JPS5922771B2 - Method for manufacturing grain-oriented silicon steel sheet with excellent repeated bending properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an advantageous method for improving the repeated bending properties, which are the mechanical properties of grain-oriented silicon steel sheets.

Normally, unidirectional silicon steel sheets are produced by hot-rolling a silicon steel material containing 4.0% or less of Si, performing annealing and one or two cold rolling processes to obtain a cold-rolled sheet with the final product thickness, and then decarburizing it. Primary recrystallization annealing was performed, which also served as
) (Develop secondary recrystallized grains in the ooB orientation, remove harmful impurities at the same time, form a forsterite-based insulating film, then apply a phosphate-based insulating film, and remove the coil set with baking.) It is manufactured through a series of processes including chemical annealing.

When grain-oriented silicon steel sheets are used as iron core materials for transformers, motors, etc., they are subjected to slitting or punching processes, and when processed into wound iron cores, they are also subjected to deformations such as small-diameter bending.

If breakage, cracks, etc. occur during such processing, the iron core cannot be manufactured.

Therefore, in order to check for such brittleness, it is prescribed that electromagnetic steel strips be tested using a repeated bending test according to JIS standard C2550-1975.

By the way, in order to prevent the occurrence of defective products that do not pass the repeated bending test, various technological improvements have been made in the past, including the addition of annealing separator additives in the final annealing process,
This was a technical improvement from the viewpoint of hydration amount or final annealing conditions.

An object of the present invention is to propose a method for improving the repeated bending characteristics of grain-oriented silicon steel sheets based on a technique different from the conventional technical improvement of the repeated bending characteristics of the grain-oriented silicon steel sheet.

Now, the present inventors have found that it is necessary to further improve the repeated bending characteristics of grain-oriented silicon steel sheets. We have found that the repeated bending properties of grain-oriented silicon steel sheets change significantly depending on the flattening annealing conditions.

Therefore, the present inventors went a step further and conducted extensive research into the relationship between flattening annealing conditions and cyclic bending characteristics, and found that among the above annealing conditions, the atmospheric dew point has a particularly large influence on cyclic bending characteristics. , and found that the repeated bending characteristics can be dramatically improved by controlling the dew point to a certain value or less, and based on this knowledge, the present invention was completed.

Conventionally, regarding flattening annealing conditions, the annealing atmosphere has been considered from the viewpoint of discoloration of the film appearance, etc., but the dew point has never been considered in relation to the repeated bending characteristics, and such flattening annealing conditions The relationship between the dew point and repeated bending characteristics is a new fact that was elucidated for the first time in the present invention.

That is, in the present invention, after decarburizing a grain-oriented silicon steel material that has been cold-rolled to the final plate thickness, an annealing separator containing MgO as a main component is applied, and final annealing is then performed. This is a method for improving the repeated bending characteristics of a grain-oriented silicon steel sheet, which is characterized by performing flattening annealing under conditions where the atmospheric dew point is 25° C. or less.

In the present invention, the atmosphere for flattening annealing is as follows:
Any one selected from neutral, weakly reducing and weakly carburizing is particularly advantageously suited.

The present invention will be specifically explained below based on the experimental results that led to the invention.

First, we will discuss the results of investigating the dew point and repeated bending characteristics of a grain-oriented silicon steel sheet manufactured under conventional general flattening annealing conditions.

After hot rolling a silicon steel material with a composition containing 3.05% Si, 0.038% C, 10,02% S to a thickness of 2.4 mm,
Cold rolling was performed twice with intermediate annealing at 900° C. for 5 minutes, resulting in a final plate thickness of 0.30 mm.

Then, after decarburizing annealing at 820°C for 3 minutes in wet hydrogen, an annealing separator of MgO + 1.5% TiO2 was applied.
It was subjected to final annealing.

During this final annealing, the temperature was changed from N2 to N2 at 600°C, the temperature was raised from 950 to 1050°C at a rate of 10°C/h, and the temperature was maintained at 1180°C for 5 hours.

After that, a coating of phosphate-based insulation film was applied, and the
After pre-drying at 0°C for 30 seconds, soaking at 800°C for 1 minute, then heating at approximately 8°C/100°C to around 100°C.
Flattening annealing was performed by cooling at a rate of sec.

The dew point during the flattening annealing at this time was 40°C, and regarding the repeated bending characteristics of the product, the minimum number of repeated bends before breakage was 1, and the incidence of bending 5 times or less was 30%.

Here, the repeated bending characteristics were evaluated based on the occurrence rate when the number of repeated bends until breakage is 5 or less, but this is even stricter than the regulations in the JIS standard mentioned above, and this value is as long as possible. The lower the value, the better.

Next, FIG. 1 shows the results of an investigation into the relationship between the dew point and repeated bending characteristics during flattening annealing.

The experiment was conducted using a small-capacity experimental reactor as follows.

After hot rolling a silicon steel material containing 3.05%Si, 10.038%C, 10.02%S to a thickness of 2.477X1L, 900℃,
Cold rolling was performed twice with an intermediate annealing period of 5 minutes in between, resulting in a final plate thickness of 0.30 mm.

After decarburization annealing in wet hydrogen at 820° C. for 3 minutes, an annealing separator of MgO + 1.5% TiO□ was applied, and final annealing was performed.

The minimum number of repeated bending cycles until breakage after annealing was 19 times.

Next, a phosphate-based insulating film is applied and pre-dried, and after soaking at 800°C for 1 minute, flattening annealing is performed at a cooling rate of approximately 10°C/sec and 40°C/sec to 100°C. provided.

The atmosphere for flattening annealing was N2 and N2, and the dew point was varied in the range of -20 to +50°C.

To control the dew point, atmospheric gas adjusted to a predetermined dew point in the range of -20°C to +30°C is rapidly blown into the furnace, and the atmospheric gas in the furnace is constantly replaced. 15 times).

In flattening annealing, even after preliminary drying, the surface of the steel sheet is oxidized by 02 derived from the moisture contained in the coating film and 00□ in the air, which inevitably enters the furnace, causing it to turn red. In order to prevent this, the atmospheric gas on the entrance side of the annealing furnace, where such evaporated moisture and air accumulate, has traditionally been replaced, but the replacement speed has been kept to the minimum necessary from the perspective of cost consumption, and is approximately 1 About 50 m° for the time-roaring furnace entrance capacity of 15 m°, that is, 3.
It was about 3 times the amount.

Now, as is clear from the results shown in FIG. 1, as the dew point increases, the incidence of repeated bending of 5 times or less increases, and embrittlement is particularly significant at dew points of 30 degrees or more.

These phenomena become particularly noticeable during rapid cooling at 40° C./sec.

Therefore, to ensure good cyclic bending properties, the dew point should be 2.
It is important to keep the temperature below 5°C, preferably below 20°C.

Regarding the annealing atmosphere, the repeated bending characteristics are better in N2 than in N2, but N
Annealing in a strongly reducing atmosphere like 2 is not preferable because it impairs the film properties such as moisture absorption resistance.

Next, in order to confirm whether the atmosphere undesirable for repeated bending characteristics is oxidizing or not, we further investigated the atmosphere for flattening annealing using air, N2, N2+DX, AX, wet AX, etc., and the results are shown in Figure 2. So, N2, N2+D
There was almost no difference between X and AX (good), but on the other hand, humidity AX (dew point 60°C) was particularly poor, and air was slightly worse.

As is clear from the above data, the most unsuitable atmosphere for flattening annealing from the viewpoint of repeated bending characteristics is a decarburizing atmosphere, not an oxidizing atmosphere.

However, when flattening annealing is performed in air or a nitrogen atmosphere containing several percent of oxygen, the appearance of the film changes to red.
As mentioned above, this is undesirable because it impairs the product value.

When the amount of C in the steel plate exceeds 0.004%, the magnetic properties deteriorate, but when a steel plate whose repeated bending properties have been deteriorated by flattening annealing is carburized, a remarkable improvement in the repeated bending properties is observed.

The deterioration of cyclic bending properties due to flattening annealing in a more decarburizing atmosphere as described above corresponds to the opposite of this phenomenon.

In this sense, like the N2+DX gas shown in Figure 2,
Flattening annealing in a weakly carburizing atmosphere eliminates the risk of decarburization (and since the heat treatment lasts at most 3 minutes, there is no risk of carburization, which is advantageous for repeated bending properties.

As mentioned above, in order to maintain the repeated bending properties of grain-oriented silicon steel sheets at a high level, the atmosphere for flattening annealing must be non-decarburizing, neutral or slightly carburizing, and the dew point must be 25°C or lower. need to be controlled.

Note that the atmospheric dew point can be controlled, as described above, by adjusting the amount of atmospheric gas blown into the furnace, by strengthening preliminary drying prior to introduction into the furnace, and the like.

Next, examples of the present invention will be described together with comparative examples.

Example 1 A silicon steel material containing 305% Si, 0.038% C, and 0.02% S was hot-rolled to a thickness of 2.4 mm, and then cold-rolled twice at 900°C with an intermediate annealing period of 5 minutes in between. It was rolled to a final thickness of 0.30 mm.

After decarburization annealing in wet hydrogen at 820° C. for 3 minutes, an annealing separator of MgO+1.5% TiO□ was applied, and final annealing was performed.

For final annealing, switch from N2 to N2 at 600°C, increase the temperature in the range of 950 to 1050°C at a rate of 10°C/h, and
It was held at 180°C for 5 hours.

The minimum number of bending cycles until breakage after final annealing is 1.
9 times, and the incidence rate of 5 times or less was 0%.

After that, apply a phosphate-based top coating liquid, and
After heating and drying at 00°C for 30 seconds, the atmospheric gas on the inlet side of the furnace was replaced with 8 times the volume for 1 hour, and soaked at 800°C for 1 minute in a N2 atmosphere with a dew point of 15°C. Thereafter, flattening annealing was performed in an N2 atmosphere with a dew point of 15°C, in which the sample was cooled to around 100°C at a rate of approximately 8°C/sec.

The minimum number of repeated bending cycles until breakage of the obtained product was 17, and the incidence of bending 5 times or less was 0%.

Example 2 The process was carried out in the same manner as in Example 1 up to the final annealing.

During flattening annealing, after heating and drying at 400° C. for 30 seconds, the amount of atmospheric gas replaced was approximately 6.5 times as much to provide N2+10% DX gas with a constant dew point of 20° C. in the furnace atmosphere.

The minimum number of repeated bending times until breakage was 10, but the incidence of bending 5 times or less was 0%.

Comparative example 1 The process was carried out in the same manner as in Example 1 up to the final annealing.

During flattening annealing, the dew point is reduced to 4 by processing according to conventional methods.
The temperature was 0° C., and the atmosphere was an oxidizing atmosphere of N2+2%02 gas.

The minimum number of repeated bending times until breakage was 0, and the incidence of bending 5 times or less was 50%.

As described above, the present invention is advantageous in that it is possible to significantly improve the repeated bending characteristics of a grain-oriented silicon steel sheet compared to the conventional method by a simple operation of adjusting the atmospheric dew point during flattening annealing.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between atmospheric dew point and repeated bending characteristics in flattening annealing, and FIG. 2 is a diagram showing the relationship between atmospheric gas and repeated bending characteristics in flattening annealing.

Claims (1)

[Claims] 1. After decarburizing a grain-oriented silicon steel material that has been cold-rolled to the final thickness, an annealing separator containing MgO as a main component is applied, and final annealing is then performed. The atmospheric dew point inside is 2.
A method for improving the repeated bending properties of a grain-oriented silicon steel sheet, the method comprising flattening annealing at a temperature of 5°C or lower. 2. The method according to claim 1, wherein the atmosphere during flattening annealing is any one selected from neutral, weakly reducing, and weakly carburizing.