JPH0739610B2 - Method for producing grain-oriented silicon steel sheet with excellent magnetic properties - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an advantageous method for producing a grain-oriented silicon steel sheet having few linear fine grains and excellent magnetic properties.

(Prior Art) A unidirectional silicon steel sheet is mainly used as an iron core material of a transformer or a generator, and it is required to have a high magnetic flux density and a low iron loss.

By the way, in recent years, reflecting the strong demand for energy saving, there has been a strong demand for inexpensive supply of grain-oriented silicon steel sheets with excellent characteristics, stabilization of characteristics, and reduction of linear fine grains that partially deteriorate the characteristics. At the same time, how to reduce the manufacturing cost is an important issue.

In order to obtain a grain-oriented silicon steel sheet having excellent magnetic properties, it is basically necessary to obtain a highly integrated secondary recrystallization structure in the {110} <001> orientation, the so-called Goss orientation. In order to develop the secondary recrystallized grains in the Goss orientation, it is necessary to use a so-called inhibitor, which is a dispersed precipitation phase that suppresses grain boundary migration appropriately, and such inhibitors include MnSe, MnS and AlN.
Are commonly used. In this case, it is very important to disperse and precipitate MnSe, MnS, etc. sufficiently during slab heating prior to hot rolling, followed by hot rolling followed by cooling under appropriate conditions to finely and uniformly disperse and precipitate. Therefore, a high slab heating temperature is required for such solid dissolution separation of MnSe, MnS and the like.

If you heat the slab to a high temperature, the inhibitor will dissolve sufficiently,
It is well known that the characteristics are improved to some extent, and great efforts have been made so far in this respect. On the other hand, however, if the slab is kept at a high temperature for a long time in order to completely dissolve the inhibitor, the slab crystal structure will become extremely coarse, and secondary recrystallization defects will occur to a greater or lesser extent due to such a non-uniform structure. Is also well known. In particular, when trying to manufacture a high magnetic flux density material in which the crystal orientation is highly uniform, which has recently become the mainstream, the adverse effect of such a nonuniform structure becomes extremely large. Such defects are also called linear fine grains and are one of the defects that must be avoided as much as possible.

Therefore, efforts have been made to effectively reduce the slab structure after heating for a long time by making the slab structure finer. As a representative technique, Japanese Patent Publication Sho 52
There are techniques disclosed in Japanese Patent Publication No. -19169 and Japanese Patent Publication No. 57-41526. For example, the technology disclosed in Japanese Examined Patent Publication No. 52-19169 reduces the proportion of columnar crystals, which are particularly harmful in the solidification structure of the slab, by making the casting temperature as close as possible to the solidification temperature to form equiaxed crystallization. Is the way to do it.
Although this technique has been recognized to some extent, it is difficult to control temperature in actual operation.
Practical application was almost impossible due to low production efficiency and high casting failure rate. Also, although it is said that the effect is observed when heating and soaking at a relatively low temperature for a long time (several hours), when it is rapidly heated to a high temperature near 1400 ° C, as described later, the equiaxed crystal part In this case, the undissolved residue of the inhibitor is often observed, which rather deteriorates the characteristics.

In addition, as a grain refining technology, Japanese Patent Publication No. 54-27820
There is also a technique disclosed in the publication. This technique is particularly effective when applied to continuously cast slabs having columnar crystals, which are considered harmful, after being heated to 750 to 1200 ° C and sized by slabbing at a rolling reduction of 5 to 50%. After that, it is a method of reheating to a higher temperature to completely dissolve the in-viter again. As described above, it is said that the process of temporarily destroying the slab structure in the slab rolling prior to the heating for the inhibitor dissolution is very effective, but in the application of such technology, the slab structure is passed through the slab rolling line prior to the normal heat rolling process. However, the increase in cost and the impediment to productivity are significant. Moreover, the application of this technology is practically impossible in a factory without a slabbing facility as recently.

As described above, in the conventional technique, it is extremely difficult to simultaneously and completely achieve complete dissolution of the inhibitor and suppression of coarsening of the slab structure, which are required to obtain good properties in the grain-oriented silicon steel sheet, Even if it could be achieved to some extent, it was inevitable that new harmful effects would occur.

In particular, recently, slab heating at 1400 ° C or higher has become possible, and heat treatment at higher temperature and shorter time than before has been studied and put to practical use. In that case, the dissolution of the inhibitor should be easier in terms of equilibrium,
In fact, it was found that this is not always the case, and that the coarsening of the structure becomes more remarkable than in the conventional low temperature long-time heating. In this case, it was also found that the dissolution of the inhibitor strongly depends on the cast structure, and that there is a portion where the inhibitor is extremely difficult to dissolve, particularly in the equiaxed crystal part where the crystal grain size is small.

The inventors, as a result of repeated studies paying attention to the above phenomenon,
It was found that there is a heating method capable of satisfying both the dissolution of the inhibitor and the suppression of coarsening of the slab structure to some extent within a very narrow range, and disclosed in JP-A-63-287520. However, in the actual operation, it cannot be said that the control is always easy, and improvement thereof, that is, further relaxation of heating conditions has been desired.

On the other hand, as a technique for refining the structure by rough rolling to improve the characteristics, for example, a method by recrystallization under high pressure rolling at 1190 to 960 ° C disclosed in JP-A-54-120214, JP-A-55 is used. No. 119126, a method disclosed in JP-A No. 119126, which is disclosed in Japanese Patent Application Laid-Open No. 57-11.
The method of making the rough rolling start temperature 1250 ° C. or lower disclosed in Japanese Patent No. 4614, and the method disclosed in Japanese Patent Laid-Open No. 59-93828.
Examples include a method in which the strain rate is 15 s ^-1 or less at 050 to 1200 ° C and the rolling reduction is 15% / pass or more. These are all 1100
Rolling in a relatively low temperature range around ℃,
It is common in that the organization is made finer. In other words, all of these findings are based on the knowledge about the phenomenon that the recrystallization initiation limit is around 1100 ° C, which was announced in "Iron and Steel" 67 (1981) S 1200, or the same technical idea, in other words, high temperature rolling. It is based on the finding that it does not contribute to recrystallization at all, and only large strain addition in the low temperature recrystallization region contributes to recrystallization. That is, this data indicates that even in a slab heated at high temperature, in order to aim at the refinement of the structure by recrystallization, it is essential to perform cooling after cooling to a temperature near 1100 ° C.

In all of these hot rolling methods, it is concluded that hot rolling at around 1100 ° C is the most preferable method.

Certainly, these rolling methods are effective to some extent for the purpose of further refining the microstructure to a certain extent by hot rolling. However, under normal slab heating conditions, the grain size becomes considerably coarse with a grain size of 10 to 70 mm. In this way, when the crystal grain coarsening is large, the effective effect is not always obtained only by applying these hot rolling methods.

It is well known that, in the production of grain-oriented silicon steel sheets, it is more important to control the dispersion of the inhibitor in addition to improving the structure.

However, in the prior art, the study from the viewpoint of optimum dispersion of this inhibitor was extremely insufficient. For example, in the above-mentioned JP-A-55-119126, only the refinement of the structure is examined as is clear from the entire description,
No mention is made of the magnetic properties in which the inhibitor plays an important role. It is a well known fact that it is essential to optimize the dispersion of the inhibitor in order to obtain good magnetic properties.

Therefore, even if the conventional texture refinement technology, which does not take into consideration the inhibitor at all, is applied to the actual process, the improvement effect is not so large as expected although the process inconvenience is great. Therefore, there has been a strong demand for development of a hot rolling method for improving such difficulties.

(Problems to be Solved by the Invention) The present invention achieves an advantageous production of unidirectional silicon steel that achieves micronization of a structure and proper dispersion of an inhibitor, and enables simultaneous improvement of surface properties and magnetic properties. The purpose is to propose a method.

(Means for Solving the Problems) The present invention has been completed based on the following new findings, based on the technical idea of controlling the temperature and the structure in the plate thickness direction, which has not been considered at all in the past. is there.

That is, the steel sheet that is actually in the hot rolling process in the factory always has a temperature distribution in the sheet thickness direction. Especially in the rough rolling process, the entrance side plate thickness is 50 to 100 mm even in the final pass,
There is a fairly large temperature change. However, in the prior art, it was general to ignore the temperature distribution and consider only the average temperature for processing. The present invention intends to obtain an optimum structure and an optimum inhibitor dispersion state by effectively and positively utilizing such temperature distribution in the plate thickness direction.

By the preliminary study, the inventors clarified the precipitation curve of the inhibitor, which was conventionally unclear, and clarified the appropriate temperature range. It was also clarified that the microstructural change in the plate thickness direction, especially the microstructural change in the plate thickness direction during rough rolling was large.
As a result, when considering the change in the average temperature of the steel sheet, there is no condition that can simultaneously satisfy the microstructure and the inhibitor dispersion, but considering the temperature distribution in the plate thickness direction, there is a rough rolling condition that can satisfy both at the same time. We found that it was small but existed.

Therefore, as a result of a more detailed examination based on the above results, in a specific material component, by controlling the temperature distribution and rolling conditions during rough rolling, the intended purpose can be advantageously achieved. I got the knowledge.

The present invention is based on the above findings.

That is, the present invention includes C: 0.05 to 0.09 wt% (hereinafter simply referred to as%), Si: 2.5 to 4.0% and Mn: 0.03 to 0.10%, and at least one selected from S, Se and Al. : 0.01〜
After heating, a silicon steel slab with a composition containing 0.10% is subjected to hot rough rolling, followed by hot finish rolling, followed by one or two cold rolling steps with intermediate annealing to obtain the final plate thickness. And, in producing a grain-oriented silicon steel sheet by a series of steps of performing final finishing annealing after decarburization annealing, the final pass of the hot rough rolling is performed from the outermost layer of the steel sheet to the sheet thickness.
A method for producing a grain-oriented silicon steel sheet having excellent magnetic properties, which is performed under a condition that a temperature up to a depth of 1/5 is 1200 to 1250 ° C and a rolling reduction is 50% or more.

Hereinafter, the experimental results that are the basis of the present invention will be described.

As a result of many experiments, the inventors have found that the precipitation state of the inhibitor greatly changes depending on the temperature. The main point is that when rolled and held at a temperature of around 1000 to 1100 ° C, the inhibitor precipitates coarsely and it is difficult to obtain good properties. Also, even if good characteristics are obtained under this condition, the range is very narrow,
I also experienced that it could not be a realistic technique. Therefore, although it depends on the component system and the temperature history until hot rolling, it was found that rough rolling at a temperature of at least 1150 ° C and generally less than 1200 ° C should be avoided.

FIG. 1 shows the experimental results obtained from the above findings. The experiment in the figure shows C: 0.03%, Si: 3.0%, Mn: 0.1% and Se: 0.024.
% Vacuum-melted steel containing 20% was used as a material, and it was carried out using a research small rolling mill.

By the way, with the conventional silicon steel rough rolling technology, 1100 to 1000 ℃
It has been said that rolling at a low temperature in the vicinity is preferable. The basic technical idea is that the hard γ phase is most likely to be generated in this temperature range, and therefore strain is accumulated in the periphery thereof to promote recrystallization. Therefore, as described above, the optimum rough rolling condition is 1000 to 11 as shown in FIG.
It is said to be in the range of 00 ° C, and the conventional rough rolling conditions have been set aiming at this range. However, this range is the most unfavorable temperature range from the viewpoint of precipitation of the inhibitor, as shown in FIG. 1 above.

Therefore, in the conventional technique, if the rough rolling is performed under the condition that the crystal structure is optimized, the precipitation of the inhibitor is necessarily inadequate.

Thus, in the past, since the average temperature of the plate was used as the processing temperature, even under the rough rolling conditions that were optimized by the conventional technology, in reality, both structure improvement and inhibitor dispersion were satisfied at the same time. It could not be done, and in the actual process, either one had to be sacrificed.

From the above experience, the inventors have introduced the idea of controlling the microstructure in the plate thickness direction in order to solve the above problems. Therefore, first, the structure of the steel sheet during rough rolling was observed in detail.

A typical example of a sheet bar structure after rough rolling of an ordinary high temperature heating material is schematically shown in FIG.

As shown in the figure, in the stage of the sheet bar after rough rolling, coarse crystal grains are highly probable from the surface layer to around 1/5 thickness. On the other hand, on the center side of that, recrystallized uniformly to form fine crystal grains.

The following process can be considered as the reason for forming such an organization.

That is, the first reason is that in a continuously cast slab, columnar crystals develop near the surface layer, and the crystal grains elongate and become large. When the slab is further heated to a high temperature, its crystal grains become coarser. The slab after heating is subjected to hot rolling, in which small crystal grains are easily recrystallized, but large crystal grains are not easily recrystallized from the beginning. Therefore, the central portion of the steel sheet recrystallizes and gradually becomes finer in each rolling pass, but the surface layer does not easily recrystallize. As a result, it is considered that a very large difference in grain size occurred between the surface layer and the central portion. And such a difference in structure becomes more remarkable when the slab is heated to an ultrahigh temperature of 1400 ° C or higher as recently. Therefore, it can be said that the need for structural improvement is especially from the surface layer to the depth of 1/5.

Therefore, it does not adversely affect the precipitation of the inhibitor 1200
Various studies were conducted on a method of refining the crystal from the outermost surface to a depth of 1/5 at a temperature of ℃ or higher.

FIG. 4 shows the effects of the rolling reduction and the C content on the recrystallization rate. This figure shows the results of examining the recrystallization rate after heating 3% Si steel with a plate thickness of 50 mm to 1200 ° C., rolling it at various reduction rates with a research rolling mill, and then examining it.

From the figure, it can be seen that the recrystallization rate greatly depends on the initial grain size. When the initial grain size is 5 mm, the recrystallization rate exceeds 70% when the rolling reduction is 50% or more, and a sufficiently fine structure is obtained. However, when the initial grain size is 30 mm, the recrystallization rate is at most about 35% even if the rolling reduction is 70%.

When the slab is heated to a high temperature, especially 1400 ° C or higher, the crystal grain size tends to become very large. Actually the particle size after heating the slab
It is extremely difficult to keep it to about 5 mm, and in most cases 10
Approximately 30 mm or even 50 mm or more.

Therefore, even if the crystal grain size is large, there is a need for a technique for surely recrystallizing and making the grains fine.

Therefore, a method for reducing the grain size even when the crystal grain size is larger was examined. At this time, it was premised that the rolling was performed at a temperature of 1200 ° C or higher from the viewpoint of preventing coarse precipitation of the inhibitor.

As a result, it was found that the intended purpose can be advantageously achieved by increasing the amount of C higher than usual and by largely reducing it. That is, for example, when the C content is increased to 0.075%, the rolling temperature is 1200 ° C, the initial grain size is 40 mm, and the rolling reduction is 50.
%, A recrystallization rate of 90% or more was obtained.

The results are also shown in FIG.

Next, an appropriate range of the C content and the reduction rate at which a high recrystallization rate was obtained at a rolling temperature of 1200 to 1250 ° C was examined.

The result is shown in FIG.

C content: 0.05% or more, reduction rate: 50, indicated by the dotted line in the figure
A recrystallization rate of 75% or more was obtained in the range of 100% or more.

Therefore, in the present invention, the C content and the rolling reduction are limited to the above ranges.

It should be noted that the above effect can be obtained by increasing the C content by γ
It is presumed that the main reason is that the phases are formed with an appropriate dispersion, but in some cases the satisfactory effect could not always be obtained, probably because the recrystallization speed decreases when the rolling temperature decreases. Therefore, from this aspect, it is necessary to set the rolling temperature to 1200 ° C. or higher. However, if the rolling temperature is too high, the grain refining effect is reduced, and sufficient cooling cannot be performed before finish rolling. Therefore, the upper limit of the rolling temperature is set to 1250 ° C.

Next, a case where the above knowledge is applied to an actual rough rolling process will be considered.

Even in the final pass of rough rolling, the plate thickness on the entry side is 50 mm or more, and in some cases about 100 mm, so it is difficult to make the temperature uniform in the plate thickness direction. The conventional idea is that uniforming the temperature in the plate thickness direction leads to homogenization of the structure,
Therefore, efforts have been made to make the temperature uniform. In other words, the conventional idea is to avoid quenching to make the temperature uniform and end rough rolling at a temperature as low as possible.
As described above, in such a method of holding the high temperature for a long time during rolling, the residence time in the precipitation critical temperature region around 1100 ° C is inevitably lengthened and the inhibitor is coarsened, as a result. The characteristics could not have been hoped for.

In order to avoid such an adverse effect, in the present invention, the surface layer 1 /
The target temperature is reached, for example, by quenching to a depth of five. Within this depth range, the target temperature can be sufficiently controlled by cooling from the surface. Therefore, the effect of the present invention is that the rolling temperature from the outermost layer to the 1/5 depth at the final pass of rough rolling is set to 1200 by cooling control during rough rolling.
It can be obtained by adjusting the temperature to ˜1250 ° C. and the rolling reduction: 50% or more.

It is preferable to use a water spray positively for cooling. Of course, if the heat radiation is large and the predetermined temperature condition can be satisfied even with air cooling, that is also acceptable. However, the method of gradually cooling before the final pass to reduce the temperature of the entire sheet bar is not the most preferable. Therefore, the method in which the time from the rolling immediately before the final pass to the final pass is as short as possible and forced water cooling is the most preferable. The specific time between passes is preferably 30 seconds or less, and not 60 seconds or more.

By rolling with such a method, good characteristics were obtained, and it was possible to significantly reduce the occurrence rate of linear fine particles due to coarse particles, which was one of the defects to be eliminated as much as possible. .

(Function) In the present invention, the reason why the component composition of the raw material is limited to the above range is as follows.

C: 0.05 to 0.09% As shown in FIG. 4 above, the range of C is set to 0.05% or more, even if the desired grain size is large, at 1200 to 1250 ° C. during rough rolling ( This is intended to improve the hot rolled structure by passing through the α + γ) range, and is limited to the appropriate range. The upper limit was set to 0.009% as a range in which the characteristics did not deteriorate.

Si: 2.5-4.0% Si is effective in increasing the specific resistance of the steel sheet and reducing iron loss,
If it exceeds 4.0%, the cold rolling property is impaired, while if it is less than 2.5%, the effect of reducing iron loss is weakened, and in addition to the secondary recrystallization and final high-temperature finish annealing performed for blunting, α-γ transformation causes crystal orientation Randomization occurs and sufficient characteristics cannot be obtained.

Mn: 0.03 to 0.10% Mn must be at least 0.03% as a lower limit amount that does not cause cracking due to hot brittleness, and the upper limit does not increase the dissociation solid solution temperature of MnS or MnSe, and from slab extraction to rough rolling. In order to prevent the coarsening of the inhibitor during the time-based process, it is limited to 0.10%.

At least one selected from S, Se and Al: 0.01 ~
0.10% S, Se and Al are finely dispersed in the steel in the form of MnS, MnSe and AlN, respectively, and act as an inhibitor. At least 0.01% is required to fully develop the function as this inhibitor. is necessary. The upper limit of 0.10% is Mn
Similar to the above case, it was determined from the viewpoints of facilitating dissociation and solid solution of the inhibitor and preventing coarsening during hot rolling. When Al is used as the western part of the inhibitor, N that balances it
It goes without saying that you need a quantity. The preferred amount of N is 0.00
It is 1 to 0.065%.

In addition to the above elements, inhibitors such as Sb, Sn, As, Pb, B
Grain boundary segregation elements such as i, Cu, Mo, and B are known, and they can be used in combination.

(Example) Example 1 C: 0.080%, Si: 3.30%, Mn: 0.054%, Se: 0.022%, Sb:
A continuous cast slab containing 0.024%, Al: 0.025% and N: 0.009%, the balance of which is substantially Fe composition, and a thickness: 200 mm, is placed in a heating furnace, and the temperature is 1400 ° C, 60 in N ₂ atmosphere. Keep burning for a minute,
After the inhibitor was sufficiently dissolved, it was immediately subjected to rough rolling. Rough rolling was performed in 4 passes to finish a sheet bar having a thickness of 50 to 30 mm. At that time, various cooling conditions and rolling conditions were changed as shown in Table 1.

After that, after making a hot rolled sheet with a thickness of 2.2 mm, primary cold rolling, intermediate annealing, secondary cold rolling to a thickness of 0.20 mm, and then secondary recrystallization,
The final product was subjected to final finishing annealing for the purpose of blunting.

Table 1 shows the results of examining the magnetic properties of the product thus obtained and the incidence of linear fine particles in the coil longitudinal direction.

The generation rate of linear fine particles is shown by the ratio of the portion that must be discarded as a product defect.

As is clear from Table 1, the products obtained according to the present invention not only have excellent magnetic properties, but also have a markedly reduced incidence of linear fine particles.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain a unidirectional silicon steel sheet which can achieve the refinement of the crystal structure and the proper dispersion of the inhibitor at the same time, and thus, the generation of linear fine grains and the excellent magnetic characteristics. It can be stably obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the effects of rolling temperature and holding time on the inhibitor precipitation state, FIG. 2 is a graph showing the effects of rolling temperature and rolling reduction on the recrystallization behavior, and FIG. 3 is Fig. 4 is a schematic diagram of a cross section of a sheet bar structure after rough rolling of a normal high temperature heating material, Fig. 4 is a graph showing the influence of the rolling reduction on the recrystallization rate by using the initial grain size and the C content as parameters, and Fig. 5 is 2 is a graph showing the relationship between the amount of C and the rolling reduction that affect the recrystallization rate.

Front page continuation (72) Inventor Toshihito Takamiya 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (1)

[Claims] 1. C: 0.05-0.09wt%, Si: 2.5-4.0wt% and Mn: 0.03-0.10wt% and at least one selected from S, Se and Al: 0.01-
After heating, a silicon steel slab having a composition containing 0.10 wt% is subjected to hot rough rolling, followed by hot finish rolling, and then subjected to one or two cold rolling steps with intermediate annealing between the final sheet. When producing a grain-oriented silicon steel sheet by a series of steps of applying a final finish annealing after decarburization annealing, the final pass of the hot rough rolling is set from the outermost layer of the steel sheet to the thickness of the sheet.
A method for producing a grain-oriented silicon steel sheet having excellent magnetic properties, which is carried out under a condition that a temperature up to a depth of 1/5 is 1200 to 1250 ° C and a rolling reduction is 50% or more.