JPH07116513B2 - Non-oriented electrical steel sheet manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a non-oriented electrical steel sheet having excellent magnetic properties and surface properties.

[Problems to be Solved by Prior Art and Invention] Non-oriented electrical steel sheets have smaller anisotropy in magnetic properties than grain-oriented electrical steel sheets, and are therefore generally used for iron cores of rotating machines. Moreover, since it is cheaper than grain-oriented electrical steel sheets, some of them are also applied to static devices such as transformers and ballasts. The characteristic values required for the non-oriented electrical steel sheet are mainly iron loss and magnetic flux density, and those having a good magnetic flux density-iron loss balance with low iron loss and high magnetic flux density are required. In addition, non-oriented electrical steel sheets are used as iron cores by laminating after punching, but if the flatness of the steel sheet is poor, the space factor will decrease and the efficiency of the rotating machine will deteriorate, so the surface quality of the steel sheet will be good. Is also required.

Heretofore, various manufacturing techniques have been disclosed in order to manufacture a non-oriented electrical steel sheet having good magnetic properties. Among the magnetic properties, the magnetic flux density has a close relationship with the texture of the steel sheet, and it is important to accumulate as many <100> axes, which are easy axes of magnetization, on the surface of the steel sheet. for that reason,
Technology for improving the structure of hot-rolled sheet by hot-rolled sheet annealing, technology for controlling the recrystallization texture during subsequent annealing by optimizing the cold pressure ratio, or performing cold pressure and annealing twice or more Discloses a technique for selecting a texture that is preferable for magnetic properties.

Here, the texture after the final annealing is largely influenced by the cold pressure in the previous process, the texture in the hot rolling and the solidification stage, and the control of the texture from the solidification stage in which the molten steel becomes a solid state. Must be done. However, any of the above-described conventional techniques is a technique for hot rolling a hot-rolled continuous casting slab having a thickness of about 200 mm, and is a technique for improving the subsequent texture, The constraints did not make an essential approach to improving the organization.

On the other hand, in recent years, a technique of casting molten steel into a slab having a thickness of a conventional hot-rolled sheet by omitting the hot rolling step and cold rolling the slab as it is (Japanese Patent Laid-Open No. 63-60227) or directly by casting A technique for obtaining a steel sheet having a final thickness (Japanese Patent Laid-Open No. 59-96219) is disclosed. When the thickness of the slab in the casting stage becomes thin, the solidification rate increases and the development of columnar crystals is promoted. It has this columnar crystal {100} <uvw> structure and contains the largest number of easy magnetization axes. Moreover, since this easy magnetization axis is randomly oriented to the steel sheet surface, it is an assembly of non-oriented electrical steel. It is preferable as an organization. The above-mentioned two techniques are techniques for maintaining such a texture having good magnetic properties as much as possible after the final annealing, but on the other hand, there is a problem that the surface properties of the steel sheet deteriorate. That is, in the former technique, since a columnar structure coarser than the conventional hot-rolled sheet structure is cold-rolled, deterioration of surface roughness and plate thickness accuracy due to occurrence of ridging is unavoidable. Also in the latter technique, since the final plate thickness is directly obtained from the molten metal, it is extremely difficult to control the plate thickness accuracy in actual production.

The present invention has been made in view of such conventional problems,
In α single-phase non-oriented electrical steel sheet where transformation does not occur,
By applying the thin slab method and optimizing the hot rolling conditions, it is possible to manufacture a non-oriented electrical steel sheet having excellent magnetic properties and surface properties.

That is, in the present invention, in weight%, Si: 1.7 to 7.0%, C ≦
0.005%, Al: 0.001-1.5%, N ≦ 0.003%, Si + Al ≦ 7.5
%, The balance Fe and unavoidable impurities, the grain interval is a thin cast piece having a thickness of 50 mm or less consisting of columnar crystals with a grain interval of 0.5 to 2 mm, and after the thin cast piece is directly or reheated, the rolling end temperature
It is characterized by performing hot rolling at 600 ° C. or higher and a reduction rate of 80 to 95%, annealing the hot-rolled sheet, followed by cold rolling or warm rolling, and then performing annealing thereafter. .

[Means for Solving the Problems]

 Hereinafter, the details of the present invention will be described together with the reasons for limitation.

In order to improve the texture of the non-oriented electrical steel sheet in terms of magnetic properties, it is important to improve the solidification texture immediately after casting the molten steel and the texture after hot rolling. Conventionally, various techniques have been disclosed as a method for controlling the texture after the hot-rolled sheet. However, there is no example that has yet been clarified about the effect of the rolling reduction during hot rolling on the texture and the surface properties of the steel sheet. Conventionally, the conventional method is to use a slab of about 200 mm by continuous casting as a material, and due to the constraint of the cold rolling reduction, the thickness of the hot rolled sheet has to be about 2 mm. This is because there was almost no room for selection for the hot rolling reduction rate.

On the other hand, the present inventors pay attention to the relationship between the rolling reduction and the texture and the surface texture of the steel sheet during the hot rolling, and determine the Si content.
In α-single phase steel containing 1.7% or more, the thickness of the slab was changed and the effect of the reduction rate in the hot rolling stage on the magnetic flux density was first examined. As a result, it became clear that the smaller the hot rolling reduction, the higher the magnetic flux density.

Magnetic flux density and waviness of steel sheet after final annealing when 1.7%, 3% and 4.5% Si steels were cast into a slab consisting of columnar crystals with a total thickness of 5 to 200 mm ^t and a slab consisting of equiaxed crystals in the same amount. The effect of hot rolling reduction on height was investigated. That is, the columnar grain spacing obtained by casting molten steel having the composition shown in Table 1 below in a water-cooled copper mold is 0.7 to 1.8 mm, and the thickness is 5 to 67 mm.
A thin slab consisting of columnar crystals with a total volume of mm ^{t, and} a total thickness of equiaxed crystals obtained by casting in a mold that was also heated to approximately 80 ° C.
A slab of ~ 200 mm ^t was prepared. After heating these slabs to 1100 ° C., subjected to hot rolling, and plate thickness 2.0~2.3mm ^t.
The hot rolling finishing temperature at this time was 690 to 810 ° C.
And, 900 ℃ × 2min (No.1
Steel), 1000 ℃ × 2min (No.2 steel), 1050 ℃ × 2min (No.3)
Was subjected to hot rolled sheet annealing of steel), cold rolled to 0.5 mm ^t (N
o.3 steel was warm-rolled at a plate temperature of 100 ° C). Annealing after cold pressure is
880 ℃ × 2min (No.1 steel), 960 in 20% H ₂ −80% N ₂ atmosphere
℃ × 2min (No. 2 steel), 1000 ℃ × 2min (No. 3 steel) soaking and air cooling.

For No. 3 steel, after casting, 60
After being placed in a heating furnace at 1100 ° C without lowering below 0 ° C, hot rolling was performed.

FIG. 1 shows the result. As is clear from the figure, the effect on the magnetic flux density of the solidified structure of the slab is that the total amount of columnar crystal slabs has a higher magnetic flux density than the total amount of equiaxed crystal slabs.

The solidification structure of steel is roughly classified into columnar crystals and equiaxed crystals. The columnar crystals are formed by nucleation from the mold contact surface in the solidification stage, and the nuclei growing from the surface to the inside of the slab in the direction perpendicular to the mold surface. Such columnar crystals have a {100} <uvw> structure in which {100} is parallel to the slab plate surface and <100> is random. <100>
Is the easy axis of magnetization in steel, and the slab consisting of columnar crystals has an ideal texture for the magnetic flux density. On the other hand, since equiaxed crystals randomly generate nuclei in the molten metal during solidification, the slab texture is also random. Therefore, as shown in FIG. 1, in the production of the electromagnetic steel sheet using the columnar crystal slab, the magnetic flux density is better than that of the equiaxed crystal slab by maintaining the texture of the slab having the good texture of the magnetic flux density. Becomes

Also, when trying to maintain the texture of the slab, this is most difficult in the hot rolling stage, but as shown in Fig. 1, by reducing the rolling reduction during hot rolling to 95% or less, It can be maintained. Therefore, as in the past
In the method of finishing a hot rolled sheet with a thickness of 2 to 3 mm from a slab with a thickness of 200 to 250 mm, it is inevitable that the reduction ratio exceeds 95% in addition to the presence of equiaxed crystals in the center of the slab. Therefore, the texture could not be made appropriate for the magnetic properties. Thus, the rolling reduction during hot rolling is the largest factor that deteriorates the magnetic properties. In the α single phase steel, the strain recovery phenomenon progresses rapidly during hot working, and it is possible to maintain a good texture originating from the texture during solidification even at a high pressure reduction rate of 90% or more. If hot rolling is performed at a rolling reduction of not less than%, the introduction of strain greatly exceeds the recovery, so that the texture during solidification is completely destroyed, and the magnetic flux density deteriorates.

On the other hand, it was newly discovered that the grain size of the slab and the rolling reduction during hot rolling greatly affect the surface properties of the steel sheet, such as ridging and roughening, which are important factors for the occupancy.
As shown in FIG. 1, when the rolling reduction is less than 80%, the waviness of the steel sheet surface after the final annealing becomes high. This is because when the rolling reduction during hot rolling becomes low, the strain introduced into the hot rolled sheet becomes small, so the ferrite grain size before cold rolling becomes large, and the coarse grain pattern caused by these large ferrite grains is caused by cold rolling. Appears, and the amount of deformation is different for each ferrite grain, which causes waviness on the surface of the steel sheet. Therefore, in order to improve the magnetic properties and surface properties of the non-oriented electrical steel sheet, the rolling reduction during hot rolling must be 80 to 95%.

Also, for unevenness of the steel plate surface called ridging,
The ferrite grain size before hot rolling, that is, the crystal grain spacing of the columnar crystals of the cast piece has an effect. In particular, it is well known that the destruction of the solidification structure is necessary to prevent ridging in the α single phase steel having a solidification structure of columnar crystals. In the method of the present invention, in order to improve the properties of the non-oriented electrical steel sheet, the rolling reduction at the time of hot rolling is made lower than that in the conventional method, and therefore ridging prevention measures must be taken. With respect to such a problem, the present inventors have found that ridging can be prevented by making the grain size of columnar crystals during solidification, that is, the grain interval of columnar crystals finer in the solidification stage, by the following tests. I found out.

By casting the molten steel shown in Table 2 below with a cast steel mold having a wall thickness of 50 to 150 mm and a water-cooled steel mold to a slab thickness of 30 mm ^t , a cast slab composed of columnar grains with a columnar grain interval of 0.3 to 18 mm was formed. It was made.

After heating these slabs to 1100 ° C, hot rolling is performed,
A hot-rolled sheet having a sheet thickness of 2.0 to 2.2 mm ^t was used. The hot rolling finishing temperature at this time was 680 to 790 ° C. Incidentally, hot-rolled sheet annealing,
The cold pressure and annealing conditions were the same as the test conditions shown in FIG.
In addition, for No. 6 steel, after casting, 60
After being placed in a heating furnace at 1100 ° C without lowering below 0 ° C, hot rolling was performed. Similarly, for No. 6 steel, as in No. 3 steel described above, after hot-rolled sheet annealing, warm rolling was performed at a sheet temperature of 100 ° C.

FIG. 2 shows the relationship between the grain spacing of columnar crystals, the ridging height of the steel sheet, and the magnetic flux density of the steel sheet obtained as described above. According to this, it is understood that the ridging can be appropriately prevented by suppressing the grain interval of the columnar crystals to 2 mm or less. On the other hand, if the grain spacing of the columnar crystals is less than 0.5 mm, the solidification rate must be increased, and it is difficult to obtain such a solidification rate in actual operation.

Further, when the grain spacing of the columnar crystals is less than 0.5 mm, a random structure called a chill crystal having a crystal orientation is formed in the surface layer portion, which causes a decrease in magnetic flux density as shown in FIG.

Molten steel shown in Table 3 below was prepared using a water-cooled copper mold and a wall thickness of 50-
150mm cast steel mold, thickness 5-200mm ^t , columnar grain spacing 0.16-1
A cast piece composed of 1 mm of columnar crystals was prepared. These slabs 1
After heating to 100 ° C., soaking, hot rolling was performed to obtain a hot-rolled sheet having a sheet thickness of 2.0 to 2.3 mm ^t . The finishing temperature at this time was 680 to 800 ° C. For steel No. 9 slabs with a thickness of 10 mm ^t or more, after casting, 1
It heated at 100 degreeC and hot-rolled. 900 ℃ × 2min (No.7 steel), 1000 ℃ × 2min (No.8)
Steel) and 1050 ° C x 2min (No.9 steel) hot-rolled sheet annealing, and then cold rolling to a sheet thickness of 0.5 mm ^t (No.9 steel was warm rolled at a sheet temperature of 100 ° C). Annealing after cold pressure is 880 ℃ × 2min (No.7 steel), 960 ℃ × 2min in 25% H ₂ −75% N ₂ atmosphere, respectively.
(No.8 steel), 1000 ℃ × 2min (No.9 steel) soaking and air cooling.

FIG. 3 shows the undulation height, ridging height and magnetic flux density of the steel sheet obtained as described above in terms of the relationship between the rolling reduction during hot rolling and the grain spacing of the columnar crystals of the slab. It can be seen that the same results as in FIGS. 1 and 2 are obtained.

Thus, by rolling a slab composed of columnar crystals having a grain structure of 0.5 to 2 mm at the time of solidification at a rolling reduction of 80 to 95% during hot rolling, excellent magnetic properties and surface properties of the steel sheet can be obtained. It becomes possible to manufacture a grain-oriented electrical steel sheet.

Next, the constitutional requirements of the present invention will be specifically described.

First, the reasons for limiting the component composition are as follows.

C: C is not only a detrimental element that deteriorates iron loss in the magnetic properties and further causes magnetic aging, but also reduces the orientation of {100} <uvw> in the columnar structure. Therefore, it is necessary to reduce the amount to 0.005% or less at the steelmaking stage.

Si: Si has the effect of increasing the specific resistance of steel and reducing iron loss. Further, by adding 1.7% or more, the α-γ transformation disappears, and the {100} <uvw> columnar structure generated during solidification can be maintained. Therefore, the lower limit of Si is 1.7%. On the other hand, even if Si is added at 7.0% or more, the magnetic properties are not particularly improved, so the upper limit is made 7.0%.

Al: Al, like Si, has the effect of increasing the specific resistance of steel and reducing iron loss, but if added in excess of 1.5%, the cold ductility decreases, so its upper limit is made 1.5%. On the other hand, in order to sufficiently deoxidize molten steel, it must be contained at 0.001% or more.

N: N forms a nitride to reduce the grain growth property of ferrite grains and deteriorates iron loss, so it is necessary to set it to 0.003% or less.

Si + Al: Si and Al are both effective elements for reducing iron loss, but if Si + Al exceeds 7.5%, the rolling property is extremely poor, and it causes cracking of the steel sheet during cold rolling.
Al needs to be 7.5% or less.

Components of the raw steel other than the above are not particularly limited, but the following ranges are preferable.

Mn: 0.01 to 2.0% P: ≤ 0.1% S: ≤ 0.005% The reason why the above components are preferable is as follows.

Since Mn: Mn precipitates solid solution S harmful to magnetic properties as MnS, it is desirable to add 0.01% or more. But,
If it exceeds 2%, the cold rolling property deteriorates significantly, so 2.0% or less is desirable.

Since P: P increases the specific resistance of steel, addition of iron can reduce iron loss, but if it exceeds 0.1%, cold ductility is significantly deteriorated, so 0.1% or less is desirable.

Since S: S deteriorates iron loss in a solid solution state, 0.005% or less is desirable.

When AlN is finely precipitated and the ferrite grain growth property is deteriorated during the final annealing, the grain growth property can be improved by adding B in the range of B / N: 0.5 to 2.0. This is because coarser BN particles preferentially precipitate than AlN particles. Further, also in the steel to which Al is not added, iron loss can be improved by adding B to fix the solid solution N as BN.

Non-oriented electrical steel sheets generally have a product thickness of 0.50 or 0.35 mm. In addition, it is generally known that, in the cold pressure of non-oriented electrical steel sheet, a good texture can be obtained by applying a reduction of 60 to 80%. In order to obtain the product sheet thickness by pressure, it is necessary to set the hot rolled sheet pressure to 1.0 to 2.5 mm. Therefore, in the present invention, the total rolling reduction during hot rolling is specified to be 80 to 95%, and therefore the thickness of the cast piece is preferably 5 to 50 mm.

Further, in the present invention, the thin slab is hot-rolled in the cooling stage by being directly or once cooled to room temperature and then reheated. In steel with Si + Al ≥ 4.5%, cracks occur due to the difference in heat shrinkage between the surface layer and the center of the slab during cooling of the slab, so after casting the slab, the temperature does not drop below 600 ° C during the cooling stage. It is desirable to carry out hot rolling directly or by reheating. In addition, in steels with Si + Al <4.5%, cracking does not occur during slab cooling, so hot rolling is possible even after cooling after casting, or after cooling to room temperature and reheating, and the magnetic properties and steel sheet There is no difference between the two processes in terms of surface texture. However, in any process, hot rolling must be completed at 600 ° C or higher. This is because when rolling is performed at 600 ° C or lower, the amount of strain introduced into the steel sheet increases, and along with this, the destruction of the {100} <uvw> structure that is favorable for the magnetic properties existing in the cast becomes significant. . By adopting the above process, it is possible to omit the rough rolling or slab reheating process compared to the conventional process of reheating a slab with a thickness of about 200 mm and performing rough rolling and finish rolling. ,
The cost can be greatly reduced.

The hot-rolled sheet thus obtained is subjected to hot-rolled sheet annealing, and then cold-rolled or warm-rolled. When hot-rolled sheet annealing is performed, agglomeration and coarsening of fine precipitate particles is promoted, and even if the annealing temperature at the final annealing is lowered, the ferrite grain growth property becomes good and iron loss can be reduced. Hot-rolled sheet annealing, if this is carried out by continuous annealing, 0.5 ~ 5 min at 750 ~ 950 ℃, in the case of open batch annealing, after pickling or mechanically removing the scale of the hot-rolled sheet surface, 700 in non-oxidizing atmosphere
It is desirable to set the temperature to 850 ° C for 1 to 10 hours.

The hot rolled sheet thus annealed is cold rolled or warm rolled to a product thickness. Where Si + Al <3.0
% Steel can be cold rolled by the usual method,
For steels with Si + Al ≥ 3.0%, the transition temperature for toughness is 5
Since the temperature is 0 ° C or higher, the hot-rolled sheet must be heated to 60 to 400 ° C and warm-rolled. The final annealing is performed to recrystallize the ferrite structure and grain growth, and to control the ferrite grains to be the optimal balance between iron loss and magnetic flux density,
As the condition, it is desirable to carry out soaking at 750 to 1000 ° C. for 0.5 to 5 minutes in a dry non-oxidizing atmosphere having a dew point of 0 ° C. or less.

〔Example〕

Example 1. A molten steel having the steel components shown in Table 4 was poured into a water-cooled copper mold to manufacture a thin cast piece having a thickness of 30 mm ^t and composed of columnar crystals having a columnar grain interval of 0.8 to 1.2 mm. This slab was soaked at 1100 ° C in the cooling stage and then hot-rolled to form a hot-rolled sheet having a sheet thickness of 1.8 to 2.3 mm ^t at the final pass outlet temperature of 750 to 820 ° C (heat reduction rate of 92 to 94%).
After the hot-rolled sheet was annealed at 1000 ° C. for 2 min and air-cooled, the hot-rolled sheets of Steels A to F were at 80 ° C., and the hot-rolled sheets of Steels G to L were at room temperature (15 to 20 ° C.). ), Each was warm-rolled or cold-rolled to a thickness of 0.5 mm. These cold-rolled sheets were in a 25% H ₂ -75% N ₂ atmosphere, steels A to F had a temperature of 950 ° C. for 2 min, and steel G to L had a temperature of 8
Annealing was performed under each condition of 80 ° C. × 2 min and air cooling. Table 5 shows the magnetic properties, the waviness height and the ridging height of the steel sheet thus obtained.

Example 2. The steel components shown in Table 6 (inventive component steels) were cast into slabs having various thicknesses and columnar grain intervals shown in Table 7,
The slab was hot rolled under various hot rolling conditions as shown in the table. This hot-rolled sheet was annealed at 930 ° C for 2 min in air, then cold-rolled at room temperature (15-20 ° C) to a thickness of 0.5 mm, and then in a 25% H ₂ -75% N ₂ atmosphere. 930 ℃ × 2mi
n, air-cooled annealing was performed. Table 7 also shows the magnetic properties, the waviness height, and the ridging height of the steel sheet thus obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of the rolling reduction during hot rolling on the magnetic flux density and the undulation height of the steel sheet. Figure 2 shows
It is a graph which shows the influence which the grain space of the columnar crystal of a cast piece has on the magnetic flux density and ridging height of a steel plate. FIG. 3 shows the undulation height, ridging height and magnetic flux density of the steel sheet in the relationship between the rolling reduction during hot rolling and the grain spacing of the columnar crystals of the slab.

Claims (1)

[Claims] 1. By weight%, Si: 1.7 to 7.0%, C ≦ 0.005%,
Al: 0.001-1.5%, N ≦ 0.003%, Si + Al ≦ 7.5%, balance F
The molten steel composed of e and unavoidable impurities has a grain interval of 0.5 to
A thin cast piece having a thickness of 50 mm or less composed of columnar crystals of 2 mm, and directly or after reheating the thin cast piece, a hot rolling at a rolling end temperature of 600 ° C. or higher and a reduction rate of 80 to 95% is performed. A method for producing a non-oriented electrical steel sheet, comprising hot rolling annealed sheet, cold rolling or warm rolling, and then annealing.