearance.

[0033]

Thus, according to the present invention, even when electromagnetic molten steel is agitated for the purpose of improving magnetic properties, the occurrence of blistering defects during slab heating is completely prevented, and good magnetic properties are obtained. And a grain-oriented silicon steel sheet having a surface appearance can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a temperature rise curve showing a slab heating procedure according to a conventional method.

FIG. 2 is a diagram showing the influence of a temperature difference between upper and lower surfaces of a slab and a heating temperature on slab swelling.

FIG. 3 is a diagram showing an influence of a temperature difference between upper and lower surfaces of a slab and a holding time at a high temperature on a slab blister.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-43936 (JP, A) JP-A-5-51638 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/12 C21D 9/46 501 C22C 38/00-38/60

Claims (1)

(57) [Claims] 1. A silicon steel slab containing N: 0.0025% by weight or more is continuously cast while stirring magnetically molten steel, and then heated in a gas combustion type heating furnace and an electric heating furnace. Rolled and then cold-rolled once or twice or more with intermediate annealing to finish to the final thickness, decarburized annealing, then apply an annealing separating agent to the steel sheet surface, then final finish annealing In a method for producing a grain-oriented silicon steel sheet, comprising a series of steps of applying a silicon steel slab to a gas-fired heating furnace at 900 to 1300 ° C.
After preheating to the temperature of 1), when heating to a temperature of 1350 ° C or more in an electric heating furnace in a non-oxidizing atmosphere, a) adjusting the set temperature of the upper and lower heating zones in the gas combustion type heating furnace The temperature difference between the upper and lower surfaces of the slab after extraction from the heating furnace is controlled to within 20 ° C. B) Heating in the electric heating furnace is performed based on the surface temperature.
A method for producing a grain-oriented silicon steel sheet, wherein the temperature is limited to 50 ° C or more and less than 60 minutes.