JPH04124218A - Production of grain-oriented silicon steel sheet excellent in magnetic property - Google Patents

Abstract

PURPOSE:To attain the refining of crystalline structure and the proper dispersion of inhibitor and to produce a grain-oriented silicon steel sheet excellent in surface characteristics and magnetic properties by exerting the final pass of the hot roughing for a slab of silicon steel with limited composition under specific conditions. CONSTITUTION:A slab of a silicon steel having a composition containing, by weight, 0.05-0.09% C, 2.5-4.0% Si, 0.03-0.10% Mn, and 0.01-0.10% of one or more elements among S, Se, and Al is heated and subjected to hot roughing. At this time, the final pass of the hot roughing is exerted under the conditions of a temp. wherein the range of temp. difference between the outermost layer of the steel plate and the position at a depth one-fifth the plate thickness is 1200-1250 deg.C and a reduction of area of >=50%. After hot finish rolling succeeding to the above roughing, the plate is cold-rolled once or cold-rolled twice while process-annealed between the cold rolling stages to the final sheet thickness, and then, decarburizing annealing and final finish annealing are performed, by which the above steel sheet can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an advantageous method for manufacturing a grain-oriented silicon steel sheet with few fine linear grains and excellent magnetic properties.

(Prior Art) Unidirectional silicon steel sheets are mainly used as core materials for transformers and generators, and are required to have high magnetic flux density and low iron loss.

However, in recent years, reflecting the strong demand for energy conservation, there has been a strong demand for inexpensive supply of grain-oriented silicon steel sheets with excellent properties, and it is necessary to stabilize the properties, especially to reduce linear fine grains that partially degrade the properties. At the same time, how to reduce manufacturing costs has become an important issue.

In order to obtain a grain-oriented silicon steel sheet with excellent magnetic properties, it is basically necessary to obtain a secondary recrystallized structure highly concentrated in the {110} <001> orientation, the so-called Goss orientation. In order to develop secondary recrystallized grains with Goss orientation, it is necessary to have a dispersed precipitated phase, so-called inhibitor, which moderately suppresses grain boundary movement, and such inhibitors include MnSe,
MnS and AIN are commonly used.

In this case, MnSe, M
It is very important to sufficiently dissociate MnSe into a solid solution, then hot-roll it under appropriate conditions and then cool it to finely and uniformly disperse and precipitate it.
, A high slab heating temperature is required for solid dissociation of MnS, etc.

If the slab is heated to a high temperature, the inhibitor will be sufficiently dissolved.
It is well known that the properties can be improved to some extent, and a great deal of effort has been made in this regard. However, on the other hand, if the slab is kept at high temperature for a long time in order to completely dissolve the inhibitor, the crystal structure of the slab will become extremely coarse, and secondary recrystallization defects will occur to some extent due to such a non-uniform structure. is also well known. In particular, when trying to manufacture high magnetic flux density materials with highly aligned crystal orientations, which have become mainstream these days, the adverse effects of such non-uniform structures can be particularly pronounced. Also called fine particles, they are a defective substance that must be avoided as much as possible.

Therefore, efforts have been made to effectively reduce the size of the slab structure after long-term heating by refining the slab casting structure. As a representative technology,
There are techniques disclosed in Japanese Patent Publication No. 2-19169 and Japanese Patent Publication No. 57-41526. For example, the special public official court 52-191
The technique disclosed in Publication No. 69 is a method of lowering the ratio of columnar crystals, which are considered to be particularly harmful, in the solidified structure of the slab by bringing the casting temperature close to the solidification temperature, thereby making the slab homobacterial.

Although this technology has been recognized to be effective to some extent, in actual operation, it is difficult to control temperature,
Its practical application was almost impossible due to low production efficiency and high casting failure rate. Furthermore, although some effects can be seen when heating and soaking at a relatively low temperature for a long time (several hours), as will be explained later, when rapidly heating to a high temperature around 1400°C, the bacterial fraction is reduced. However, residual inhibitor dissolution is often observed, which may actually cause property deterioration.

In addition, as a grain refining technology, Japanese Patent Publication No. 54278
There is also a technique disclosed in Publication No. 20. This technique is particularly effective when applied to continuously cast slabs with columnar crystals, which are considered harmful, and after heating to 750-1200'C, 5-50
This is a method in which the particles are sized by blooming at a rolling reduction ratio of 1,000,000, and then reheated to a high temperature to once again completely dissolve the inhibitor. Although the treatment of further destroying the slab structure through blooming rolling prior to heating to dissolve the inhibitor is considered to be very effective, in applying this technique, the blooming line must be completed before the normal hot rolling process. There is a need to pass through the system, which increases costs and impedes productivity significantly. Moreover, it is virtually impossible to apply this technology in modern factories that do not have blooming equipment.

As described above, with conventional technology, it is extremely difficult to completely dissolve the inhibitor and suppress coarsening of the slab structure, which are required to obtain good properties in grain-oriented silicon steel sheets. , and even if they were able to achieve a certain degree, they could not avoid the occurrence of new adverse effects.

In particular, recently it has become possible to heat slabs to 1400° C. or higher, and the possibility of heat treatment at higher temperatures and shorter times than before is being studied and put into practical use.

In this case, should the inhibitor be dissolved more easily in terms of equilibrium theory, but in reality this is not necessarily the case, and moreover, the coarsening of the tissue will be more significant than in conventional low-temperature, long-term heating. found. In this case, it has been found that the dissolution of the inhibitor strongly depends on the cast structure, and that there are parts in which the inhibitor is extremely difficult to dissolve, especially in bacterial parts such as small crystal grains.

As a result of repeated studies focusing on the above phenomenon, the inventors found that
It has been discovered that there is a heating method that satisfies both the dissolution of the inhibitor and the suppression of coarsening of the slab structure to a certain extent, albeit within a very narrow range, and this was disclosed in JP-A-63-287520. However, in actual operation, this control is not necessarily easy, and there has been a desire for improvement, that is, further relaxation of the heating conditions.

On the other hand, as a technique to refine the structure and improve properties by rough rolling, for example, Japanese Patent Application Laid-Open No. 54120214
The method of recrystallization and high pressure rolling at 1190 to 960°C disclosed in Japanese Patent Publication No. 119126/1984, and the method of 1150 to 10500C disclosed as preferred conditions in JP-A-55-119126.
and a method of rolling with a high reduction of 30% or more in a state containing 3% or more of γ phase, a method of reducing the rough rolling start temperature to 1250°C or less disclosed in JP-A-57-114614, and a method of Examples include the method disclosed in JP-A-59-93828 in which the strain rate is 15 s"-' or less at 1050 to 1200 DEG C. and the rolling reduction is 1596/pass or more.

All of these have in common that rolling is performed in a relatively low temperature range centered around 1100° C. in order to refine the structure. In other words, all of these are based on the knowledge regarding the phenomenon that the recrystallization initiation limit is around 11008C, as announced in Tetsu to Hagane J 67 (1981) S L200, or the same technical idea, in other words, rolling at high temperatures does not lead to re-crystallization. This is based on the knowledge that it does not contribute to crystallization at all, and only large strain addition in the low-temperature recrystallization region contributes to recrystallization. In other words, this data indicates that even in a slab heated to a high temperature, in order to refine the structure through recrystallization, it is essential to cool the slab to a temperature around 1100° C. and then roll it.

It has been concluded that for all of these hot rolling methods, 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 a somewhat fine structure by hot rolling. However, under normal slab heating conditions, the crystal grains become quite coarse, with a grain size of about 10 to 70 mm. In cases where the crystal grains are coarsened or coarsened as described above, the application of these hot rolling methods alone does not necessarily provide an effective effect.

Furthermore, it is well known that in producing grain-oriented silicon steel sheets, in addition to improving the structure, it is even more important to control the dispersion of inhibitors.

However, in the conventional techniques, studies from the viewpoint of optimal dispersion of this inhibitor have been extremely insufficient. For example, in the above-mentioned Japanese Patent Application Laid-open No. 119126/1983, as is clear from the overall content, only the microstructure is considered, and there is no mention of the magnetic properties that play an important role as an inhibitor. . It is a well-known fact that in order to obtain good magnetic properties, it is essential to optimize the dispersion of the inhibitor.

Therefore, even if the conventional fine graining technology, which does not take inhibitors into account at all, was applied to the actual process, the improvement effect was not as great as expected, although the process was inconvenient. Therefore, there was a strong desire to develop a hot rolling method that would overcome these difficulties.

(Invention or Problem to be Solved) The present invention provides an advantageous method for manufacturing unidirectional silicon steel that achieves finer structure and proper dispersion of inhibitors, and simultaneously improves surface properties and magnetic properties. The purpose is to propose a method.

(Means for solving the problem) This invention was completed based on the following new findings and the technical idea of controlling the temperature and structure in the thickness direction, which had not been considered at all in the past. It is something.

That is, a steel plate actually undergoing hot rolling in a factory always has a temperature distribution in the thickness direction. In particular, in the rough rolling process, even in the final pass, the plate thickness on the entry side is 50 to 1.00 mm, so there is a fairly large temperature change. However, in conventional techniques, it has been common to completely ignore the temperature distribution and process only the average temperature. This invention attempts to obtain an optimal structure and an optimal inhibitor dispersion state by effectively and actively utilizing such temperature distribution in the thickness direction.

Through preliminary studies, the inventors have now clarified the inhibitor's precipitation curve, which was previously unclear, and clarified the appropriate temperature range. It was also revealed that the structural changes in the thickness direction, especially during rough rolling, were large.

As a result, when considering changes in the average temperature of the steel plate, there is no condition that can satisfy the structure and inhibitor dispersion at the same time, but when considering the temperature distribution in the thickness direction, there are rough rolling conditions that can satisfy both at the same time. I discovered that it exists even in a small space.

Therefore, as a result of further detailed study based on the above results, we found that the desired objective could be achieved advantageously by controlling the temperature distribution and rolling conditions during rough rolling for specific material components. I gained knowledge.

This invention is based on the above knowledge.

That is, this invention contains C: 0.05 to 0.09 wt% (hereinafter simply indicated as %), Si: 2.5 to 4.0%, and Mn: 0.03 to 0.10%, and S , Se and AI:
After heating, a silicon steel slab having a composition containing 0.01 to 0.10% is subjected to hot rough rolling, followed by hot finish rolling, and then cold rolling once or twice with intermediate annealing in between. When manufacturing grain-oriented silicon steel sheets through a series of steps including decarburization annealing and final finish annealing, the final pass of the hot rough rolling is carried out from the outermost layer of the steel sheet to the final thickness. This is a method for producing a grain-oriented silicon steel sheet with excellent magnetic properties, which is carried out at a temperature of 1200 to 1250 DEG C. to a depth of 1.15 DEG C. and at a rolling reduction of 5006 DEG C. or more.

The experimental results that formed the basis of this invention will be explained below.

As a result of many experiments, the inventors have found that the precipitation state of the inhibitor changes greatly depending on the temperature. The key point is that when rolled and held at temperatures around 1000 to 1100°C, the inhibitor precipitates coarsely.
Either it has good properties or it is difficult to obtain. It has also been experienced that even if good characteristics were obtained under these conditions, the range would be very narrow and it would not be a practical technology. Therefore, depending on the component system and the temperature history during hot rolling, it was found that rough rolling at temperatures below 1200°C should be avoided at least below 11508°C.

The experimental results from which the above findings were obtained are shown in FIG. The experiment shown in the same figure was conducted using C: 0.03%, Si: 3.0%, Mn: o,
i% and Se: Thickness 12 containing 0.024%
This experiment was conducted using a small research rolling mill, using zero plate vacuum melted steel as the material.

By the way, with conventional silicon steel rough rolling technology,
It was considered preferable to roll at a low temperature around 000°C. The basic technical idea is that in this temperature range, the hard γ phase is most likely to be formed, and therefore strain accumulates around it, promoting recrystallization. Therefore, as described above, the optimum rough rolling conditions are said to be in the range of 1000 to 1100'C as shown in FIG. 2, and conventional rough rolling conditions were set aiming at this range. However, as shown in FIG. 1 above, this temperature range is the most unfavorable temperature range from the viewpoint of inhibitor precipitation.

Therefore, in the conventional technology, if rough rolling is performed under conditions that optimize the crystal structure, inhibitor precipitation inevitably becomes inappropriate.

In this way, in the past, the average temperature of the plate was used as the processing temperature, so even with the rough rolling conditions that were considered optimal in the conventional technology, in reality it was not possible to simultaneously satisfy both microstructure improvement and inhibitor dispersion. Therefore, in the actual process, one or the other had to be sacrificed.

Based on the above experience, the inventors introduced the idea of controlling the structure in the thickness direction in order to solve the above problems. For this purpose, the structure of the steel sheet during rough rolling was first observed in detail.

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

As shown in the figure, at the stage of the sheet bar after rough rolling, there is a high probability of coarse grains from the surface layer to around 115 mm thick. On the other hand, closer to the center, it recrystallizes uniformly and becomes fine crystal grains.

The reason for such an organization may be the following process.

In other words, the first reason is that in continuously cast slabs, columnar crystals develop near the surface layer, and the crystal grains are elongated and large. When the slab is further heated to a high temperature, its crystal grains become even coarser. The heated slab is subjected to hot rolling, during which time small crystal grains easily recrystallize, but large crystal grains do not easily recrystallize from the beginning. Therefore, either the center of the steel plate recrystallizes and becomes finer and finer with each rolling pass, or the surface layer does not recrystallize easily. As a result, it is thought that a very large difference in particle size occurred between the surface layer and the center. These differences in structure become more noticeable when slabs are heated to ultra-high temperatures of 1400° C. or higher, as has been the case recently. Therefore, it can be said that there is a particular need for tissue improvement in the layers from the surface layer to a depth of 115 mm.

Therefore, it does not have a negative effect on inhibitor precipitation12
Various methods were investigated for recrystallizing the surface layer to a depth of 115 mm at a temperature of 00° C. or higher.

FIG. 4 shows the effects of rolling reduction and C content on the recrystallization rate. The figure shows a plate thickness of 50[[II[l of 3%S1
After heating the steel to 1200°C, it was rolled at various reduction ratios in a research rolling mill, and the subsequent recrystallization rate was investigated.

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

When a slab is heated to a high temperature, particularly 1400° C. or higher, the crystal grain size tends to become very large. In reality, it is extremely difficult to suppress the particle size after heating the slab to about 5 mm, and in many cases it is about 10 to 30 plates, and even more than 50 mm.

Therefore, a technique is required to reliably recrystallize and refine grains even in the case of large grain sizes.

Therefore, we next investigated a method for making the grains finer even when the grain size is large. At this time, it was assumed that rolling was carried out at a temperature of 1200° C. or higher from the viewpoint of preventing coarse inhibitor precipitation.

As a result, it was found that the desired objective can be advantageously achieved by increasing the amount of C and reducing the pressure by a large amount. That is, for example, when the C content is increased to 0.075%, rolling temperature: 1200°C 1 initial grain size: 40
A recrystallization rate of 90% or more was obtained at a rolling reduction rate of 50%.

The results are also shown in Figure 4.

Next, we investigated the appropriate ranges of C content and rolling reduction that would allow a high recrystallization rate to be obtained at a rolling temperature of 1,200 to 1,250°C.

The results are shown in FIG.

In the range of C content: 0.05% or more and rolling reduction: 50% or more, as shown by the dotted line in the figure, a recrystallization rate of 75% or more was obtained.

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

Note that the above effect can be achieved by increasing the C content.
The main reason may be that the phase is formed with appropriate dispersion, or perhaps because the recrystallization rate decreases as the rolling temperature decreases, but in some cases, a satisfactory effect was not always obtained. Therefore, from this point of view, it is necessary to set the rolling temperature to 1200'c or higher. However, if the rolling temperature becomes too high,
The upper limit of the rolling temperature is set at 1250° C. because the grain refining effect becomes small and it becomes impossible to cool the material sufficiently before finishing rolling.

Next, we will discuss the case where the above knowledge is applied to an actual rough rolling process.

Even in the final pass of rough rolling, the plate thickness on the entry side is 50 mm or more, and in some cases as much as 100 mm, making it difficult to make the temperature uniform in the thickness direction. The conventional idea was that making the temperature uniform in the thickness direction of the plate would lead to a uniform structure, and therefore efforts to equalize the temperature have continued. That is, the conventional way of thinking is to avoid rapid cooling in order to equalize the temperature and to finish rough rolling at as low a temperature as possible.

As mentioned above, in this method of holding high temperature for a long time during rolling, the residence time in the precipitation dangerous temperature range around 1100°C becomes long and the inhibitor becomes coarse. , good characteristics could not be expected.

In order to avoid such disadvantages, this invention
The target temperature is reached by rapid cooling, for example, to a depth of /'5. Within this depth range, the target temperature can be sufficiently controlled by cooling from the surface. Therefore, the effects of this invention can be obtained by setting the rolling temperature from the outermost layer to 115 depth during the final pass of rough rolling to 1200 to 1250°C by cooling control during rough rolling, and by setting the rolling reduction ratio to 5094 or more. can.

For cooling, it is preferable to actively use water spray. Of course, heat radiation or air cooling may be used as long as the predetermined temperature conditions can be satisfied. The least preferable method is to lower the temperature of the entire bar by slowly cooling it before the final pass. Therefore, the most preferable method is to shorten the time from the rolling immediately before the final pass to the final pass as much as possible, and to perform forced water cooling. Specifically, the inter-pass time is preferably 30 seconds or less, and is not preferably 60 seconds or more.

By rolling with this method, not only were good properties obtained, but the incidence of linear fine grains caused by coarse grains, which was the defect that we wanted to eliminate as much as possible, could be significantly reduced. .

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

C: 0.05-0.09% The reason for setting the C range to 0.05% or more is that, as shown in Figure 4 above, even if the initial grain size is large, 1200% C is set during rough rolling. It is intended to improve the hot-rolled structure by passing through the (α+γ) region at ~1250°C, and is limited to the appropriate range. The upper limit was set at 0.0996 as a range in which the characteristics did not deteriorate.

Si: 2.5 to 4.0% Si is effective in increasing the resistivity of steel sheets and reducing iron loss, 4.0! If it exceeds 2.5%, the cold rollability will be impaired.
%, the effect of reducing iron loss will be weakened, and the final high-temperature finishing annealing for secondary recrystallization and purification will
-γ transformation causes randomization of crystal orientation, making it impossible to obtain sufficient properties.

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 embrittlement, and the upper limit is MnS and Mn.
The content is limited to 0.10% in order not to increase the dissociation solid solution temperature of nSe and to prevent the inhibitor from becoming coarse during the time control process from slab extraction to rough rolling.

At least one selected from S, Se and AI:
0.01-0.10% S, Se and AI are respectively MnS, MnSe,
It is finely dispersed in the steel in the form of AIN and acts as an inhibitor, and in order to fully exhibit its function as an inhibitor, it needs to be at least 0.01%. In addition, the upper limits of o and io% were determined in the same manner as in the case of Mn from the viewpoint of facilitating dissociation and solid solution of the inhibitor and prevention of coarsening during hot rolling. Note that when AI is used as an inhibitor component, it goes without saying that a balanced amount of N is required. The preferred amount of N is 0.001 to 0.06
It is 5%.

In addition to the above elements, examples of inhibitors include sb and Sn.

Grain boundary segregation elements such as As, Pb, Bi, Cu, Mo, and B are known, and it is also possible to use these in combination.

(Example) Example I C: 0.080%, Si: 3.30?4, Mn:
0.05496, SeO, 022%, Sb: 0.
024%, Al: 0.025% and NO,009
Thickness containing 96 and the remainder being substantially Fe:
A 200 mm continuous cast slab was placed in a heating furnace and heated at 1400° C. for 960 minutes in a N2 atmosphere to sufficiently dissolve the inhibitor, and immediately subjected to rough rolling. Rough rolling was performed in 4 passes to produce a sheet bar with a thickness of 50 to 30 mm. At that time, the cooling conditions and rolling conditions were varied as shown in Table 1.

After that, it was hot-rolled to a thickness of 2.2 mm, then subjected to primary cold rolling, intermediate annealing, and secondary cold rolling to a thickness of 0.20 mm, followed by secondary recrystallization and final finish annealing for the purpose of purification. The final product was obtained.

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

Incidentally, the incidence of linear fine particles was expressed as the proportion of the part that had to be discarded as a product defect.

As is clear from Table 1, the products obtained according to the present invention have excellent magnetic properties and the incidence of linear fine grains is significantly reduced.

(Effects of the Invention) Thus, according to the present invention, it is possible to simultaneously achieve refinement of the crystal structure and proper dispersion of the inhibitor, thereby producing a unidirectional silicon steel sheet that does not generate linear fine grains and has excellent magnetic properties. It is possible to obtain it stably.

[Brief explanation of the drawing]

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

Claims (1)

[Claims] 1. Contains C: 0.05 to 0.09 wt%, Si: 2.5 to 4.0 wt% and Mn: 0.03 to 0.10 wt%, and contains S, Se and Al. At least one of these:
After heating, a silicon steel slab having a composition containing 0.01 to 0.10 wt% is subjected to hot rough rolling, followed by hot finish rolling, and then cold rolling once or twice with intermediate annealing in between. When manufacturing grain-oriented silicon steel sheets through a series of steps including decarburization annealing and final finish annealing, the final pass of the hot rough rolling is carried out from the outermost layer of the steel sheet to the final thickness. Temperature up to 1/5 depth is 1200~12
A method for producing grain-oriented silicon steel sheets with excellent magnetic properties, characterized in that the method is carried out at a temperature of 50° C. and a rolling reduction of 50% or more.