JPH02159319A - Manufacture of grain-oriented silicon steel sheet excellent in surface characteristic and magnetic property - Google Patents

Abstract

PURPOSE:To obtain a grain-oriented Si steel sheet excellent in surface characteristics and magnetic properties by treating a slab of an Si-containing steel having a specific composition under the prescribed process conditions. CONSTITUTION:A slab of an Si steel containing, by weight, 2-4.5% Si and 0.004-0.080% Sb is subjected to soaking treatment in an internally heated-type heating furnace, such as an induction heater into which nonoxidizing gas is introduced, at a temp. in the region of >=1400 deg.C for <=60min. Subsequently, the above slab is subjected to hot rollings including roughing where rolling at >=40% draft is carried out by one or more passes at >=1250 deg.C. Then, the hot rolled plate is subjected to a single cold rolling or is cold-rolled twice while process-annealed between the cold rolling stages, and the resulting sheet is subjected to decarburizing annealing and is subjected, after subjected to the application of a separation agent at annealing to the steel-sheet surface, to finish annealing. By this method, deterioration in the surface characteristics of a product due to the internally heated-type heating of the Si steel slab at high temp. can be avoided, and the above steel sheet having high quality can be obtained.

Description

[Detailed Description of the Invention] (Industrial Field of Application) This invention relates to a method for producing grain-oriented silicon steel sheets having excellent magnetic properties in the rolling direction, and in particular to an advantageous method for solving problems associated with high-temperature heating applied to silicon-containing steel slabs. This is an attempt to provide the results of development research regarding solutions.

(Prior art) A grain-oriented silicon steel sheet has a (110) (
001) Secondary recrystallized grains with highly aligned orientations are formed in the final annealing. Prior to the final annealing, fine particles such as MnS, MnSe and AfN called inhibitors are added to suppress the growth of normal grains. The precipitate is uniformly precipitated and dispersed (and (110)
The crystal structure other than the (001) orientation should be fine-grained;
In particular, it is necessary to destroy the (100) <1 mn> structure, which tends to form a fibrous structure. In order to satisfy these requirements, the influence of the hot rolling process cannot be ignored, and especially when using a silicon-containing steel slab obtained by continuous casting, the influence of heating the slab is extremely large.

In other words, in order to finely precipitate and disperse the inhibitor,
Since the inhibitor must be completely dissolved in the solid solution when heating the slab, heating the slab to a high temperature will coarsen the crystal structure of the slab and change the crystal structure to (100) <i,
This results in the disadvantage that a fibrous structure of mn> appears.

Therefore, as for slab heating, a method of heating for a short time at a high temperature of 1300° C. or more and less than 1450° C., as disclosed in Japanese Patent Application Laid-Open No. 60-190520, is becoming mainstream.

Heating of such a slab is described in Japanese Utility Model Publication No. 58-24397, Japanese Patent Application Laid-open No. 60-145318, and Japanese Patent Application Laid-open No. 60-1.
Although it is efficient to conduct heating for a short time using an internal heating type heating furnace such as an induction heating furnace or a direct current heating furnace disclosed in Publication No. 28210, even though the heating is performed for a short time, a large amount of heat is deposited on the slab surface. A molten scale is generated, and this molten scale not only impairs the operability of the heating furnace, but also becomes so-called slag and causes surface flaws. As a method to solve this problem, for example, Japanese Patent Application Laid-Open No. 145318/1983 describes a method to suppress the formation of molten scale on the slab surface and improve the surface quality of the product by reducing the Oz concentration in the slab heating atmosphere to 1% or less. Improvements have been disclosed, but it is difficult to completely seal the furnace that heats large slabs weighing more than 10 tons at high temperatures, so it is difficult to reduce the 0□ concentration in the atmosphere to 1% or less in actual operations. Also, when the slab is heated to a temperature of 1400℃ or higher, a surface defect called a linear pattern appears on the product, which is different from the surface defect caused by conventional slag. Even if it was reduced to 0.1% or less, the occurrence could not be avoided.

(Problem to be solved by the invention) Therefore, silicon steel slabs containing inhibitors such as MnS, MnSe, and AffiN have the advantage of not causing surface defects called linear patterns even after being heated in a high temperature range of 1400°C or higher. It is an object of the present invention to provide a manufacturing method.

(Means for Solving the Problem) The inventors investigated various means for improving the surface properties of products that cannot be improved by the conventional regulation of 0□ concentration in the atmosphere, and found that The inventors have newly found that controlling the components in the steel and the rough rolling conditions when heating is extremely effective in improving the surface properties of the product, leading to the completion of this invention.

That is, this invention heats a silicon steel slab containing St: 2 to 4.5 wtX and Sb: 0.004 to 0.080 wtX to a temperature of 1400° C. or higher in an internal heat-generating furnace into which a non-oxidizing gas is introduced. 6 in this temperature range.
Soaking for 0 minutes or less, followed by hot rolling including rough rolling at a temperature of 250°C or higher and a rolling reduction of 40% or higher for at least one pass, followed by one round or two intermediate annealing steps. This is a method for producing a grain-oriented silicon steel sheet with excellent surface properties and magnetic properties, in which the steel sheet is cold rolled twice, then decarburized annealed, then an annealing separator is applied to the surface of the steel sheet, and then finish annealed. .

Now, the starting material in this invention is a grain-oriented silicon steel slab, and the following components are advantageously suitable for its composition.

C is necessary to improve the crystal structure of the steel plate, and is 0.02w.
If it is less than tX (hereinafter simply referred to as %), there is no effect, and if it exceeds 0.08%, decarburization performance deteriorates, so it is usually 0.
．． The range is 0.02% to 0.08%.

Si is necessary to increase the specific resistance of steel sheets and reduce iron loss.
If it is less than 2%, α-γ transformation will occur and the crystal orientation will not be aligned during final annealing, while if it exceeds 4.5%, cold rollability will deteriorate, so it is usually in the range of 2 to 4.5%.

Mn should be 0.0 in order to act as an inhibitor.
2% or more is necessary, and if it exceeds 0.12%, the solid solution temperature will rise and the slab heating temperature will become too high, so it is usually in the range of 0.02 to 0.12%.

In addition to the above, the steel contains S as an inhibitor component.

Se, ^It, Cu, Sn, Mo+ P+ Cr
, Te, and Bi.

Furthermore, in this invention, in order to suppress linear patterns in the product, it is particularly necessary to contain sb in an amount of 0.004% or more. However, if it exceeds 0.080%, the decarburization properties in decarburization annealing will deteriorate, so the content is set at 0.004 to 0.080%.

Further, the slab may be a slab produced by blooming an ingot or a slab produced by continuous casting.

It goes without saying that the target also includes slabs that have been continuously cast and then re-blushed.

The slab containing the above components is heated to a temperature of 1,400°C or higher in an internal heat-generating heating furnace, such as an induction heating furnace or a direct current heating furnace.However, in consideration of economic efficiency, conventional gas It may be heated in a combustion furnace. However, 140
Since it is nearly impossible to heat a slab weighing several tons to a high temperature of 0°C or higher using a gas combustion furnace, it is ultimately necessary to use an internal heating type heating furnace, which is also essential in this invention. .

At this time, 0 in the atmosphere! Concentrations do not need to be strictly regulated because sb is included, so there is no need to take special measures such as sealing the furnace as in the past, and Ar and N2
It is sufficient if a non-oxidizing gas such as
□ If the concentration is high, work efficiency may decrease due to slag accumulation in the furnace, so it is preferable that the oz1 degree is 5% or less.

In addition, a soaking temperature of 1400°C or higher is required for solid solution of the inhibitor, but practically it is 1400-1470"C.
is commonly used. The soaking time is preferably 10 minutes or more, but it should be determined by the soaking temperature; on the other hand, if it exceeds 60 minutes, the slab structure will become coarse and the magnetic properties will deteriorate, so the soaking time should be within 60 minutes.

The heated slab is then rough rolled into a sheet bar, and then turned into a hot coil by a finishing tandem rolling mill, but the rough rolling conditions are important in order to prevent the generation of linear patterns in the product. That is, in the rough rolling stage, it is necessary to perform at least one pass of rolling at a temperature of 1250° C. or higher and a rolling reduction of 40% or higher in order to prevent the generation of linear patterns in the product.

Then, the hot-rolled sheet is subjected to normalization annealing if necessary, and then cold-rolled once or twice with intermediate annealing in between to reach the final thickness. After that, decarburization and primary recrystallization annealing were performed, and after applying an annealing separator to the steel plate surface,
After secondary recrystallization and purification are performed by final annealing at around 00'C, an insulating coating is applied to produce a product.

(Effect) The reason for applying high temperature heating to the slab is to harden the inhibitor as mentioned above. This is to reduce the Mn contained in the steel.
S SMnSe and AI! , N, etc., it is necessary to make the temperature sufficiently high. Therefore, since the slab charged into the internal heating type heating furnace is exposed to a high temperature atmosphere, molten scale generated by surface oxidation becomes slag and causes surface defects. Therefore, it is important to suppress surface oxidation of the slab, and this can be done by strictly regulating the 0□ concentration in the atmosphere, but if the heating temperature exceeds 1400°C, a new surface defect called a linear pattern will occur on the product. This surface defect occurs when the structure of the forsterite coating on the surface of the product plate is disturbed, and a part of the base metal surface is exposed, resulting in a structure in which forsterite forms an eyebrow inside the base metal, and the average coating thickness is thin. However, the occurrence of surface defects is advantageously avoided by performing slab heating and hot rolling in accordance with the present invention.

Next, the experimental results that led to this invention will be described.

St: 3.35χ, C: 0.035%, Mn:
Silicon steel slab (8) with a thickness of 180ff I1 containing 0.070% and S: 0,020% and Si
: 3.36%C: 0.032%, Mn: 0.
068% and S: 0.021% and further Sb:
A 180 mm thick silicon steel slab (B) containing 0.008% was heated from 1320°C to 146°C in an induction heating furnace.
Soaking treatment was carried out for 40 minutes each at a predetermined temperature in the temperature range of 0"C, and then rough rolling was carried out at 01380C to a thickness of 120mm.
mm (rolling rate 33%) 01290℃ to thickness 60mm (rolling rate 50%) ■1210℃ thickness 45ff 11m
(Reduction rate: 25%) ■ After 4 passes of rolling at 1120"C to a thickness of 35 mm (Reduction rate: 22%), it was hot-rolled to a thickness of 2.0 mm at once using a 7-stand finishing mill. The coils were finished into steel strips.At this time, the gas tightness in the induction heating furnace was changed to change the 0□ concentration to two levels, 0.1% and 3%.These coils were rolled using the conventional two-rolling method. The product was made by
In order to determine the surface quality, the product coil was divided into blocks of 100 m each, and the presence or absence of surface defects was determined for each block, and the number of blocks with the corresponding defect was divided by the total number of blocks and multiplied by 100 to determine the flaw occurrence rate. FIG. 1 shows the relationship between the flaw occurrence rate, the linear pattern occurrence rate, and the slab induction heating temperature.

As shown in the figure, the number of scratches, which are conventional types of defects, is 0! It can be effectively reduced by regulating the concentration to 0.1% or less, but regarding the new defect that is the problem here, linear patterns, in the case of a slab (A) that does not contain sb, the slab heating temperature increases rapidly when the temperature exceeds 1400°C. This linear pattern is created by containing o, ooa% of sb, as shown in slab (B) in the same figure.
It becomes possible to significantly reduce it to 0% or less. Also sb
Another feature of the containing steel is that 3% of 0□
Even when it comes to density, the conventional defects, such as flaws, are reduced, and it has been found that it is possible to significantly relax the regulation of 0□ density.

In addition, the above slab containing 0.008% sb (thickness 1
80 mm) to investigate the effect of the rough rolling reduction ratio on the linear pattern in hot rolling. The slab was heated using an induction heating furnace and subjected to inhibitor solution treatment at 1460°C for 20 minutes, at which time the 0□ concentration in the atmosphere was 5%.
And so. The rough rolling conditions are as shown in Table 1 below. Table 1 also shows the results of investigating the linear pattern occurrence rate of products obtained according to each condition.

From the same table, in rough rolling, by performing one pass or more of rolling at a temperature of 1250°C or higher and a rolling reduction rate of 40% or higher,
It can be seen that the incidence of linear patterns is drastically reduced. Conventional 140
When heating the slab below 0'C, it was necessary to heat the slab for a long time to complete the solutionization of the inhibitor, so the slab structure after heating inevitably became coarse, and the aim was to destroy this coarse grain. Thus, controlled recrystallization rolling was performed during hot rolling. That is, as disclosed in Japanese Patent Publication No. 60-37172, 96
Since there is a region where recrystallization occurs due to rolling with a high reduction rate of 30% or more in the temperature range of 0 to 1190°C, the first stage of finish rolling in hot rolling and the second half of rough rolling can be performed at a high reduction rate. It was needed. Therefore, rough rolling in a high temperature range inevitably lowers the rolling reduction rate, while in a low temperature range it increases the rolling reduction rate.

In the hot rolling according to the present invention, the reduction rate may be low in the recrystallization temperature range of 1190"C or lower, but this can be achieved by regulating the soaking time of the slab internal exothermic heating to within 60 minutes. To prevent deterioration of magnetic properties,
This was discovered through the following experiment.

C: 0.040%, Si: 3.36%, Mn:
0.070%, Se: 0.025% and Sb:
A slab containing 0.015% was heated to 1450'C in an induction heating furnace with N2 introduced, and heated for 20 minutes, 40 minutes, and 60 minutes.
After soaking for 80 minutes and 100 minutes, hot rolling was performed to obtain a hot rolled steel strip having a thickness of 2.3 mm. At this time, the 02 concentration in the atmosphere was 8%. The rough rolling schedule was set to the condition (2) in Table 1 above.

20 Hot-rolled steel strip is normalized and annealed at 950°C for 2 minutes, pickled, rolled to an intermediate thickness of 0.70mm, and then
Intermediate annealing was performed at 000″C for 1 minute, and then 0.23
It was cold rolled to a final thickness of +ma. Then 800℃
After decarburizing annealing for 2 minutes at
Finish annealing was performed at ℃ for 10 hours. Table 2 shows the measurement results of the linear pattern occurrence rate and magnetic properties of the obtained product.

Table 2 As shown in the table, magnetic properties are maintained well by slab heating treatment within 60 minutes of soaking time.

Normally, when the reduction ratio in rough rolling is high, it is thought that intergranular cracking is induced due to high deformation during hot rolling, and the surface quality is deteriorated. On the other hand, the influence of rolling in this invention shows a completely opposite tendency, and this is a phenomenon that was discovered for the first time in the experiments of this invention.

Furthermore, in order to clarify the mechanism of this phenomenon, the inventors
The two types of slabs mentioned above were treated with N containing a 0□ concentration of 3%.
After annealing at 1430°C for 30 minutes in a 2 atmosphere, the structure of the surface layer of the slab was observed, as shown in Figure 2.
In m(b), which does not contain b, there was a layer of slag on the surface, and a thick subscale layer was also present on the base iron side. This subscale layer develops rapidly at temperatures above 1400°C. On the other hand, a(a) containing sb is characterized in that the slag layer on the surface is thin, and the subscale layer on the base metal side is also thin, resulting in a smooth interface.

The subscale structure of steel (a) containing sb is
It easily flakes off due to heavy reduction in the next process of rough rolling, exposing the bare steel interface, whereas steel with a thick, congested subscale structure, such as steel (b) that does not contain sb, peels off easily during hot rolling. It is difficult to remove it even by the strong reduction during rough rolling during rolling, and it is thought that the effect remains until the final product. On the other hand, subscales such as the structure of rope (a) can be peeled off by strong working of 40% or more, but unless the temperature is higher than ff11250℃, the adhesion between the subscale and the base steel will deteriorate. It is assumed that the subscales are strengthened and the subscales do not fall off.

FIG. 3 shows the results of an experiment conducted regarding the appropriate content of sb effective for producing such a subscale structure and reducing the linear pattern of the product.

The above slab (Cu: 0.036%, Si: 3.
16%, Mn2 0.064%, S: 0.018%)
200 with various sb contents in the composition of
Each mm-thick slab was heated at 1460℃ for 2 hours by induction heating.
After being heated in a N2 atmosphere containing 2% concentration of 02 for 0 minutes, it was heated at 1320 °C to a thickness of 160 mm (reduction rate 20%).
-1270℃ to 90mm thickness (reduction rate 44%) →11
After rough rolling at 90° C. to a thickness of 55+++ m (reduction ratio: 39%), finish rolling was performed by tandem rolling with 7 stands. The hot-rolled steel strip was then made into a product by a conventional two-step cold rolling method. As shown in FIG. 3, by containing 0.004% or more of sb, it is possible to drastically reduce the incidence of linear patterns.

(Example) C obtained by continuous casting on backing: 0.050%, Si
: 3.25%, Mn: 0.078%, S: 0.
020%, Sn: 0.10%, Cu: 0.08
%, AI! , : 0.025%, N : 0.008
A slab of 200 threshold thickness containing 3% and Sb: 0.009% was charged into a gas combustion furnace and heated to 1200°C, and then immediately raised to 1400"C in an induction heating furnace and held for 20 minutes. After that, hot rolling was performed.At this time, the atmosphere in the induction heating furnace was N2 gas containing 3% 0□.

In addition, hot rolling is 140 m at 200°C - 1350°C.
m thickness (reduction rate 30%, same below) → 80 at 1280℃
mm thickness (43%) → 40 mm thickness at 1200℃ (50
%), and then a 2.6M thick hot-rolled steel strip was obtained using a 7-stand finishing mill. After pickling, the hot rolled steel strip was cold rolled to a thickness of 1.50 mm, annealed at 1150°C for 2 minutes, rapidly cooled, and then cold rolled at 150°C to form a 0.23 mm plate. 840℃ after thickening
After 3 minutes of decarburization annealing, the surface of the steel plate is After applying an annealing separator mainly composed of IgO, it was heated at 120°C in hydrogen.
Finish annealing was performed for 0''C x 20 hours.On the other hand, as a comparative example, C: 0.055%, obtained by continuous casting.
St: 3.30%, Mn: 0.080%, S
: 0.021%, Sn: 0610%, Cu: 0.09
%, 8Z: 0.025% and N: 0.008
A 200 mm thick slab containing 3% was processed in the same manner as above to produce a product steel strip. Table 3 shows the results of investigating the linear pattern occurrence rate and magnetic properties of both.

C20.040%, Si obtained by continuous casting:
3.35%, Mn: 0.080%, Se: 0.
024%, Sb: 0.018% and Mo: 0
．． Two 210 mm thick slabs containing 0.010% were made into a 180 aun thick slab by blooming recompression, and then
After heating to 200°C in a gas combustion furnace, the temperature was immediately raised to 1420"C in an induction heating furnace, and after soaking for 40 minutes, each slab was subjected to the following hot rolling. That is, one slab was , 180 rm thickness → 150 at 1320℃
mm thick (17%) -+80 mm thick at 1280°C (47%) → 60 mm thick at 1195°C (25%) → 1
After rough rolling to 45mm thick (25%) at 060℃,
A hot-rolled steel strip with a thickness of 2.2mm (conforming example) is made by finish rolling, and the other slab is 180mm thick → 135mm thick at 1320°C (25%) → 100mm thick 1 at 1270″C ( 2
6%) → 75mm thickness at 1180'C (25%) → 11
60mm thick at 00℃ (20%) → 45m at 1060℃
After rough rolling (25%) to m thickness, 2.
A hot-rolled steel strip (comparative example) with a thickness of 2 mm was used.

Both were then normalized at 1000°C for 1 minute and then normalized at 950'C with an intermediate thickness of 0.60mm.
After performing intermediate annealing for 1 minute, the plate was cold rolled to a final thickness of 0.23 mm.

After decarburization annealing in wet hydrogen at 800"C for 3 minutes, an annealing separator containing MgO as a main component was applied to the surface of the steel plate, and final annealing was performed in hydrogen at 1200°C for 15 hours. Ta.

Table 4 shows the results of examining the linear pattern occurrence rate and magnetic properties of the product thus obtained.

Table 4 C obtained by continuous casting: 0.060%, Si
: 3.35%, Mn: 0.080%, Se: 0
．． 020%, Sb: 0.020%, Cu: 0.
08%, Mo: 0.015%, A1: 0.0
25% and N: 220 containing 0.0076%
A slab with a thickness of ffI [Il is
After heating to 0°C, it was immediately placed in a slab induction heating furnace and heat-treated at 1450°C for 20 minutes. At this time, the atmosphere in the induction heating furnace was N2 and contained 6% 02.

Next, the heated slab was heated to a thickness of 220 mm → 11011II11 thickness at 1310°C (50%) → 60 mm at 1270°C.
mm thickness (45%) → 40 mm thickness at 1120℃ (33
%), and then processed into a 2.7 mm JVO hot-rolled steel strip using a 7-stand tandem rolling mill, followed by pickling.
4 The intermediate plate thickness was TM1, and after intermediate annealing at 1050°C for 2 minutes, it was rapidly cooled by mist cooling. After that, the final thickness of the steel plate is 0.20 mm by cold rolling, and after decarburization annealing in wet hydrogen, the surface of the steel plate is An annealing separator containing IgO as a main component was applied, and final annealing was performed at 1200° C. for 20 hours. The linear pattern occurrence rate of the product obtained was 2%.

C: 0.035%, Si obtained by continuous casting
: 3.05%, Mn: 0.075%, S: 0.01
215 mm containing 7% and Sb: 0.014%
After a thick slab was heated to 1180°C in a gas combustion furnace, it was immediately charged into a slab induction heating furnace into which N2 gas was introduced and heat treated at 1440°C for 30 minutes. 70 soaking time for slab
The slab was heated in the same manner except for a minute.

When heating both by induction, the 02 concentration in the furnace was set to 15%. After heating the slab, both have a thickness of 215 rrrm → 13
160 mm thick at 50℃ (26%) →1280'c
95mm thick at (41%) → 75mm thick at 1200℃ (21%) → 60mm thick at 1130℃ (20%) → 1
The steel strip was roughly rolled to a thickness of 45 mm (25%) at 080'C, then made into a hot rolled steel strip of 2.6 InIn thickness using a 7-stand tandem rolling mill, and then pickled to a thickness of 0.8. After making the intermediate thickness of 0.30 mm and performing intermediate annealing at 950°C for 2 minutes,
Cold rolled to a final thickness of mm and rolled at 840”C to 2
After performing decarburization annealing for 1 minute, an annealing separator containing MgO as a main component was applied to the surface of the steel sheet, and then final annealing was performed at 1200° C. for 10 hours. Table 5 shows the results of examining the linear pattern occurrence rate and magnetic properties of the product thus obtained.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the slab heating temperature in an induction heating furnace and the surface texture of the product.
Graph showing the gold content; Figure 2 is a metallographic photograph showing that the appearance of the surface layer oxide changes depending on the presence or absence of sb when the slab is heated in an induction heating furnace; Figure 3 is the content of sb. It is a graph showing the relationship between the linear pattern occurrence rate and the linear pattern occurrence rate. Patent Applicant: Kawasaki Steel Corporation (Effects of the Invention) According to this invention, the deterioration of the surface quality of the product resulting from the internal heat generation type heating at the flat temperature of the silicon steel slab can be advantageously avoided, thereby improving the quality of the product. It is possible to produce silicon steel sheets. (b)

Claims (1)

[Claims] 1, Si: 2 to 4.5 wt% and Sb: 0.004 to
A silicon steel slab containing 0.080wt% was heated to a temperature of 1400°C or higher in an internal heat-generating heating furnace into which non-oxidizing gas was introduced, and subjected to soaking treatment within this temperature range for 60 minutes, and then soaked at 1250°C. Hot rolling including rough rolling is performed at least one pass of rolling with a rolling reduction of 40% or more at a temperature of ℃ or higher, followed by cold rolling once or twice with intermediate annealing, and then decarburization. A method for producing a grain-oriented silicon steel sheet with excellent surface properties and magnetic properties, which comprises annealing the steel sheet, applying an annealing separator to the surface of the steel sheet, and then subjecting the steel sheet to final annealing.