JPH083126B2 - Manufacturing method of thick plate electromagnetic soft iron - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing thick plate electromagnetic soft iron, for example, a thick plate electromagnetic soft iron suitable for blocking leakage magnetism and having excellent magnetic characteristics.

[0002]

2. Description of the Related Art With the remarkable progress of science and technology in recent years,
For example, in the field of medical equipment, a nuclear magnetic resonance tomography apparatus (MRI) utilizing the nuclear magnetic resonance phenomenon has been put into practical use and actively introduced. However, when using this nuclear magnetic resonance tomography apparatus, it is necessary to block a large amount of leakage magnetism that occurs. Therefore, the steel sheet used as a structural member of this nuclear magnetic resonance tomography apparatus is required to have magnetic blocking characteristics, and the thickness of the steel sheet is 20 mm or more from the necessity of having strength as an apparatus. Many.
On the other hand, electromagnetic soft iron for thick plates, which has excellent magnetic cutoff characteristics, that is, high magnetic permeability, is used not only for the above-mentioned nuclear magnetic resonance tomography apparatus but also for magnetic shields of large scientific experimental equipment such as cyclotrons, nuclear fusion equipment or medical equipment. It is a material that is also used as a cover and a structural member for performing.

Therefore, in order to further advance the achievements of recent science and technology, such electromagnetic soft iron for thick plates is required to have excellent mechanical properties which are originally contradictory to each other and magnetic properties represented by magnetic permeability or magnetic flux density. It is strongly desired from each field to satisfy both of the above. An electromagnetic soft steel plate is known as a steel plate having such magnetic blocking characteristics, and a thin plate generally used for a transformer is well known. This is the conventional JIS C 2503 or JIS C 2504 steel material with excellent magnetic properties.
Electromagnetic soft iron bars and electromagnetic soft iron plates specified in. JIS C 25
Those specified in 03 are rods with a diameter in the range of 1.0 to 16 mm, and those specified in JIS C 2504 are 0.6 to
It is a thin plate with a thickness of 4.5 mm, both of which are intended for application to small parts for relays or electromagnets. In addition, there is an example in which S10C, which is a carbon steel material for machine structure defined in JIS C 4051, which is not classified for magnetism, is hot-processed to a width of 250 mm and used as a magnetic material.
Further, as disclosed in JP-A-60-96749, JP-B-63-45442 or JP-B-63-45443, the content of sol.Al is as large as 0.005-1.00% by weight,
In recent years, a thick plate for direct current magnetization, which is an Al deoxidized ultra-low carbon steel in which Si is reduced to some extent, has been proposed.

[0004]

However, these known means are excellent in that, as will be described below, for example, the leakage magnetism can be blocked when a nuclear magnetic resonance tomography apparatus (MRI) is used. It is impossible to stably provide electromagnetic soft iron for thick plates having excellent magnetic characteristics. (i) The electromagnetic soft iron bar or electromagnetic soft iron plate shown in JIS C 2503 or JIS C 2504 is intended for small parts as described above, and the mechanical characteristics as structural members are not considered at all. . Therefore, when applying this electromagnetic soft iron plate to the above-mentioned nuclear magnetic resonance tomography apparatus, it is necessary to stack several tens of these electromagnetic soft iron plates in order to secure the strength of the apparatus, the manufacturing cost, the product In terms of quality, practical implementation cannot be achieved. (ii) In the example using the carbon steel material for machine structure shown in JIS C 4051, since no consideration was given to the magnetic properties, the maximum magnetic permeability μ was as low as 1800 (B / H) or less. Has not been done. Therefore, it is still impossible to provide the desired electromagnetic soft iron for thick plates. (iii) In the method proposed by JP-A-60-96749, JP-B-63-45442 or JP-A-63-45443, in order to improve the magnetic characteristics of the obtained electromagnetic soft iron, Cr is used. It is necessary to significantly reduce the impurities in the steel, such as, etc., during the refining stage. For example, the above-mentioned JP-A-60-9674.
In the method proposed by Japanese Patent No. 9, the upper limit of the Cr content is limited to 0.05% by weight.

However, refractory materials other than acidic refractory materials, such as magnesia.
When a chromium refractory is used, impurities such as Cr that are unavoidably mixed in the furnace wall and the like are mixed in the molten steel, so that 0.05% or more of Cr is contained in the molten steel. Therefore, JP-A-60-96749 and JP-B-63-45442.
In the means proposed in Japanese Patent Publication No. 63-45443 or Japanese Patent Publication No. 63-45443, the refractory used for the furnace wall and the like is xAl ₂ O ₃
-Limited to acidic refractory materials containing y SiO ₂ (SiO ₂ ) (x and y represent compounding ratios) as a main component. On the other hand, magnesia-chromium refractories have a maximum usable temperature of about 1650 ° C of the above-mentioned acidic refractories, while they are generally said to be able to withstand high temperatures of 1870 ° C. As a refractory material, it has extremely suitable characteristics and its application is extremely wide.

Therefore, even when the steel for electromagnetic soft iron is smelted, if the magnesia-chromium refractory is used, the smelting temperature can be further raised and the life of the refractory can be extended, but in this case, As described above, the amount of impurities such as Cr mixed in the molten steel increases. In other words, when magnesia / chromium refractory is used during melting of electromagnetic soft iron for thick plates, Cr will necessarily be contained in molten steel in an amount of 0.05% or more. If the magnetic characteristics of can be sufficiently improved, as in the past,
It is not necessary to strictly control the amount of impurities mixed, in other words, it is possible to manufacture the electromagnetic soft iron for thick plates without deteriorating the magnetic characteristics even when impurities are mixed. Here, an object of the present invention is to provide a thick plate excellent in mechanical characteristics as well as having excellent magnetic characteristics capable of blocking leakage magnetic field when using, for example, a nuclear magnetic resonance tomography apparatus (MRI). Stable electromagnetic soft iron for use with magnesia-chromium refractory in the melting stage, that is, without deteriorating the magnetic characteristics even when mixed with more Cr than the conventional electromagnetic soft iron for thick plates. It is intended to provide a method of manufacturing an electromagnetic soft iron for thick plates, which can be manufactured.

[0007]

In order to solve the above problems, the present inventor has conducted various studies and as a result, in order to improve the magnetic properties and to improve the homogeneity of the magnetic properties in the plate thickness direction, the It is effective to minimize the content of components that increase the magnetic susceptibility, and in the heat treatment after hot working, in order to facilitate the growth of crystal grains, processing strain is added to the crystal grains during hot working. It was noted that the above is effective and that the heat treatment temperature is set just below the austenite-ferrite transformation temperature so that transformation does not occur.

Based on these facts, as a result of further studies, the present inventor has found that a steel ingot having a composition containing sol.Al: 0.10 to 0.5% and a relatively large amount of Al has a finishing temperature of steel ingot. Hot working so that the temperature becomes a ferrite single phase,
For example, rolling, forging, or drawing is performed, and after processing to a predetermined size, in order to remove the processing strain of the crystal grains and to grow the crystal grains, heating and holding is performed immediately below the austenite-ferrite transformation temperature, and thereafter, it is gradually heated. By cooling, for example, electromagnetic soft iron for thick plates having excellent magnetic characteristics, which can block leakage magnetism when using a nuclear magnetic resonance tomography apparatus (MRI), is magnesia-treated at the melting stage. The present invention has been found that it can be stably provided by using a chromium-based refractory, that is, it can be provided without deteriorating the magnetic characteristics even when mixed with more Cr than the conventional electromagnetic soft iron for thick plates. Was completed.

Here, the gist of the present invention is, in% by weight, C: 0.01% or less, Si: 0.30% or less, Mn: 0.30% or less, P: 0.01% or less, S: 0.01% or less, sol. .Al: 0.10 to 0.50%, Cr: 0.25% or less, steel ingot having a steel composition consisting of the balance Fe and unavoidable impurities is hot worked at a finishing temperature of 740 to 850 ℃, and then at 800 ℃ or more and Ac ₃ or less. A method for producing a thick sheet electromagnetic soft iron having coarse grains with a ferrite grain size of 1 or less, which comprises heating and holding for 30 minutes or more and gradually cooling to a Curie point or less at a rate of 200 to 30 ° C./h. In the present invention, "hot working" refers to rolling, forging, drawing or the like. Further, in the present invention, the “steel ingot” is a base material of a product obtained by the hot working, and specifically refers to a steel ingot such as a slab or a bloom.

[0010]

The present invention will now be described in more detail with reference to the accompanying drawings. In this specification, "%" means "% by weight" unless otherwise specified. First, the reason for limiting the composition of the steel ingot used in the present invention will be described. C is an element having the highest demagnetization rate of magnetic properties, and it is desirable to reduce it as much as possible in order to improve the magnetic properties. However, since it requires a considerable cost to reduce it, the allowable upper limit of the content is made 0.01% or less.
Addition of more than 0.01% causes deterioration of magnetic properties. In the Al deoxidizing type electromagnetic soft iron for thick plates, which is the object of the present invention, Si inhibits the coarsening of crystal grains during heat treatment due to the grain size regulating action. Therefore, the content should be 0.30% or less.

As is well known, Mn is an element that greatly contributes to ensuring the strength of steel materials, and, like C, is an element that enhances the demagnetization rate. Therefore, it is desirable to reduce the demagnetization rate from the viewpoint of reducing the demagnetization rate. The allowable upper limit is 0.30%. Since P and S both form non-metallic inclusions in steel and cause segregation, the smaller the content, the better. Further, it is an element that increases the coercive force with the increase of its content and deteriorates the magnetic characteristics. Therefore, the smaller the content, the better, but the reduction of these elements always involves an increase in cost, so P is 0.01% or less and S is 0.01% or less.

Al is an element that acts as a deoxidizing material in steel, and it is necessary to contain 0.005% or more in order to exert such an effect. However, according to the findings of the present inventors, It has been found that if the amount is too small, fine AlN 3 is precipitated and grain growth is prevented, so in the present invention, 0.10% or more is added. However, if added in a large amount, inclusions are formed and the properties of the steel are impaired, so the upper limit is made 0.50% or less. Therefore, the content of Al is set to 0.10 to 0.50%. It is preferably 0.20 to 0.40%.

Since Cr is an element that deteriorates the magnetic properties, it has been conventionally desired that the smaller the amount, the more desirable. However, Cr is an element that is inevitably mixed from the furnace wall and the like, and it takes a very large cost to remove Cr.
Especially when magnesia / chromium refractory is used for the furnace wall, the refractory reacts with the molten steel and Cr, Ni, Mo, etc. are mixed in the steel. However, in the present invention, by appropriately limiting the production conditions as described later, it is not necessary to take a prominent reduction of the content of Cr, but if the amount is too large, the magnetic properties deteriorate and the allowable upper limit is 0.25%. To do. Therefore, the Cr content range is 0.25% or less, generally 0.05
Limit to ~ 0.25%.

A steel ingot having the above composition is manufactured under the following conditions. These conditions will be described in detail.
FIG. 1 is a diagram showing a temperature pattern of thermomechanical processing according to the present invention. First, a steel ingot having the above composition is hot-worked at a finishing temperature of 740 to 850 ° C at which the steel structure becomes a ferrite single phase. Generally, adding processing strain to crystal grains during hot working is an effective means for growing crystal grains after heat treatment, as proposed in Japanese Patent Publication No. 60-208417. is there. Since coarsening of crystal grains is one of the factors that improve the magnetic properties of steel products, the present invention also utilizes such means to improve the magnetic properties of the products obtained. As an example of the hot working, any working method such as rolling, forging or drawing, which can give working strain to the steel ingot at the time of working, is not particularly limited. Further, in the present invention, the temperature at which the steel structure becomes a ferrite single phase is 900 ° C. or less from the Fe—C binary system phase diagram.

Thus, the finishing temperature for hot working is set to the ferrite-austenite transformation temperature or lower. In other words, the work strain is applied to the ferrite grains, and a heat treatment is performed after the work to release the work strain and coarsen the crystal grains. Therefore, the finishing temperature is preferably 740 to 850 ° C, and more preferably 750 to 800 ° C from the above viewpoint. Next, the steel ingot thus hot-worked is heat-treated for the purpose of releasing work strain and coarsening crystal grains. The heating temperature of this heat treatment is the ferrite-austenite transformation temperature (Ac ₃ points).
The following is good, and the area immediately below it is good. That is, when heated to a temperature higher than the transformation temperature, the structure becomes a transformation texture and the magnetic properties are significantly deteriorated. Therefore, the heating temperature for heat treatment is set to Ac ₃ point or lower, generally 940 ° C or lower. If the temperature is below 800 ° C, the processing strain cannot be removed. Therefore, the heat treatment temperature is 80
It is limited to 0 ° C or higher and Ac ₃ points or lower. The holding time of heating is
30 minutes or more. This is the minimum value required to remove distortion. The upper limit is not particularly limited, but considering the actual production process, it is preferable to limit it within t / 25.4 (where t: plate thickness) time.

After this, gradual cooling, specifically cover gradual cooling, is performed for dehydrogenation. The cooling rate of the gradual cooling at this time is generally about 200 to 30 ° C./hr. . It is preferably 100 to 30 ° C / h. In particular, according to the present invention, since a relatively large amount of Al is blended, grain growth is promoted, dehydrogenation is promoted, and the process proceeds easily, resulting in a cooling rate of 200-
It can be relatively high at 30 ℃ / hr. Further, during the heat treatment cooling, it is important not to perform dehydrogenation treatment but to gradually cool from the high temperature holding temperature to below the Curie point for improving magnetic properties. The Curie point is 770 ° C in the steel composition range targeted by the present invention. After that, it can be taken out of the furnace and air-cooled. Thick electromagnetic soft iron with coarse grains of ferrite grain size 1 or less is produced.

As described above, according to the present invention, an electromagnetic soft iron for a thick plate is melted by using a magnesia / chromium refractory or the like by combining an appropriate composition of steel ingot and manufacturing conditions,
Even if a steel ingot containing Cr in an amount of more than 0.05% is used, it is possible to produce electromagnetic soft iron for thick plates having excellent magnetic properties. Further, the present invention will be described in detail together with examples, but this is an exemplification of the present invention, and the present invention is not limited to this.

[0018]

EXAMPLES Steel ingots having the compositions shown in Table 1 (Ru of the present invention example
n No. 1 to Run No. 14 and Run Nos. 1 to 15 of Comparative Example)
Similarly, hot working (rolling) was performed under the conditions shown in Table 1, and thereafter, heat treatment was performed under the heat treatment conditions also shown in Table 2, followed by gradual cooling to obtain electromagnetic soft iron for thick plates.

[0019]

[Table 1]

[0020]

[Table 2]

For each of these test materials, B ₁ (magnetic field 1 O
The magnetic properties of magnetic flux density (e) and μ _max (maximum magnetic permeability) were evaluated. The results are also shown in Table 2. The mechanical properties of all of these test materials were in the following ranges, and it was found that they also have sufficient properties as structural materials.・ YP (yield point) = 12 to 20 kgf / mm ²・ TS (tensile strength) = 25 to 34 kgf / mm ²・ El (elongation) = 28 to 45% vE ₀ ^Ave (V notch Charpy impact at 0 ° C The low temperature toughness expressed by the absorbed energy value) was 28.8 kgf · m.

FIG. 2 shows Run No. 1 and Run No. 4 of the example of the present invention.
The relationship between the heat treatment temperature and the magnetic properties when the heat treatment temperature is changed under the standard treatment conditions is shown. FIG. 3 shows the Run of the example of the present invention.
The relationship between the cooling rate and magnetic characteristics for No. 4 is shown. Further, in FIG. 4, Run No. 1 of the example of the present invention was investigated.
Shown below is the relationship between the temperature at which the furnace was taken out and the magnetic properties when it was soaked for 2 hours at ℃, cooled in the furnace at 150 ℃ / h, taken out from the furnace at each temperature, and air-cooled (cooling rate = 700 ℃ / h). Similarly, FIG. 5 shows the relationship with the ferrite grain size. FIG. 6 is a graph showing the influence of the magnetic properties on the change in the sol.Al content when Run No. 1 of the present invention was used as the standard treatment condition. As is clear from the above, according to the present invention, even if Cr is inevitably mixed, it has excellent magnetic properties, toughness, and mechanical properties.

[0023]

As described above in detail, according to the present invention, the electromagnetic soft iron for thick plates having excellent magnetic characteristics can be stably prepared by using a magnesia / chromium refractory at the melting stage, that is, the conventional method. It is now possible to provide it by containing more Cr than the electromagnetic soft iron for thick plates. The practical significance of the present invention having such an effect is remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing the processing heat treatment conditions of the present invention.

FIG. 2 is a diagram showing a relationship between heat treatment temperature and magnetic characteristics in the example of the present invention.

FIG. 3 is a diagram showing a relationship between a cooling rate and magnetic characteristics in the example of the present invention.

FIG. 4 is a diagram showing the relationship between the temperature taken out from the furnace and the magnetic characteristics in the example of the present invention.

FIG. 5 is a diagram showing the relationship between ferrite grain size and magnetic characteristics in an example of the present invention.

6 is a diagram showing the relationship between the sol.Al content and magnetic characteristics in the example of the present invention.

Claims (2)

[Claims] 1. By weight%, C: 0.01% or less, Si: 0.30% or less, Mn: 0.30% or less, P: 0.01% or less, S: 0.01% or less, sol.Al:0.10 to 0.50%, Cr: A steel ingot with a steel composition consisting of 0.25% or less and the balance Fe and unavoidable impurities is hot-worked at a finishing temperature of 740 to 850 ℃, heated to 800 ℃ or more and Ac ₃ or less, and held for 30 minutes or more to Curie point. A method for producing thick plate electromagnetic soft iron having coarse grains with a ferrite grain size of 1 or less, characterized by gradually cooling the following to a rate of 200 to 30 ° C / h. 2. The method for manufacturing a thick plate electromagnetic soft iron according to claim 1, wherein the plate thickness is 6 to 100 mm.