JPH03115525A - Production of grain-oriented silicon steel sheet excellent in magnetic property - Google Patents

Abstract

PURPOSE:To obtain a fine and uniform structure and to provide uniform and superior magnetic properties free from linear fine grains by applying hot rolling to a silicon-containing steel slab after raising the temp. in a position at a specific depth from a surface layer by means of induction heating under specific conditions. CONSTITUTION:Prior to the hot rolling of a silicon-containing steel, a slab is subjected to induction heating at 1380-1470 deg.C average temp. under the conditions satisfying relationships in an inequality. By the above heating, a temp. in a position at a depth of 1/10 from a surface layer is regulated so that it is higher by 15-50 deg.C than the temps. in the central part and the outermost surface layer. In the successive hot roughing stage, a crystalline structure is controlled in a thickness direction and grain growth in the abnormal grain growth of equiaxed crystals in the center of thickness is inhibited. By this method, a grain-oriented silicon steel sheet having a grain size distribution uniform in a sheet-thickness direction and also having superior magnetic properties can be obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing an electrical steel sheet with excellent magnetic properties, and in particular, by adding innovation to slab heat treatment,
The crystal structure in the thickness direction is controlled in the hot rough rolling process, thereby advantageously improving the magnetic properties.

(Prior Art) As is well known, grain-oriented silicon steel sheets are used as core materials for transformers and other electrical equipment, and have masks called Goss grains with (110) planes and <001> axes aligned in the rolling direction. It is composed of secondary recrystallized grains.

In order to develop secondary recrystallized grains with such crystal orientation, fine Mn5I MnSe+ called inhibitors are required.
It is necessary to disperse precipitates such as AIN in steel and effectively suppress the growth of crystal grains other than the Goss orientation during high-temperature finish annealing. For this purpose, the inhibitor dispersion form is controlled by once solidly dissolving these precipitates during slab heating prior to hot rolling, and then hot rolling with an appropriate cooling pattern.

Here, the role of hot rolling is to refine the slab casting structure by recrystallization and obtain an optimal texture for secondary recrystallization. Conventional techniques attempt to achieve solid solution of inhibitors or microstructural refinement individually, and many related techniques have been proposed to date.

For example, regarding inhibitor solid solution, JP-A-63-1
As disclosed in Japanese Patent Application No. 0911, when the slab surface temperature is kept in the temperature range of 1420 to 1495°C for 5 to 60 minutes, at 1320″C or higher,
It is said that by increasing the temperature at a rate of 8° C./min or more until the temperature reaches 495° C., a homogeneous silicon steel sheet with few surface defects and good properties can be obtained.

This method certainly makes it possible to achieve complete solid solution of the inhibitor, and in principle also suppresses the coarsening of slab surface grains and improves the surface quality. is difficult to achieve in practice, especially since it is impossible to completely suppress grain coarsening over the entire length of the slab, and some grain refinement during hot rolling is necessary to ensure uniformity of the structure. It is necessary to take action.

On the other hand, regarding microstructure, for example, Japanese Patent Application Laid-Open No. 54-12
Method by recrystallization high pressure rolling at 1190 to 960'C disclosed in Japanese Patent Application Laid-open No. 1191/1983
The method of rolling with a high reduction of 30% or more at 1230 to 960°C in a state containing 3% or more of γ phase disclosed in '26, and the start of rough rolling disclosed in JP-A-57-11614. Method for lowering the temperature to 1250°C or less, JP-A-59
-1050 to 1200°C disclosed in Publication No. 93828
A method is already known in which the strain rate is 15 s-' or less and the rolling reduction is 15%/bath or more. All of these have in common that they are rolled under high pressure in a temperature range of around 1200° C. to refine the structure. In other words, these are all "Tetsu to Hagane J 67 (1981) S
It is based on the knowledge regarding the recrystallization limit published in 1200, or the same technical idea. Figure 1 illustrates this finding. This figure shows that rolling at high temperatures does not contribute to recrystallization at all, and only large strain addition in the recrystallization region at low temperatures contributes to recrystallization. In other words, this shows that even with a slab heated to a high temperature, it is essential to cool it to 1250° C. or lower and then roll it in order to achieve microstructural refinement through recrystallization. In all of these techniques, the heating temperature is set at 1250° C. or higher, and no upper limit is specified. They are common in that by holding the slab in a furnace for a long time, the inhibitor is dissolved into solid solution, allowing slab grain growth to some extent, and refining by hot rolling.

However, when considering the reality of these technologies, heating the slab at high temperatures to completely dissolve the inhibitor requires a cooling device on the hot strip mill, and additional mill power is required for low-temperature hot rolling. This contradicts the idea of a hot strip mill, which aims to save energy and increase productivity. Furthermore, the effects of low-temperature rolling were not always clear.

In other words, the effectiveness of these methods was small and too many problems remained to be applied to actual processes.

(Problems to be Solved by the Invention) This invention advantageously solves the above-mentioned problems by making the most of the mass-productivity advantage of a hot strip mill and applying high-temperature heating that is advantageous for complete solid solution of the inhibitor. The purpose of the present invention is to propose an advantageous manufacturing method for grain-oriented electrical steel sheets that reliably obtain a completely fine and uniform structure even under such conditions and have uniform and excellent magnetic properties without linear fine grains.

(Means for Solving the Problems) That is, the present invention heats a silicon-containing steel slab, then hot-rolls it, then cold-rolls it once or twice with intermediate annealing in between, and then decarburizes and decarburizes it. In a method for producing a grain-oriented silicon steel sheet, which consists of a series of steps of primary recrystallization annealing and final annealing, the slab heating prior to hot rolling is performed by induction heating, and the slab average temperature is in the range of 1380 to 1470 ° C. By heating under conditions that satisfy the following relational expression, the temperature at the 1,710-thickness position from the surface layer is raised 15 to 50 degrees Celsius higher than the temperature at the center and the outermost layer, and crystallization occurs in the subsequent hot rough rolling process. This is a method for producing electrical steel sheets with excellent magnetic properties, which involves controlling the structure in the thickness direction.

Note 20 ≦ x −V′; “7τ〒+273) ≦ 60
(14Q/P) Here, Explain in detail.

As a result of numerous experiments and studies on recrystallization behavior in high-temperature ranges, the inventors have found that the amount of strain is sufficient even in high-temperature ranges, which have traditionally been considered to be strain recovery regions and have not been the subject of research at all. We have newly discovered that if the size is large, recrystallization will proceed sufficiently.

There have been no reports on this point so far. This is because high-temperature heating has been extremely difficult in an industrial setting, and even in the laboratory, high-temperature rolling requires heating to high temperatures, but it is difficult to prevent scale formation and repair of experimental furnaces. This is because there were problems such as these, and it was extremely difficult. There are many experimental reports on ordinary steel, but 12
The high temperature region of 00° C. or higher is considered to be a dynamic recovery region where recovery or dynamic recrystallization is the main activity, and further studies have not been conducted sufficiently. Especially in the case of grain-oriented silicon steel sheets,
Since it contains about 3 wt% (hereinafter simply expressed as %) of Si, most of it is α phase, and this α phase is said to be easy to recover, so dynamic recrystallization will not occur.
It was not a subject of interest at all.

However, the inventors had doubts about the above-mentioned conventional wisdom, developed a heating furnace capable of ultra-high temperature heating with little influence of scale, and after conducting various experiments, discovered the above-mentioned phenomenon for the first time. .

Below, the experimental results that led to this invention will be explained.

c: o, o4%, Si: 3.36%, Mn: 0
．． A silicon steel slab containing 0.05% and 70.022% Se, with the remainder being essentially Fe, was heated at 1350″C.
The specimens were heated for 30 minutes at a predetermined temperature, and when the temperature reached a predetermined temperature, they were rolled for one pass at various rolling temperatures and reduction ratios, and after cooling with water, the cross-sectional structure was observed and the recrystallization rate was measured.

The survey results thus obtained are shown in Figure 2.

As is clear from the figure, even at high temperatures such as 1350°C, where conventional knowledge suggests that no recrystallization occurs, 30°C
It was found that recrystallization progresses if the rolling reduction is % or more.

This phenomenon can be understood as follows. First, it was observed that subgrains consisting of a coarse network-like dislocation structure were formed within the unrecrystallized grains after rolling. Therefore, it is presumed that recovery is completed fairly quickly after rolling. Moreover, the roughness of this network, that is, the dislocation density, differs between crystal grains. Therefore, this difference in dislocation density is considered to be the driving force for recrystallization. At high temperatures, grain boundaries are thermally activated and become movable, and if the migrated grain boundaries have a curvature above a certain level, they can become recrystallization nuclei. The above-mentioned phenomenon confirmed that recrystallization is actually possible even in high-temperature ranges, where it was previously thought that strain would not remain to the extent that dynamic recrystallization would occur. However, the driving force for this recrystallization behavior is very small because the dislocation density in the unrecrystallized region is low as described above. However, when the mobility of grain boundaries is very high, that is, when the temperature is high (1280° C. or higher), sufficient recrystallization becomes possible, although it takes some time.

This phenomenon is quite different from the conventional well-known static recrystallization.

The point described so far is the recrystallization mechanism in the case of rolling in a temperature range of 1300° C. or higher with 3% 5ijil, that is, in the α phase single phase state, and this is a point that has been clarified for the first time.

On the other hand, the reason for the recrystallization limit line shown in Figure 1, which was previously known for 3% silicon steel, is that the hard γ phase precipitates and recrystallization is promoted only in its vicinity. This is the case. In other words, in the past, data was obtained from rolling experiments, but the heat treatment method before rolling was too often omitted.
It is thought that the results obtained were different from the experimental results that formed the basis of this invention. That is, conventionally, a sample solution-treated at high temperature was once cooled to room temperature and then reheated to a predetermined rolling temperature and subjected to rolling.

In this case, some γ phase is always generated in the structure, but the γ phase is preferentially generated near the grain boundaries of α grains, where recrystallization easily progresses. However, even in this case, if the original grain size is coarse, such as slab cast grains, recrystallization is difficult to complete, and unrecrystallized portions tend to remain in the center of the old core. In addition, the γ phase fraction and its dispersion are determined not only by temperature but also by
It also depends greatly on the amount of strain, amount of strain, and cooling rate (holding time). Therefore, it is thought that even a slight difference in processing conditions can significantly change the effect. This is presumed to be a major reason why the pile machine thinning effect by low-temperature hot rolling has not been stably obtained in the past. On the other hand, there is also a drawback that increasing c4 (increasing coarse carbide) makes it difficult to obtain a rolled texture with a high degree of integration in the subsequent process.

However, the recrystallization behavior in the α single phase region at high temperatures that the inventors discovered this time is different from conventional recrystallization in the presence of the γ phase at low temperatures, and does not use the γ phase as the recrystallization nucleation site.
Grain boundaries simply serve as nucleation sites, and the recrystallized grain size tends to become relatively large, so it is difficult for unrecrystallized parts to remain.
It is easy to obtain a uniform recrystallized grain structure. If there are no unrecrystallized grains at this point, no linear fine grains will appear.

Figure 2 also shows the influence of the initial grain size before rolling, and as can be seen from previous knowledge on recrystallization behavior, the larger the initial grain size, the more difficult it is to recrystallize. However, it was also found that sufficient recrystallization can be achieved by increasing the rolling temperature.

Next, the inventors investigated grain growth behavior in a case where the structure is non-uniform in the thickness direction, such as in a continuously cast slab.

As a result, the grain growth rate (grain size after growth) becomes thicker as the temperature increases, but the difference between columnar crystals (surface layer of slab) and equiaxed grains (center of slab) We obtained the knowledge that the temperature dependence is significantly different.In other words, the grain growth of columnar crystals progresses slowly as the heating temperature increases, but in the case of equiaxed crystals, grain growth progresses slowly up to a certain temperature that depends on the composition. It was found that grain growth was suppressed, and when the temperature exceeded that temperature, the growth rate increased rapidly, and in some cases it became coarser than the columnar part.Therefore, the slab was heated to the inhibitor solid solution temperature range by the usual method. In other words, even if you try to heat uniformly in the thickness direction, it is very difficult to keep the difference in grain size between the surface layer and the center constant.Therefore, even if the rolling conditions are constant until the end of rough rolling, Because the initial conditions often change in this way, it is difficult to obtain a uniform structure throughout the thickness.Furthermore, during rolling, the thermal history in the thickness direction also changes, meaning that the cooling rate is faster in the surface layer. Therefore, it can be concluded that it is difficult to obtain a uniform structure when the slab temperature is uniform in the thickness direction.

As is clear from the above-mentioned knowledge regarding high-temperature recrystallization behavior, grain growth behavior during slab heating, and temperature history in the thickness direction during rolling, in order to obtain a uniform structure in the thickness direction, temperature distribution is extremely important.

This invention was developed as a result of repeated research regarding the above points.

In other words, in this invention, when heating a silicon-containing steel slab before hot rolling, if the slab grain growth is controlled in the thickness direction by intentionally heating it to have a temperature distribution in the thickness direction, the above-mentioned effect can be achieved during hot rolling. This follows the recrystallization behavior discovered by the inventors, so if we consider the distribution of slab cooling in the thickness direction during rolling, a sheet bar with a complete recrystallized structure uniform in the thickness direction can be obtained at the end of rough rolling. Furthermore, it is intended to impart good characteristics free of linear fine particles to the product obtained through the subsequent normal steps.

Thickness temperature control during slab heating that satisfies the above requirements is difficult with the conventional continuous butcher type gas heating furnace, and was only possible with the rapid heating method using the induction heating method. be. In other words, with normal slab heating using a gas heating furnace, only the temperature distribution shown in Figure 3(a) can be obtained, so the temperature at the outermost layer becomes too high and the columnar crystals extend from the surface layer, causing the surface layer to become uniform. On the contrary, it becomes more likely to become coarse than at high temperatures.

The preferred temperature distribution in this invention is the temperature distribution shown in FIG. 3(b), and for this purpose, it is essential to use an induction heating method that can control the heating temperature in the thickness direction.

Moreover, short-term heating is more effective in suppressing the decrease in temperature difference between the surface and the center due to thermal diffusion.

Hereinafter, the configuration of the present invention will be explained in more detail.

In this invention, first, a silicon steel slab having the composition described below is charged into an induction heating furnace and heated. At this time, the solid solution temperature of the inhibitor varies somewhat depending on its type and amount, but if it is 1380''C or higher, all inhibitors can be almost completely dissolved in solid solution.
The heating was to be at a temperature of at least 1380''C.

On the other hand, if the heating temperature becomes too high, the slab may crack and begin to melt depending on the component system, so the upper limit was set at 1470'C.

However, what is important here is to give a temperature distribution in the thickness direction when heating the slab, especially from the surface layer where columnar crystals are likely to develop.
The purpose is to increase the temperature of the 10th layer and to suppress the abnormal grain growth of the equiaxed product at the center of the thickness.

Experimental results regarding this point are shown in FIGS. 4 and 5.

According to FIG. 4, the temperature difference between the outermost layer and the center portion decreases during rolling, and it can be seen that the manner in which it decreases is determined by the initial temperature difference.

According to Figure 5, the difference in initial average grain size depends on the temperature difference between the surface layer and the center, but by rough rolling, the grain sizes at the end of rough rolling can become almost equal. . Note that the average grain size in FIG. 5 is calculated from the abundance ratio of the recrystallized grain size and the unrecrystallized grain size. When looking at the average grain size, the relationship between the initial grain size and the recrystallized grain size may be reversed, and the grain size in the thickness direction may become more uniform.

As a result of a more detailed study, it was found that the uniformity of the grain size in the thickness direction as shown in this example is achieved when the temperature of the 1710 layer is higher than that of the center, although it depends on the occupancy rate of columnar crystals in the slab structure. It turned out that this was necessary.

The reason why the temperature at the 1710-thickness position from the surface layer was particularly considered here is because this position is particularly suitable as a representative position of the surface layer portion. In other words, once the temperatures at the three points of the outermost layer, the 1/10 thickness layer, and the center are determined, the temperature distribution in the thickness direction is almost uniquely determined.

In order to make the grain size uniform in the thickness direction, the temperature difference between the 1710th layer from the surface and the center must be 15 to 50"C.
It is important to do so. This is because in order to obtain a uniform structure, it is necessary that the difference in the average grain size at each position is within 20%, but as shown in Figure 6, the difference in the average grain size at each position and the central part after rolling is This is because in order to keep the difference in average crystal grain size within 20%, it is necessary to set the heating temperature difference between the two to be 15 to 50°C.

Therefore, in order to find a heating method that satisfies the above range for the temperature difference between 1/101g from the surface layer and the center and outermost layer, we investigated the temperature distribution in the thickness direction of the slab, including the slab thickness. We conducted a number of experiments by varying various factors such as heating furnace frequency, slab average temperature, slab heat radiation amount, and input power, and investigated and examined the effects of these factors. The following regression equation was obtained as a condition that can be achieved.

In other words, when the average slab temperature is in the range of 1380 to 1470'c, ) P: If heating is carried out under conditions that satisfy the relationship of input power it (kW), 17
The temperature difference between the 10th layer and the center and outermost layer is set to the desired 15~
It was possible to keep the temperature within the range of 50°C.

Here, X・~r; +273) is a measure of the skin effect (power concentration on the surface layer) in induction heating, and as this value increases, the skin effect increases and current concentrates on the surface layer. If this value is small, the skin effect will be small and the current will flow uniformly from the surface layer to the center, and the slab will be heated uniformly.

If the above temperature difference is applied during heating of the slab, there is no particular change in the subsequent rough rolling conditions, and rolling can be carried out under the normal rolling method (based on the idea of energy saving and mass production) in a hot strip mill. For example, it is possible to obtain a sheet bar that always has a uniform grain size distribution in the thickness direction.

The subsequent hot finish rolling conditions are not particularly different from normal conditions. Uniform structure before finish rolling (no unrecrystallized grains)
If the result is (α+γ)2 in the first stage of finish rolling,
Recrystallization occurs in the phase region, and microstructure can be easily achieved. Finish-rolled hot-rolled steel strips are annealed and pickled if necessary, and cold-rolled once or twice with intermediate annealing to achieve a 0.115
The final plate thickness is approximately 0.50 mm.

Next, following conventional methods, decarburization and primary crystal annealing are performed, followed by application of an annealing separator containing MgO as the main component, followed by final finish annealing consisting of secondary recrystallization annealing and purification annealing to produce the final product. shall be.

It goes without saying that a top insulating coating or the like may be applied after that.

(Function) As the silicon-containing steel that is the material of this invention, any conventionally known compositions are suitable, but typical compositions are as follows.

C: Q, 01 to 0.10% C is an element useful for the development of Goss surroundings, which results in uniform refinement of the a weave during hot rolling and cold rolling, and is an element that is useful for the development of Goss surroundings during hot and cold rolling. Addition is preferred. However, 0.1
If the content exceeds 0%, the Goss orientation will be disturbed, so the upper limit is preferably about 0.10%.

Si: 2.0 to 4.5% Si increases the resistivity of the steel sheet and effectively contributes to reducing iron loss, but if it exceeds 4.5%, cold rollability is impaired;
If it is less than 0%, not only will the resistivity decrease, but also randomization of crystal orientation will occur due to α-T transformation during the final high-temperature annealing performed for secondary recrystallization and purification, resulting in a sufficient iron loss improvement effect. is not obtained, the amount of Si is preferably about 2.0 to 4.5%. .

Hn: 0.02-0.12% Mn is at least 0.02% to prevent hot embrittlement
It is preferable to set the upper limit at about 0.12%, since too much content deteriorates the magnetic properties.

Inhibitors include so-called MnS, MnSe, and AIN inhibitors. In the case of MnS, MnSe, S
At least one selected from e, S: 0.0
05 to 0.06% Se and S are both effective elements as inhibitors that control secondary recrystallization of grain-oriented silicon steel sheets. From the perspective of securing suppressive power, at least 0.005% is required, but if it exceeds 0.06%, the effect will be impaired, so the lower and upper limits are 0.01% and 0, respectively.
．． It is preferable to set it to about 0.06%.

^In case of IN type, Al: 0.005-0.10%, N: 0.0
Regarding the range of 04 to 0.015% AI and N, due to the same reason as in the case of MnS and MnSe systems mentioned above,
It is set within the above range. Here, the above-mentioned MnS, MnS
The e system and the AIN system can each be used in combination.

As inhibitor components, the above-mentioned S, Se, AI
In addition to Cu+ Sn+ Crs Ge, Sb, Mo
, Te, Bi and P etc. are also advantageously suited, so
They can also be contained together in small amounts. Here, the preferred addition ranges of the above components are Cu, Sn, and C.
r: 0.01-0.15%, Ge, Sb, Mo
, Te, Bi: 0.005-0.1%, P: 0
．． 01 to 0.2%, and each of these inhibitor components can be used alone or in combination.

Note that the target slab is one that has been continuously cast or one that has been bloomed from an ingot, but it goes without saying that the target also includes slabs that have been continuously cast and then re-blushed.

(Example) C: 0.040%, Si: 3.30%, Mn:
0.054%, Se: 0.022% and Sb:
A continuous cast slab with a thickness of 215 mm containing 0.024% and the remainder substantially made of Fe was preheated in a continuous heating furnace and then charged into an induction heating furnace under the conditions shown in Table 1. 30 minutes after charging, the material was controlled to have a temperature distribution in the thickness direction as shown by A-D in FIG. 7, and immediately subjected to rough rolling.

After rough rolling, it is made into a sheet bar with a thickness of 30 mm, and after that, it is finished with a finishing tandem mill. A hot-rolled steel plate of 〇-thickness was used.

After annealing and pickling, this hot-rolled steel sheet is first cold-rolled, and after intermediate annealing,
Secondary cold rolling was performed to produce a product with a thickness of 0.23 mm.

Thereafter, after decarburization annealing was performed, an annealing separation agent containing MgO as a main component was applied, and a final product was obtained through secondary recrystallization and final finish annealing for the purpose of purification.

Table 1 shows the results of examining the magnetic properties and presence or absence of linear fine particles of the product obtained.

As is clear from the table, the products obtained according to the present invention exhibit good electromagnetic properties.

(Example) Slabs with a thickness of 215 mm having various compositions shown in Table 2 were preheated in a continuous heating furnace, then charged into an induction heating furnace, and heat-treated under conditions A and B in Table 3. Thereafter, it was immediately subjected to rough rolling. After rough rolling, it is made into a sheet bar with a thickness of 35ffiI11, and after that it is rolled into a 2.4m sheet bar with a finishing tandem mill.
A hot-rolled steel plate with a thickness of m. After pickling, this hot rolled steel sheet was first cold rolled to a thickness of 0.851, and after intermediate annealing at 950° C. for 2 minutes, it was second cold rolled to a product thickness of 0.30 mm. After that, decarburization annealing was performed at 820°C for 13 minutes in wet hydrogen, an annealing separator containing MgO as the main component was applied, and a final finish annealing was performed at 1180°C for 7 hours in the base wood. , and the final product.

Table 4 shows the results of examining the magnetic properties of the thus obtained product and the presence or absence of linear fine particles.

(Effects of the Invention) Thus, according to the present invention, even under high-temperature slab heating conditions, it is possible to obtain a grain-oriented silicon steel sheet that has a fine and uniform structure throughout the thickness direction and has uniform and excellent magnetic properties. I can do it.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the rolling reduction and rolling temperature on the recrystallization rate in the (α+r) two-phase region, and Figure 2 is a graph showing the relationship between the rolling reduction and rolling temperature on the recrystallization rate in the α single-phase region. A graph showing the influence of the initial grain size on the relationship with rolling temperature as a parameter. Figure 3 is a temperature distribution diagram in the plate thickness direction during slab heating. Figure 4 is a graph of the surface layer 1710 layer and center layer during hot rolling. Figure 5 is a graph showing the change in grain size in the surface layer 1/10 m and the center before and after rough rolling, and Figure 6 is a graph showing the temperature change in the surface layer 1.
Figure 7 is a graph showing the relationship between the temperature difference between the 10 layers and the center and the average grain size ratio at the repositioning. FIG. same

Claims (1)

[Claims] 1. A silicon-containing steel slab is heated, then hot rolled, and then cold rolled once or twice with intermediate annealing in between,
In the method for producing grain-oriented silicon steel sheets, which consists of a series of steps of subsequent decarburization, primary recrystallization annealing, and final finish annealing, the slab is heated by induction heating prior to hot rolling, and the slab average temperature is 1380 to 1,380. 1470℃
In the range of , by heating under conditions that satisfy the following relational expression, the temperature at the 1/10th thickness position from the surface layer is made 15 to 50 ° C higher than the temperature at the center and the outermost layer,
A method for manufacturing an electrical steel sheet with excellent magnetic properties, characterized by controlling the crystal structure in the thickness direction in a subsequent hot rough rolling process. Note 20≦X・√(ω/(T+273)≦60(1+Q/P
)) Here, X: Slab thickness (mm) ω: Heating furnace frequency (Hz) T: Slab average temperature (℃) Q: Slab heat radiation (kW) P: Input power (kW)