Steel Slab Containing Silicon for Use in Electrical Sheet and Strip Manufactured by Continuous Casting and Method for Manufacturing Thereof

1. Field of the Invention

This invention relates to a steel slab containing silicon for use as an electrical steel sheet and strip manufactured by continuous casting and a method for manufacturing thereof.

2. Description of the Prior Art

In recent years, electrical steel sheets and strips have been manufactured by continuous casting, and the surface defects due to blisters in the final product have been a problem.

As used herein, blister means a puffy defect on the surface of the sheet and strip caused by the expansion of gases occluded in the steel in the heat treatment of the sheet and strip. Due to the occurrence of blisters, the commercial value of the electrical sheet and strip used as a material for laminated cores is almost lost.

While it is well known that low silicon- or non-silicon electrical steel sheets and strips have been utilized as electrical sheet and strips for cores of electrical instruments in recent years, high class materials represented by grain-oriented electrical sheet and strips contain silicon, which silicon content is generally in the range of 2.5 - 4%.

On the other hand, the material contains up to 1% of Al for improving the magnetic properties.

When the Al is less than 0.04% in a high class electrical sheet and strip containing 2.5 - 4% of Si, the surface defect called blisters occur frequently. It has been ascertained that the lower the Al content, the more frequently the blister occurs and that this frequently occurs in the continuous casting material.

While the reason of the occurrence of blister has mainly been regarded as due to hydrogen occluded in the steel, this was not the conclusion reached from our studies.

The frequent occurrence of blisters in the continuous casting material as above mentioned may also be considered to be due to the influence of mold lubricant (or thermal insulator) used in the continuous casting. However, no effect from the lubricant could be observed at all in our detailed experiments.

The object of this invention is to obtain, in the manufacture of electrical sheet and strips by continuous casting, a slab having no surface defects due to blisters in producing final products.

Another object of this invention is to offer a method for the continuous casting of molten steel containing silicon whose nitrogen or oxygen content has been controlled.

A further object of this invention is to offer a continuous casting slab for producing grain-oriented electrical steel sheets and strips wherein the occurrence of surface defect due to blister is minimized.

Still further objects of this invention may be seen from the description in this specification and the attached drawings.

FIG. 1 is a graph showing the change of nitrogen content in the molten steel from the tapping to the pouring.

FIG. 2 is a graph showing the influence of the Al- and N contents on the occurrence of blisters in the final product.

FIG. 3 is a graph showing the influence of the contents of Al, N and O on the occurrence of blisters in the final product.

Applicants have discovered a method for producing a steel slab for use as cube-on-edge grain oriented electrical steel sheets and strip wherein the blister defect is minimized. Particularly, the present process comprises the continuous casting of a molten steel slab containing 2.5 - 4% of Si and less than 0.04% of Al, to cast continuously said molten steel in a condition so as to hold the hydrogen content within 3 ppm and the nitrogen content within [Al(%) .times. 10.sup.3 + 50] ppm, or further to hold the oxygen content within 80 ppm.

When the oxygen content is not restricted, the nitrogen content is preferably less than [Al(%) .times. 10.sup.3 + 40] ppm. However, the nitrogen content can be eased up to [Al(%) .times. 10.sup.3 + 50] ppm by defining the oxygen content to be less than 80 ppm. Thus, no formation of blisters occurs at all by easing the nitrogen content up to about 80 ppm even when the Al content is less than 0.01% and it becomes possible to consistently and uniformly manufacture electrical sheets and strips having no blisters by continuous casting particularly in the low Al region.

More particularly, the present invention relates to cube-on-edge grain oriented electrical steel sheets having excellent magnetic properties and high stability by continuously casting a steel slab containing 2.5 to 4.0 wt.% Si, less than 0.04 of Al, less than 3 ppm of hydrogen or less than 3 ppm of hydrogen together with less than 80 ppm of oxygen and less than [Al (%) .times. 10.sup.3 + 50] ppm of nitrogen with the remainder being iron. The slab is heated at a temperature not less than 1,200.degree. C., and is then hot rolled, cold rolled with a reduction of not less than about 40%. It is then subjected to decarburization annealing and annealing at a temperature not lower than 1,100.degree. C.

The silicon content is defined as 2.5 - 4% in this invention. This is because the invention aims at a high class electrical steel sheet and strip represented by grain-oriented electrical sheet and strip.

The Al content in said silicon steel is usually up to 1%. However, the Al content of a steel whose nitrogen content should be controlled strictly in order to prevent the occurrence of blisters is less than 0.04%, and therefore, the object of this invention relates to a steel containing less than 0.040% and preferably, less than about 0.010% of Al. Generally, the minimum amount of Al is about 0.002%.

As for other components, it is desirous that the steel comprises less than 0.060% of carbon, less than 0.40% of manganese, less than 0.03% of phosphorous and less than 0.03% of sulphur, with the remainder being iron and unavoidable impurities.

In order to prevent the occurrence of blisters in the final product, it is quite important to restrict the amount of hydrogen in the molten steel to less than 3 ppm while at the same time the steel is maintained under the condition that the nitrogen content is less than [Al(%) .times. 10.sup.3 + 50] ppm, preferably less than [Al(%) .times. 10.sup.3 + 40] ppm, or less than [Al(%) .times. 10.sup.3 + 50] ppm when the oxygen is restricted within 80 ppm. The object cannot be attained at all when only the hydrogen content is restricted as was previously thought.

The object cannot be attained also when merely nitrogen, or nitrogen and oxygen, are restricted. Only when the contents of hydrogen and nitrogen, or oxygen, satisfy the above mentioned condition, can the occurrence of blisters in the final product be prevented.

Usually, about 3 - 5 ppm of hydrogen is contained in the tapping of molten steel. To reduce the content to less than 3 ppm, the conventional vacuum degassing treatments may be applied.

On the other hand, the vacuum degassing treatment can be expected to have little effect on the nitrogen contrary to the case of hydrogen. To restrict the nitrogen content in the range of this invention, the nitrogen is reduced during the smelting in the steel making furnace by adjusting the amount of Al.

To prevent the pick-up of nitrogen from the tapping to the pouring, molten steel is protected by an inert atmosphere (in vacuum or inert gas). In some instances, when the amount of nitrogen is reduced satisfactorily during the smelting in the steel making furnace, there is no need to prevent the pick-up.

To reduce the oxygen content and to diminish oxide inclusions, the following methods may be applied: for instance, to reduce the amount of deoxidation product by lowering the oxygen content in the molten steel before deoxidation, to accelerate the floating-up of deoxidation products by elevating the temperature of the molten steel or by agitating the molten steel, to prevent the increase of oxygen (oxide inclusion) by the pick-up of atmospheric oxygen from the tapping (deoxidation) to the pouring, or a suitable combination of these methods.

As above mentioned, it is the essential point of this invention to restrict the final nitrogen content in the molten steel (slab) to be less than [Al(%) .times. 10.sup.3 + 50] ppm.

The reason why the nitrogen content is restricted in the range as above mentioned will be explained.

FIG. 1 shows an example of the change of nitrogen content from the tapping of molten steel to the pouring.

In FIG. 1, (1) shows the result when molten steel is subjected to alloy-addition by adding Si and Al during vacuum treatment, and said molten steel is sealed with argon gas from the ladle to the mold in the continuous casting; (2) shows the result when the molten steel in (1) is cast continuously without argon gas sealing; (3) is the case when molten steel is subjected to deoxidation and alloy-addition by adding Si and Al during tapping, and said molten steel is sealed with argon gas from the ladle to the mold in the continuous casting; and (4) is the case when the molten steel in (3) is cast continuously without argon gas sealing.

It is observed from FIG. 1 that the nitrogen in the steel has a tendency to increase significantly after the tapping. Therefore, the above mentioned methods are applied selectively according to the circumstances in order to restrict the nitrogen content in molten steel before tapping as well as in the molten steel (slab) in accordance with the Al content within the range of this invention.

FIG. 2 shows the occurrence of blisters in the final products which are prepared by cold rolling and annealing continuous casting materials having varying nitrogen and Al contents as usual after hot rolling. It is observed that nitrogen and Al have a distinct influence on the occurrence of blisters. When the Al content is large, the occurrence of blisters is controlled even if the nitrogen content is relatively large. However, as the Al content diminishes, the nitrogen content should be kept small. Thus, the inventive condition that the nitrogen content is less than [Al(%) .times. 10.sup.3 + 40] ppm is deduced.

Next, the influence of the oxygen content will be discussed.

The influence of oxygen content on the occurrence of blisters differs largely but should be kept under about 80 ppm.

When the oxygen content is more than about 80 ppm, the occurrence of blisters is accelerated. However, it is proved that less than about 80 ppm of oxygen has almost relation to the occurrence of blisters. It is possible that this may perhaps be due to the fact that oxide inclusions formed by the presence of oxygen segregates when the oxygen content increases, and the blisters occur by the accumulation of gaseous hydrogen and nitrogen at the site of segregation.

For instance, in a continuous cast silicon-steel strip containing Si 3.15%, Al 0.004%, N 0.0052%, O 0.0085% and H 0.0002%, the analyses of oxide inclusions were carried out at locations where blisters occurred and did not occur. The result is as shown in Table 1.

As is obvious from Table 1, a large amount of oxide inclusion is observed at the site where blisters occur.

FIG. 3 shows the effect of the nitrogen content on the occurrence of blisters when the oxygen content is greater and less than 80 ppm.

FIG. 3 shows the occurrence of blisters in final products which are prepared by cold rolling and annealing continuous cast materials having varying nitrogen, oxygen and Al contents after hot rolling. It is observed that nitrogen, oxygen, and Al have a distinct influence on the occurrence of blisters.

As the Al content diminishes, it is necessary to reduce the nitrogen content. However, by restricting the oxygen content within 80 ppm, the nitrogen content for controlling the occurrence of blisters can be raised higher than in the case when the oxygen content is not controlled.

Thus, it is concluded that the nitrogen content can be eased up to [Al(%) .times. 10.sup.3 + 50] ppm when the oxygen content is restricted within 80 ppm.

As above explained, the present invention is characterized in controlling the nitrogen content in connection with the contents of nitrogen, or hydrogen and oxygen, as well as Al in order to prevent the occurrence of blisters in the final product.

With respect to producing the slab defined herein, the slab is heated at a temperature not lower than about 1,200.degree. C. It is then hot rolled, cold rolled with a reduction of not less than about 40% and subjected to decarburizaton annealing. Thereafter it is annealed at a temperature not less than about 1,100.degree. C.

Preferably, the cold rolling is performed two or more times with an intermediate annealing step.

In order to produce a grain oriented electrical steel sheet having excellent magnetic properties with high stability, a slab heating temperature of not lower than about 1,200.degree. C. is necessary, although the blister readily occurs in this temperature range in the conventional art.

The greater than about 40% cold rolling reduction is required for producing a grain oriented electrical steel sheet having excellent magnetic properties.

After the grains of a grain oriented electrical steel sheet is oriented by the secondary recrystallization, the annealing must be effected at a temperature not lower than 1,100.degree. C. for removing the impurities, and since blister more readily occurs at higher temperatures.

Examples of this invention will be explained in the following.

In the following examples, the conditions used to prepare the slabs are summarized in Table 2 hereinbelow:

Necessary amounts of Si and Al were added when molten steel was tapped from a 100-ton converter to a ladle. Thus, the deoxidation and alloy-addition was carried out at the tapping. Then, said molten steel was degassed in a vacuum and cast continuously. In the continuous casting, a first half of the casting, about 50 tons, was sealed with argon gas from the ladle to the tandish and from the tandish to the mold, and a latter half, about 50 tons, was cast for comparison without argon gas sealing. The analysis of the slab obtained was: C 0.035%, Si 3.15%, Mn 0.05%, P 0.010%, Al 0.024% and H 1.5 ppm. Said slabs were hot rolled to an intermediate thickness, and treated by a conventional two-stage cold rolling method to obtain grain-oriented electrical strips with a thickness of 0.30 mm. The change of nitrogen content from molten steel to slab of the said test materials and the occurrence of blisters in the final products were as shown in Table 3 hereinbelow:

As is obvious from Table 3, no blister formation occurs in the slab having the nitrogen content within the range of this invention obtained under the sealing with argon gas from the ladle to the mold in the continuous casting.

Magnetic properties of the final products are shown in Table 4 hereinbelow:

After tapping molten steel from a 100-ton converter to a ladle, degassing, deoxidation and alloy-addition treatment was carried out in a vacuum treatment apparatus. In the continuous casting of said molten steel, a first half of the casting, about 50 tons, was sealed with argon gas as an inventive example as in Example 1, and a latter half, about 50 tons, was cast for comparison without argon gas sealing. The analysis of the slab obtained was: C 0.042%, Si 3.22%, Mn 0.06%, P 0.012%, S 0.019%, Al 0.002% and H 1.2 ppm. Then, said slabs were hot rolled to an intermediate thickness, and treated by a usual two-stage cold rolling method to obtain a grain-oriented electrical strip with a thickness of 0.30 mm. The change of nitrogen content from the molten steel to the slab of said test materials and the occurrence of blisters in the final products were as shown in Table 5 hereinbelow:

As is obvious from Table 5 above, no blister formation occurs in the slab having the nitrogen content range of this invention obtained under the sealing with argon gas from the ladle to the mold in the continuous casting.

Two test materials, A and B, were smelted in a 100-ton converter. In tapping each of molten steel A and B, the deoxidation and alloy-addition treatment was carried out under the addition of the necessary amounts of Si and Al. In the continuous casting of each of molten steel A and B after a vacuum degassing treatment, a first half of the molten steel, about 50 tons, was sealed with argon gas from the ladle to the tandish and from the tandish to the mold, and a latter half of each of them, about 50 tons, was cast without argon gas sealing. The analyses of the slabs obtained were as shown in Table 6 hereinbelow:

Said slabs were hot rolled to an intermediate thickness, and treated by a usual two-stage cold rolling method to obtain grain-oriented electrical strips with a thickness of 0.30 mm.

The occurrence of blisters in the final products were as shown in Table 7 hereinbelow:

From Table 6 and Table 7, particularly by comparing argon gas sealed materials A and B, it is obvious that the occurrence of blister can be avoided by reducing the oxygen content, e.g., 52 ppm, even when the nitrogen content is relatively high, such as, 44 ppm.

The magnetic properties of the final products, treated similarly as in Example 1, are shown in Table 8 hereinbelow:

By a similar treatment as in Example 1, test materials C and D as shown in Table 9 (hereinbelow), having a relatively high Al content, were prepared. The occurrence of blister in the final products from these slabs were as shown in Table 10.

Among these test materials, argon gas sealed samples C and D are within the scope of this invention. In sample C, N<[Al(%) .times. 10.sup.3 + 50] ppm and 0<80 ppm, and sample D is an example of N<[Al(%) .times. 10.sup.3 + 40] ppm. There is no occurrence of blister in the final products from them. On the contrary, samples without argon gas sealing are without the scope of this invention, sample C being high in nitrogen content and sample D being high in nitrogen and oxygen contents, and blisters occurred in both samples.

The magnetic properties of the final products, treated similarly as in Example 1, are as shown in Table 11 hereinbelow: