JPS6231050B2 - - Google Patents

Description

[Detailed description of the invention]

In particular, in the manufacturing process of unidirectional silicon steel sheets containing Al, the present invention involves completing secondary recrystallization in high-temperature short-time annealing after decarburization annealing, or further performing high-temperature annealing after that. The present invention provides a method for stably manufacturing a unidirectional silicon steel plate having an extremely high magnetic flux density. The (110) [001] texture, which is the crystal structure of the Al-containing unidirectional silicon steel sheet generally used in the present invention, is
It is obtained by the secondary recrystallization phenomenon during the final annealing process after decarburization annealing. This final annealing is usually done in a batch type in the form of a coil, and the magnetic flux density is _extremely high.
Various proposals have been made to obtain characteristics greater than 1.90. For example, as described in JP-A-52-78615, box annealing is carried out under various conditions, such as a method in which the nitrogen content of the atmospheric gas during final annealing is limited to a certain range and secondary recrystallization is carried out. Although the technology has been disclosed, since the heating rate changes depending on the position of the coil, that is, the inside and outside of the coil, and the condition of the atmosphere inside the furnace changes, it is difficult to manufacture a silicon steel plate with a uniformly high magnetic flux density in the longitudinal direction of the coil. This requires extremely sophisticated atmosphere control and temperature rise control technology. On the other hand, if secondary recrystallization can be performed using a continuous annealing method, uniform heating conditions and atmospheric conditions can be provided to any position of the coil. A specific method of secondary recrystallization using a continuous annealing method is disclosed in Japanese Patent Application Laid-Open No. 49-98721,
Although it is described in Japanese Patent Application Laid-Open No. 49-95816,
With these methods, the obtained magnetic flux density is 1.90T or less, and the main objective is to produce unidirectional silicon steel sheets at low cost and stably, although the quality is inferior to box annealing. It was hot. Furthermore, JP-A-51-10119 discloses a treatment method in which decarburization annealing is followed by continuous annealing in a high temperature range, but this is for the purpose of stabilizing the formation of secondary recrystallization in the subsequent finish annealing. I have it too. The present invention is based on a viewpoint completely different from the prior art described above, and is based on heat treatment and atmospheric conditions for short-time annealing after decarburization annealing, for example, continuous annealing (the short-time annealing of the present invention will be explained below using an example of continuous annealing). Through proper control of the process, surface crystal grains are developed during the continuous annealing process, and secondary recrystallized grains are generated, thereby providing a method to stably obtain an extremely high magnetic flux density, that is, a value of _B8 of 1.90 or more. be. In addition to developing secondary recrystallized grains, the purpose of conventional final annealing is to form a glassy insulating film through the reaction between the oxide film on the surface of the steel sheet and the annealing separator, MgO, and to eliminate impurities in the steel sheet. This included the removal of In order to satisfy these objectives, the present invention further adds high-temperature annealing in a simple cycle at a temperature of 1100°C or higher, such as box annealing, after the continuous annealing process, which is extremely stable and extremely stable compared to conventional methods. This makes it possible to manufacture unidirectional silicon steel sheets with high magnetic flux density. When this high-temperature annealing is performed for the purpose of dulling and forming an insulating film, it is not necessary to pay particular attention to the temperature increase rate, the _N2 partial pressure of the atmospheric gas, and the dew point as in the past, making the work easier. Therefore, the annealing time can be reduced to less than 1/3. The present inventor has obtained the following new knowledge as a result of various studies on secondary recrystallization behavior due to continuous annealing. That is, when we investigated the distribution of crystal grains in the thickness direction of the plate immediately before the start of secondary recrystallization during the continuous annealing process, we found that the crystal grains in the surface layer that had grown as shown in Figure 1a were As shown in Figure a', the orientation of the secondary recrystallized grains is improved and the magnetic flux density _B8 is high. On the other hand, as shown in Fig. 1b, the crystal grains in the surface layer do not grow and undergo secondary recrystallization, as shown in Fig. 1b'.
It was found that the secondary recrystallization orientation was poor and _B8 was also low. Based on this new knowledge, the present invention has completed a method of controlling the temperature and atmosphere of continuous annealing to promote grain growth in the surface layer and then performing secondary recrystallization. The present invention will be explained in detail below. In the present invention, the cold-rolled steel sheet obtained in a normal process can be obtained, for example, as follows. That is, first, molten steel refined by a conventional method is formed into an ingot by a continuous casting method or a steel ingot method, and a slab obtained by these methods is made into a hot rolled sheet by a conventional method. The components of the hot rolled sheet are C≦0.08% and Si≦4 in weight%.
%, Al _sp1 0.01 to 0.065%, and Mn and S as impurities. It may also contain trace components such as Sb, Se, V, Cr, Ni, and Cu as inhibitor components. The hot-rolled sheet is annealed if necessary, and then cold-rolled by a one-step method or a two-step method (including intermediate annealing) to obtain the final thickness. In the present invention, a cold-rolled sheet is obtained by such a conventional process, and a suitable amount of AlN is precipitated in the steel.
As described above, the present invention is characterized in that, after decarburization annealing, continuous annealing is performed at a relatively high temperature, and the desired structure is generated by this continuous annealing. The Al-containing unidirectional silicon steel of the present invention uses AlN precipitates and auxiliary MnS precipitates as inhibitors, and also uses conventionally used inhibitors such as grain boundary segregation type elements such as Sb. However, if the strength of the inhibitor is too strong, secondary recrystallization will not occur, and if it is too weak, so-called normal grain growth will occur and secondary recrystallization will not occur. According to the present invention, secondary recrystallization is generated by continuous annealing, but the amount and size of AlN precipitated in the secondary recrystallization annealing are determined by the temperature of the secondary recrystallization annealing and the PN ₂ /PH ₂ , PH ₂ O / in the atmosphere gas.
Varies significantly with PH ₂ . First, it is necessary to coarsen the crystal grains in the surface layer of the steel sheet without causing secondary recrystallization in the first half of the continuous annealing process.To do this, the partial pressure of _N2 in the atmosphere must be set to 10% or more, and the temperature must be set to 1150℃. Must be as follows. When the N ₂ partial pressure is set to 10% or less, secondary recrystallization occurs near the outermost surface layer before the crystal grains in the steel sheet surface layer grow coarsely, resulting in a (110) [001] secondary recrystallized structure with a low degree of integration. becomes. If the N ₂ partial pressure is too high, high-temperature and long-time processing must be performed, but in order to perform short-term continuous annealing at 1150°C to be industrially advantageous, the N ₂ partial pressure should be less than 50%. There is a need. The reason why the temperature was set at 1150°C or less is that at temperatures higher than this, the amount of precipitated AlN, MnS, etc. in the steel sheet decreases rapidly. This is because crystals are formed. The reason for setting the temperature above 950°C is that at temperatures below this, even if the _N2 partial pressure is appropriately controlled, it will take a long time to coarsen the crystal grains in the surface layer, which is industrially disadvantageous. be. Further, it is more effective to adjust the humidity in an atmosphere containing 10% or more of N ₂ so that the dew point of the atmosphere is within the range of 0°C to 60°C. in the atmosphere
By injecting H ₂ O, Si and Al on the surface of the steel sheet are selectively oxidized, and this oxide film forms a surface layer that makes it difficult for N to diffuse, reducing absorption of N from atmospheric gas. This is because the amount of AlN precipitated in the surface layer decreases as the concentration of Al decreases, and the inhibitor effect of the surface layer begins, promoting coarsening of crystal grains in the surface layer. By maintaining the temperature and atmospheric conditions described above, it is possible to coarsen only the crystal grains in the surface layer without causing secondary recrystallization by annealing for a short time, preferably within 10 minutes. In other words, the concentration of AlN is reduced only in the surface layer, causing the crystal grains in the surface layer to grow coarsely, but in areas other than the surface layer, the inhibitor effect is strong and it is possible to create a situation in which secondary recrystallized grain growth does not occur. I can do it. The optimum range of annealing temperature, atmospheric _N2 partial pressure, and dew point is determined by the precipitation contained in the steel sheet after primary crystal annealing.
Needless to say, it must be changed depending on the amount of AlN, free Al, and free N. That is, AlN or
If the amount of Al or N is small, secondary recrystallization becomes easier, so it is necessary to relatively increase the _N2 partial pressure to cause grain growth without secondary recrystallization.AlN or Al, If the amount of N is large, it is necessary to actively reduce the AlN in the surface layer by increasing the temperature, reducing the relative N ₂ partial pressure, blowing H ₂ O, etc. It is. Next, in the material in which the crystal grains in the surface layer have become coarse, secondary recrystallized grains grow in a short period of time in the latter half of continuous annealing. As mentioned above, in order for secondary recrystallized grain growth to occur, the strength of the inhibitor must be within a certain range. The reason why secondary recrystallized grains do not grow during the short-time annealing in the first half is because the strength of the inhibitor is too strong, and the strength of this inhibitor can be weakened in the second half. Based on this knowledge, we conducted various experiments and found that the annealing atmosphere for secondary recrystallization in the second half should be made with a lower atmospheric _N2 partial pressure than in the first half.
It was found that by lowering the N ₂ partial pressure, secondary recrystallized grain growth occurred because the concentration of AlN gradually decreased from the surface layer toward the center layer. In this case, it has been found that it is more effective to uniformly lower the relative strength of the inhibitor and to slightly increase the temperature in the latter half than in the first half in order to more easily cause secondary recrystallization. When a material in which the crystal grains in the surface layer have become coarse (at least three times the crystal grain size in the center layer) is subjected to secondary recrystallization, most of the difference between the crystal grain size in the surface layer and the crystal grain size in the center layer is The reason why the crystal orientation is better compared to the case of secondary recrystallization of a material without any material has not been essentially investigated yet. There is a limit to the thickness effect of this coarse grain layer, approximately 100μ
When it is more than m, there are almost no crystal grains with the (110) [001] orientation, which serve as nuclei for secondary recrystallization, so they become so-called "fine grains" and the magnetic flux density decreases significantly. Further, according to the present invention, for example, box annealing type annealing (hereinafter referred to as final annealing) of a secondary recrystallized steel sheet, which is performed as necessary, will be described. The magnetic flux density of the steel sheet secondary recrystallized by the method of the present invention is about 1.90 to 1.94 T, which is significantly higher than that of the material secondary recrystallized by the conventional continuous annealing method. However, in this state, grains first grow on the surface layer of the steel sheet (110).
In some cases, orientations other than the [001] orientation remain uneaten by the (110) [001] orientation in which secondary recrystallized grains have grown, and the magnetic flux density is about 300 Gauss low. If a method such as increasing the continuous annealing time or increasing the temperature is adopted, these crystal grains will be eaten by (110) [001] crystal grains during the continuous annealing process, and
Although it is possible to completely eliminate the crystal grains, it is more effective to perform the final annealing using box-shaped annealing, which can eliminate crystal grains on the surface and purify them, than continuous annealing. The final annealing performed in the present invention is to remove N and S that are harmful to the magnetic properties of the steel sheet, and to improve the surface of the steel sheet.
Since SiO ₂ and MgO are reacted to create an insulating glass film, the heating rate has no effect on magnetism, and N and S can be diffused and removed from the steel sheet. There are no particular restrictions, except that the soaking temperature should be 1100°C or higher and the partial pressure of _H2 in the atmosphere during soaking should be 100%. Batch
Generally, annealing is performed in a type of annealing furnace, but in the present invention, temperature increase rate, dew point control, and atmosphere control are unnecessary, so by using a tunnel furnace, etc.
Continuous annealing is possible while maintaining the coil form, and its economical effects are tremendous. Next, the present invention will be explained based on examples. Example 1 C: 0.050%, Si: 2.91%, acid soluble Al: 0.029
%, S: 0.023%, Mn 0.08% by hot rolling a 200 mm thick slab manufactured by the continuous casting method.
A hot-rolled sheet with a diameter of 1 mm was prepared and annealed at 1100°C for 5 minutes. Then, after cold rolling to 0.30mm, 850
Decarburization annealing was performed in a wet hydrogen atmosphere for 4 minutes at ℃, followed by continuous annealing under 4 conditions of A, B, C, and D. MgO was applied and the temperature was rapidly raised at 200℃/hr.
A final blunting annealing was performed at 1200°C for 20 hours.

[Table] When the distribution of crystal grains in the sheet thickness direction was investigated in the first half of continuous annealing under condition A, no growth of crystal grains in the surface layer was observed as shown in FIG. 1b. When switching to 100% H ₂ in the latter half of continuous annealing, 2
After recrystallization, no remaining crystal grains were observed in the surface layer. _B8 before final purification annealing is
_B8 was 1.885T, which was relatively poor even after purification annealing. When the distribution of crystal grains in the thickness direction during the first half of continuous annealing under condition B was investigated, crystal grain growth in the surface layer was observed as shown in Figure 1a, but secondary recrystallization had not yet occurred. When this was switched to 100% H ₂ , secondary recrystallized grain growth started immediately, but some of the crystal grains that had become coarse in the surface layer in the first half remained uneaten. _B8 at that time was extremely high at 1.945T. When it was further purified and annealed, all of the crystal grains in the surface layer were eaten by (110) [001] grains, so _B8 improved to 1.975T. When the distribution of crystal grains in the sheet thickness direction during the first half of continuous annealing under condition C was examined, growth of crystal grains in the surface layer was observed, but compared to condition B, the depth was shallower. When the cross-sectional crystal grain distribution of the plate was examined after the second half of continuous annealing, some remaining grains were observed in the surface layer and center layer, but secondary recrystallized grain growth had almost been completed. When we measured the magnetic flux density at this time, we found that
It was 1.920T. After the final purification annealing, there were no uneaten grains and _B8 was 1.950T. Examination of the crystal grain distribution in the cross section of the plate after continuous annealing under condition D revealed that the entire thickness of the plate was made up of crystal grains with (110) [001] orientation, and there were no crystal grains left behind in the surface layer. The parts other than the surface layer are left uneaten,
Some nearly circular crystal grains other than (110) [001] were observed. The magnetic flux density was measured at B ₈ .
It was 1.752T. After purification annealing, it improved slightly to 1.767T. It can be seen from the above that by growing the crystal grains in the surface layer before starting the secondary recrystallization and then performing the secondary recrystallization, the orientation becomes better and the magnetic flux density (B ₈ ) becomes higher. Example 2 C: 0.045%, Si2.95%, acid soluble Al: 0.020
After blooming and hot rolling a steel ingot containing S0.027% and S0.027%, a 2.3 mm hot-rolled plate was made, which was annealed at 1120°C for 5 minutes and then quenched in hot water at 100°C. . Subsequently, the steel sheets were cold rolled to 0.3 mm and decarburized annealed at 850°C for 4 minutes in a wet hydrogen atmosphere. Four steel plates A, B, C, and D were prepared and treated as follows. After applying MgO, A and B were heated at a heating rate of 20°C/hr, and B was heated at a rate of 200°C/hr to 1200°C for 20 hr.
Annealing was performed. C is 100% H ₂ DP<-10 annealed at 1100℃ for 10 minutes
The test was carried out in a hydrogen atmosphere at ℃. D is 1100℃ for 5 minutes75
Annealing is performed in an atmosphere of % _H2-25 % _N2DP <-10℃, followed by annealing at 1100℃ for 5 minutes at 100% _H2DP <-10℃.
The experiment was carried out in a hydrogen atmosphere. Next, MgO in C and D
was coated, heated at 200°C/hr, and annealed at 1200°C for 20 hours. The magnetic properties of each sample are shown below.

【table】

[Table] In this way, D has a heating rate of 200℃/hr for box annealing.
Despite this speed, the magnetic flux density is significantly higher than that in normal process A. _B8 is even lower in C than in B. This is because in the case of B, there is secondary recrystallized grain growth during the box annealing process.
While the surface layer undergoes secondary recrystallization after grain growth, continuous annealing was performed in a 100% H ₂ atmosphere. Condition C is continuous annealing, and since there is no room for grain growth in the surface layer and secondary recrystallization occurs immediately, the orientation deteriorates despite continuous annealing and secondary recrystallization.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the crystal grain distribution A in the cross-sectional direction of the plate before the start of secondary recrystallization in the continuous annealing process and the crystal orientation after secondary recrystallization.
a′ is an example in which the surface layer undergoes secondary recrystallization a′ after grain growth, and the magnetic flux density is extremely good; b and b′ are examples in which there is no grain growth in the surface layer, b secondary recrystallization occurs, and b′ has a poor magnetic flux density. It is a micrograph diagram of the metal structure shown.

Claims (1)

[Claims] 1 A cold-rolled steel sheet obtained through a normal process in the production of Al-containing unidirectional silicon steel sheet is decarburized and annealed, and then
When performing continuous annealing in the temperature range of 950℃ to 1150℃, first, annealing is performed in a mixed atmosphere of N ₂ + H ₂ with a partial pressure of N ₂ of 10% or more, and the crystal grain size of the surface layer of the plate cross section is adjusted to the crystal grain size of the center layer. After grain growth from the diameter, it is subsequently annealed in a mixed atmosphere of H ₂ and N 2 or H ₂ atmosphere with a lower N ₂ partial pressure to cause secondary recrystallization, _and then subjected to purification annealing. A method for producing an Al-containing unidirectional silicon steel sheet with extremely high magnetic flux density.