JPH03104822A - Method for heating slab for non-oriented electrical sheet - Google Patents

Abstract

PURPOSE:To produce the electrical steel sheet of a low iron loss valve with good productivity by heat treating the slab for the non-oriented electrical sheet under specific conditions, thereby effectively decreasing the effect of suppressing grain growth at the time of finish annealing in the production process of the electrical steel sheet. CONSTITUTION:The slab for the non-oriented electrical sheet is first heated to the temp. below 1150 deg.C, is then reheated up to a 1150 to 1200 deg.C range at >=7 deg.C/min heating up rate and is hot rolled to form a hot rolled sheet at the time of once cooling the slab down to ordinary temp., then hot rolling the slab. The slab is otherwise heated to <=1250 deg.C first, is cooled down to <1156 deg.C at <30 deg.C/min average cooling rate in a range at least form this temp. down to (heating temp.-100 deg.C), is then reheated up to 1150 to 1200 deg.C at >=7 deg.C/min heating up rate and is hot rolled. The slag heating temp. is otherwise set over 1250 deg.C and the slab is cooled down to <1150 deg.C at <30 deg.C/min average cooling rate in at least either of the temp. range of the range from this temp. down to 1250 deg.C or the range therefrom down to 1250 to 1150 deg.C. This slab is reheated up to 1150 to 1200 deg.C at >=7 deg.C/min heating up rate and is hot rolled. After this sheet is annealed, the sheet is subjected to cold rolling and annealing. The non-oriented electrical sheet having the small iron loss value is thus produced.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for heating a slab for non-oriented electrical steel sheets, and in particular, the invention aims to reduce iron loss and increase productivity by adding innovations to slab heating treatment. It is an attempt to maintain sexuality.

(Prior Art) Non-oriented electrical steel sheets are mainly used for cores of rotating machines and transformers, and recently there has been a strong desire for energy savings in these electrical devices. To achieve this, it is necessary to lower the iron loss value, and one approach is to lower the iron loss by increasing the Si content of the material.

However, as the Si content of the material increases, the magnetic flux density decreases accordingly, so methods for reducing iron loss include not only increasing the St content, but also methods for reducing impurities in the material (Japanese Patent Publication No. 56 122931, JP-A-59-74223), ■ A method of making impurities harmless by adding trace elements (Japanese Patent Publications No. 54-36966 and JP-B No. 59-20731), ■ Making impurities harmless by heat treatment How to convert into
-35885, Special Publication No. 18045, Special Publication No. 56-18045, Special Publication No. 18045
Various iron loss reduction methods have been proposed, such as Japanese Patent Laid-Open No. 6-33451 and Japanese Patent Application Laid-Open No. 123825/1982).

Impurities in steel mainly refer to N and S, and if these elements remain in the steel, they will form nitrides and sulfur in the hot rolled sheet, respectively.
It is dispersed and precipitated as sulfide and inhibits grain growth during finish annealing after cold rolling.

(Problem to be solved by the invention) Method (2) reduces impurity elements at the steel manufacturing stage and is highly effective, but requires a large cost and cannot be applied to all electromagnetic control plate materials. .

Moreover, since it is not possible to completely eliminate impurities industrially, the remaining impurities have an adverse effect on magnetism depending on their amount.

Of the methods (2), the method disclosed in Japanese Patent Publication No. 54-36966 is a method of adding rare earth elements (REM) to aggregate sulfides, and the method disclosed in Japanese Patent Publication No. 59-20731 is a method of agglomerating sulfides in steel. This method eliminates the grain length suppressing effect of AIN by reducing AI to 0.1% or less and adding B, but since both REM and B are expensive elements, it is not only economically disadvantageous, but also Specific impurities (REM→S
, B-N).

Among the methods of ■, Special Publication No. 56-18045,
6-33451 and Japanese Patent Application Laid-Open No. 58-123825, the continuously cast slab is kept warm at a high temperature (800 to 1150°C) L, and the precipitates are coarsened and rendered harmless, and then heated directly. Although this is a method of performing inter-rolling, it is difficult to adopt this method in factories that do not have the above-mentioned direct hot-rolling equipment because it is necessary to always maintain a tie between continuous casting and hot rolling.

Furthermore, the method disclosed in Japanese Patent Publication No. 50-35885 aims to agglomerate AIN by combining low-carbon and low-temperature slab heating (below 1200°C), but the method suitable for aggregation depends on the concentration and type of impurities. Since the temperature changes and the actual slab heating furnace is a continuous furnace, there was a disadvantage that the temperature could not be freely selected. Moreover, in order to obtain sufficient effects with this method, the slab heating temperature must be lower than 1200°C (for example, lower than 1150°C), but the load on the hot rolling mill increases as the temperature decreases, which reduces productivity. This results in a decrease in

By the way, the inventors previously filed the patent application No. 63-257372.
In the specification, even if slabs with different concentrations of impurity elements are mixed in the heating furnace or the slab heating temperature is inappropriate, the adverse effects of precipitates can be significantly reduced and iron loss can be lowered. We proposed a method for heating slabs for non-oriented electrical steel sheets based on controlling the cooling rate after heating the slab.

In other words, "1. When heating a slab for non-oriented electrical steel sheets, the slab heating temperature should be 1250°C or less, and the average cooling rate should be 30°C/min or less in the temperature range from at least this heating temperature to (heating temperature - 100°C) A method for heating a slab for a non-oriented electrical steel sheet, characterized by cooling it to below 1150°C.

2. When heating a slab for non-oriented electrical steel sheets, the slab heating temperature is set to exceed 1250°C, and from this heating temperature to 1250°C.
Heating of a slab for a non-oriented electrical steel sheet characterized by cooling at an average cooling rate of 30°C/min or less for at least one of the temperature ranges from 1250°C to 1150°C. Method. ”.

According to the above method, if within the predetermined cooling condition range,
Rough rolling can be carried out at a temperature of 1,150°C or higher, which can prevent a decrease in productivity to some extent, but even with this method, in order to sufficiently prevent the coarsening of precipitates, it is still necessary to heat the slab to 1,150°C. It is preferable to reduce the temperature to a temperature range of less than 10,000 yen. Therefore, this has not yet been a fully satisfactory solution from the viewpoint of improving productivity while ensuring good magnetic properties.

This invention aims to suppress the grain size during final annealing of cold-rolled steel sheets, which is caused by impurities that normally remain in the steel during the manufacturing process of non-oriented electrical steel sheets on an industrial scale as described above. The advantages of non-oriented electrical steel plate slabs are that they can be effectively reduced without using special additive elements, without having to adjust the timing of continuous casting and hot rolling, and without reducing productivity. The purpose is to propose a heating method.

(Means for solving problems) First, the background to the elucidation of this invention will be explained.

Non-oriented electrical steel sheets are generally manufactured through the following process.

That is, after tapping molten steel produced in a converter, it is degassed, and ferroalloy etc. are added to adjust the precepts. The molten steel after this weight adjustment is poured into an independent mold, and the resulting steel ingot is either bloomed into a slab or directly formed into a slab by continuous casting.

After the slab thus obtained is once cooled to room temperature,
Alternatively, the hot slab is usually reheated to a temperature of 1250° C. or lower (slab heating), and then hot-rolled into a hot-rolled sheet. Thereafter, the hot-rolled sheet is subjected to hot-rolled sheet annealing if necessary, and then becomes a product through cold rolling and annealing. In some cases, cold rolling and annealing are repeated,
In some cases, the product is made into a product as is after cold rolling.

Now, impurities in the material of non-oriented electrical steel sheets become precipitates that inhibit grain elongation during final annealing, but the hot rolling process has a particularly large influence on the form of these precipitates. As measures to make precipitates harmless in the hot rolling process, the above-mentioned Japanese Patent Publication No. 50-35885 and Japanese Patent Application No. 1983-
No. 257372, etc., but as mentioned above, both of them required a reduction in the hot rolling temperature and tended to reduce productivity.

Focusing on these points, the inventors conducted extensive research into methods for ensuring productivity in the hot rolling process while making precipitates harmless. If the cooled slab is rapidly heated again,
It was discovered that the productivity of the hot rolling process could be ensured without impairing the effect of making the precipitates harmless, and the invention was completely abandoned.

This invention is based on the above knowledge.

That is, when heating a slab for a non-oriented electrical VA steel plate, this invention involves heating the slab to a temperature below I150°C, and then reheating it to a temperature in the range of 1150 to 1200°C at a heating rate of 7°C/min or more. A method of heating a slab for a non-directional electromagnetic control plate (first invention).

Furthermore, when heating a slab for a non-oriented electrical steel sheet, the slab heating temperature is set to 1250°C or lower, and the average cooling rate is 30°C/min at least in a temperature range from this heating temperature to (heating temperature - 100°C). After cooling to below l150℃, increase the temperature to 1150~1 at a heating rate of 7℃/min or more.
This is a method of heating a slab for a non-oriented electrical steel sheet (second invention), which comprises reheating to a temperature in the range of 200''C.

Further, in the present invention, when heating a slab for a non-oriented electrical steel sheet, the slab heating temperature is set to exceed 1250°C, and at least one of the temperature ranges from this heating temperature to 1250°C or from 1250°C to 1150°C is provided. About the range Average cooling rate: After cooling to less than 1150°C at 30°C/min or less, 1 x 50 at a heating rate of 7°C/min or more
This is a method for heating a slab for a non-oriented electrical steel sheet (third invention), which comprises reheating to a temperature in the range of ~1200°C.

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

A slab having the composition shown by the steel ingot symbol a in Table 1 is 1100
℃ and 1200℃, respectively, and the 200℃ heated slab was cooled at approximately 20℃/min to II 00℃ using a heat insulating cover, and then these two types of slabs were heated at various heating rates in an induction heating furnace. After reheating to 1200°C and soaking for 5 minutes, it was rolled to a thickness of 2.5mm by normal hot rolling.
It is made into a hot-rolled sheet of 3 rolls, then cold-rolled to 0.5 m.
After forming a cold-rolled sheet with a thickness of m, it was annealed for so"c, l.For comparison, sheets were heated at 1200°C at iioo°C, and after heating at 1200°C, cooled at about 20°C to 1100°C without reheating. Similar treatment was performed for each slab.

was processed.

The results of investigating the iron loss value of each product plate thus obtained are shown in FIG. 1 in relation to the temperature increase rate during slab reheating.

As is clear from the figure, when the heating rate is 7°C/min or more, there is less deterioration in iron loss than in the corresponding case without reheating, and even when heated at 1200°C without reheating, It can be seen that the iron loss is a sufficiently low value compared to the case where

Here, the slab temperature becomes 1200° C. by reheating, so productivity is significantly improved compared to the case without reheating.

Next, a set of slabs indicated by the same steel ingot symbol a is 110
Heating to 0℃ and 1200℃ respectively, 1200℃
The heated slab was cooled to 1100°C using a heat insulating cover at a rate of approximately 20°C/min, and then the two types of slabs were heated in an induction heating furnace to 1130°C to 1250°C at a heating rate of approximately 10°C.
After reheating at a rate of .degree. C./min and soaking for 5 minutes, the same treatment as in the above experiment was performed to obtain a product board.

The results of examining the iron loss value of each product sheet thus obtained are shown in FIG. 2 in relation to the slab reheating temperature.

As is clear from the figure, the reheating temperature is 1130~12
In the range of 00°C, there was almost no deterioration in core loss compared to the corresponding cases without reheating.

However, the passability in the hot rolling process, that is, the processing time from rough rolling to winding, is
In the temperature range of 1,250°C, rolling could be done in the required time as per standard operation, but at 1,130°C, the rolling time was about 20% longer.

That is, from the viewpoint of both iron loss characteristics and productivity, the appropriate range of reheating temperature is 1150 to 1200°C.

From the above experimental results, the inventors found that after detoxifying impurities by methods such as slab low-temperature heating or controlled cooling treatment after slab heating, the
They discovered that by reheating to a temperature range of ~1200°C, it was possible to reduce barring loss and ensure productivity, and brought this invention to fruition.

(Function) Slab materials targeted by this invention include pure iron, silicon steel containing about 3.5% or less of Si,
Furthermore, all materials commonly used as materials for non-oriented electrical steel sheets are suitable.

In addition, in the first invention, the reason why the slab heating temperature is set to be less than 1150°C is to sufficiently coarsen the precipitates, but if it is too low, the coarsening of the precipitates cannot be achieved. A heating temperature of about °C is necessary.

In the second and third inventions, the slab heating conditions are limited to the above-mentioned range in order to sufficiently coarsen the precipitates, as in the first invention.

The second invention is a case of general slab heating where the heating temperature is 1250°C or less, and in this case, from the slab heating temperature (
Average cooling rate in the temperature range up to heating temperature -100℃: 3
It is essential to cool to less than 1150°C at 0°C/min or less.

In Table 1, the slab with steel ingot symbol e is 1100
℃, 1200℃ and 1250℃, and after soaking, the cooling rate from this heating temperature to (heating temperature - 100℃) is basically air-cooled, depending on the time until rough rolling and the rough rolling mill. After various changes were made by adjusting the rolling reduction distribution, a hot rolled sheet with a thickness of 2.3 mm was obtained. The sheet was then cold-rolled to obtain a cold-rolled sheet with a thickness of 0.5 nm, and then annealed at 800° C. for 1 minute.

FIG. 3 shows the results of examining the iron loss value of the non-oriented electrical steel sheet thus obtained.

As is clear from the figure, the slower the cooling rate, the lower the iron loss is, and especially when the cooling rate is 30° C./min or less, excellent iron loss characteristics are obtained.

In addition, in the normal process, from the slab heating temperature (heating temperature -
The cooling rate to 100° C.) varies depending on the situation, but is generally in the range of 40 to 60° C./min.

The cooling rate here refers to the rate of change of the slab average temperature, and means the average value in the target temperature interval (hereinafter,
similar).

Next, comparing heating to 1200°C and heating to 1100°C, if the cooling rate is the same, heating to 1100°C has lower core loss, but even when heating to 1200°C, the cooling rate is 30°C/30°C.
It can be seen that if the temperature is less than 1,100° C., a good iron loss comparable to or higher than that of normal cooling with heating at 1100° C. can be obtained.

Recognize.

By setting the cooling rate after heating the slab to 30°C/min or less between the slab heating temperature and (slab heating temperature - 100°C), stable iron loss can be achieved regardless of the high or low slab heating temperature. This shows that it is possible to reduce In other words, there is no need to finely adjust the slab heating temperature depending on the impurity concentration in the steel. In actual operation,
It is often difficult to finely adjust the slab heating temperature for each slab, and it takes a lot of effort to know in advance the appropriate temperature to render impurities harmless. By following the above method, these practical difficulties can be overcome, and stable iron loss can be achieved regardless of whether the slab heating temperature is high or low.

Next, the third invention is a case where slab heating is performed at a heating temperature in a higher temperature range than usual.

A cold slab having the components indicated by f in Table 1 above was heated at a slab heating temperature of 1350°C for 2 hours, cooled under the cooling conditions of Hiroshi 1 to 4 in Table 2, and then hot rolled to form a 2ffl!
It was made into a hot rolled sheet of I+ thickness. As a comparative example, a case where the slab was heated at 1250°C for 2 hours and then cooled under the conditions in box 5 of Table 2 was also included. At this time, the slab cooling rate can be adjusted by using a heat insulating cover (Table 2, kl, 3) and air cooling (N(12,4) from 1350°C to 1250°C, and by air cooling from 1250°C to 1150°C. This was carried out by adjusting the time until rough rolling and the distribution of the rolling reduction of the rough rolling mill, and then hot rolling was performed to obtain a hot rolled sheet with a thickness of 2 inches.

After that, each hot-rolled sheet was coiled, cooled, annealed at 950°C for 2 minutes, then cold-rolled into a cold-rolled sheet with a thickness of 0.5 mm, and then heated at 970°C in a non-oxidizing atmosphere. Annealing was performed for 2 minutes.

Table 2 also shows the measurement results of the iron loss values of the product plates thus obtained.

As shown in the table, No. 1, which is a normal example. For 5,
1350"C→1250"C and 1250℃→11
Although no improvement in iron loss was observed with Nα2 where the cooling rate at 50°C was both 30°C/min or more, Nα1 and 3 where at least one cooling rate was 30°C/min or less
．． Iron loss was improved in all cases of No. 4, and especially in Nα3 in which the cooling rate was set to 30° C./min or less in both cases, iron loss characteristics were significantly improved.

That is, the average cooling rate between 1250°C and 1150°C,
By setting the heating temperature to 30℃/min or less, the slab heating temperature can be
The iron loss can be improved even when the temperature is 1250℃ or higher, and the iron loss can be improved even if the average cooling rate up to 1250℃ is 30℃/min or less. It has been found that core loss can be further reduced by reducing the speed to 30° C./min or less.

Contrary to the conventional wisdom of heating slabs at low temperatures to reduce iron loss, it has been shown that heating slabs over 1250°C effectively contributes to lowering iron loss under certain cooling conditions. This is new and unknown knowledge.

Furthermore, in both the second and third inventions, in order to sufficiently coarsen the precipitates, the slab after heat treatment must be heated to 1150°C.
It is preferable to lower the temperature below the
, in the third invention, the cooling temperature after heating the slab is 115
The temperature was limited to below 0°C.

Note that when reheating the slab, as mentioned above, a temperature increase rate of 7° C./min or more is required, so induction heating or electrical heating is suitable as the reheating method. This is because the combustion type heating furnace normally used for heating slabs cannot achieve the above temperature increase rate. Although the holding time in the reheating temperature range is not particularly specified, it is desirable to keep it as short as possible.

(Example) 2 Benefit ■ A slab with the composition shown in the steel ingot symbol C in the construction table 1 is 105
After heating at 0°C, it was reheated to 1190°C in an induction heating furnace at a heating rate of 14°C/min, and after soaking for 5 minutes, it was made into a hot rolled sheet with a thickness of 2.0 mm by normal hot rolling.

This hot-rolled plate was heated to 950°C. After annealing for 30 seconds, it was cold-rolled to a thickness of 0.5 mm and rolled at 900°C for 30 seconds.
S annealing was performed.

Table 3 shows the results of investigating the iron loss characteristics and productivity (evaluated by the time required for hot rolling) of the product sheet thus obtained. For comparison, Table 3 also shows the results of a survey on product boards obtained by processing in the same manner as above without reheating the slab.

Table 3 As is clear from the same table, the iron loss was almost the same level, but the hot rolling processing time was at the normal level with slab reheating, whereas it was at the normal level with slab reheating. The length of the sheet was significantly longer, and the two goals of hot-rolled sheets could not be achieved, resulting in an excessive thickness of 2.4 mm.

123
After heating to 0°C and cooling from this temperature to 1100°C using an insulated car at an average cooling rate of 25°C/min, reheating in an induction heating furnace to 1190°C at an average heating rate of 18°C/min. Immediately hot rolling was performed to obtain a hot-rolled blank with a thickness of 2 to 3 mm. This hot-rolled sheet was cold-rolled into a cold-rolled sheet with a thickness of 0.5 mm, and then annealed at 810° C. for 1 minute.

Table 4 shows the results of examining the iron loss characteristics and productivity of the product sheets thus obtained.

For comparison, Table 4 also shows the results of a survey on product boards obtained by processing in the same manner as above without reheating the slab.

Table 4 As is clear from the table, the iron loss was at almost the same level. However, in terms of productivity, the slab with reheating was able to be rolled in the required time according to the operating standard, whereas the slab without reheating required twice as much time.

The slabs shown in Table 1 with the steel ingot symbol b were heated in an induction heat furnace at a heating temperature of 1350°C for 2 hours, and then
After cooling down to 00°C at an average cooling rate of 25°C/min, it was heated again to 1170°C at a temperature increase rate of 8°C/min.
It was held for 0 minutes. This slab was rolled to 2.0 mm by normal hot rolling.
After forming a hot-rolled sheet with a thickness of mm, it was annealed at 1000°C for 30 seconds, and then cold-rolled into a cold-rolled sheet with a thickness of 0.5mm, which was then annealed at 980°C for 1 minute.

Table 5 shows the results of examining the iron loss characteristics and productivity of the product sheets thus obtained.

For comparison, Table 5 also shows the results of an investigation on product sheets obtained by processing in the same manner as above without reheating the slab.

Table 5 As is clear from the table, the increase in iron loss is extremely small even after reheating. On the other hand, the time required for hot rolling becomes significantly longer without reheating. In this respect, the time required in the case of reheating is at the normal level.

These results also show that according to the method of the present invention, productivity can be ensured while reducing iron loss.

(Effects of the Invention) Thus, according to the present invention, it is possible to suppress grain length during final annealing of cold-rolled steel sheets caused by process impurities that normally remain during conditioning during the manufacturing process of non-oriented electrical steel sheets on an industrial scale. The effects can be effectively alleviated without using expensive special additive elements, without having to adjust the timing of continuous casting and hot rolling, and without causing a decrease in productivity.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between heating rate and iron loss during slab reheating. Figure 2 is a graph showing the relationship between slab reheating temperature and iron loss. Figure 3 is a graph showing the relationship between slab reheating temperature and iron loss. Figure 3 is a graph showing the relationship between slab reheating temperature and iron loss. 3 is a graph showing the relationship between cooling rate and iron loss.

Claims (1)

[Claims] 1. When heating a slab for non-oriented electrical steel sheets, the slab is heated to a temperature below 1150°C, and then reheated to a temperature in the range of 1150 to 1200°C at a heating rate of 7°C/min or more. A method for heating a slab for non-oriented electrical steel sheet, characterized by: 2. When heating a slab for non-oriented electrical steel sheets, the slab heating temperature is 1250°C or less, and the temperature range from this heating temperature to (heating temperature -100°C) is at least 1150°C at an average cooling rate of 30°C/min or less. A method for heating a slab for a non-oriented electrical steel sheet, which comprises cooling the slab to less than 0.degree. C. and then reheating it to a temperature in the range of 1150 to 1200.degree. C. at a temperature increase rate of 7.degree. C./min or more. 3. When heating the slab for non-oriented electrical steel sheets, the slab heating temperature is set to exceed 1250°C, and from this heating temperature to 1250°C.
Average cooling rate for at least one of the temperature ranges from 1250°C to 1150°C: after cooling to less than 1150°C at 30°C/min or less, at a heating rate of 7°C/min or more A method for heating a slab for a non-oriented electrical steel sheet, comprising reheating the slab to a temperature in the range of 1150 to 1200°C.