GB2093480A - Non-oriented silicon steel sheet - Google Patents

Abstract

A sheet with a low watt loss value as well as uniform magnetic properties contains in wt.% Si 1.5-3.5 Mn 0.05-1 Al 0.005-0.08 N </= 0.005 B sufficient to provide a B:N ratio of 0.4:1 to 2:1 Rare earth metal 0.001-0.02

Description

SPECIFICATION High-grade non-oriented silicon steel sheet The present invention relates to a high-grade non-oriented silicon steel sheet, and more particularly, to a non-oriented silicon steel sheet having excellent magnetic properties. The sheet contains 1.5 - 3.5% Si, not more than 0.015% C, and not more than 0.08% acid soluble Al (referred to as "sol Al" hereinafter), and which has B added thereto to obtain a balanced ratio of B to N and further contains a rare earth element.

Silicon steel sheets which are soft magnetic materials include grain-oriented silicon steel sheet composed of a recrystallized collective texture crystallographicallyexpressed as (110) [001] having its (1 10) plane on the rolling plane and its [001] orientation in the rolling direction; and a non-oriented silicon steel with negligible orientation. These materials are used for iron cores of electrical devices and are chosen for specific applications in accordance with their properties (magnetic, mechanical etc.) and their production cost. For instance, grain-oriented silicon steel sheet is generally used for large capacity transformers and apole transformers regardless of its high cost because it has excellent magnetic properties.In particular, it is very easily magnetizable iri the rolling direction, that is in the [001] direction, has a very low value of watt loss (consists chiefly of hysteresis loss and eddy current loss), and has very high permeability.

Non-oriented silicon steel sheet with which this invention is concerned is usually categorized into low-grade non-oriented silicon steel sheet having a Si content between near zero and 1.5%, and highgrade non-oriented silicon steel sheet containing more than 1.5% Si. As these materials have an orientation so small as to be negligible and are low in cost, they are widely used for small-sized electric motors, medium-sized transformers, and medium-to-large-sized rotating machines. A key property required of non-oriented silicon steel sheet is a low watt loss value. In non-oriented silicon steel sheet, hysteresis loss is most effectively reduced by increasing the crystal grain size and eddy current loss can be reduced by increasing the specific resistance of the steel, in other words by adding an alloying element such as Si or the like.One means for enlarging the crystal grain size is by high-temperature annealing carried out over an extended period of time. This is, however, economically disadvantageous and difficult in practice because sufficiently long annealing times are difficult to obtain with normal continuous annealing processes.

Inhibited growth of crystal grain size is known to be closely related to dispersed inclusions and precipitates. The size of the inclusions and precipitates and the state of their dispersion are very significant. It is considered in general that the presence of many fine inclusions and precipitates is not beneficial to the growth of the crystal grains. Substances present in the dispersed phase in non-oriented silicon steel sheet include various oxides, such as Al203, SiO2 and the like, and various sulphides and nitrides, such as MnS and AIN.

As for the oxides, it is only necessary to collect and float them upwardly as deoxidized products in the steel making step and to prevent oxidizing at the time of casting the steel melt into ingot moulds.

Owing to recent progress in steel making techniques, inclusion of oxides can be easily prevented.

A widely used method for contending with sulphides is to thoroughly carry out desulphurizing in the steel making step while, at the same time, adding a suitable amount of Mn or the like, and then maintaining the heating temperature of the slab prior to the hot rolling step so low that the MnS can be converted to a substance harmless to the growth of the crystal grains. Because of the use of this method, sulphides to not pose a problem.

To hold down the inclusion of nitrides, it is best to keep the nitrogen content as low as possible in the steel making step. It is, however, very difficult to reduce the nitrogen content in silicon steel to consistently less than 10 ppm, and about 30 ppm of nitrogen is usually present.

The combination of N with Al used for deoxidizing or with Al contained in a silicon alloy results in the formation of AIN. As an Al content in the range of 0.005 - 0.1 5% is detrimental to the growth of the crystal grains, it has been usual in the production of low-grade non-oriented silicon steel sheet containing less than about 1.0 - 1.5% Si to reduce the Al used for deoxidizing as much as possible, preferably to less than 0.005%. But in high-grade non-oriented silicon steel sheet containing more than about 1.0 - 1.5% Si the Al content is controlled to be more than 0.15% and AIN is precipitated in a form harmless to the growth of the crystal grains when the steel slab is heated.

However, the former case gives rise to such difficulties as insufficient deoxidation in the steel making step or the occurrence of defects on the surface of the steel sheet. In the latter case, the production cost inevitably rises due to the consumption of a large quantity of expensive Al.

As an effective method for stabilizing deoxidation in the steel making process while avoiding the rise in cost caused by the use of large amounts of Al, the inventors previously developed an inexpensive method for manufacturing non-oriented silicon steel sheet having an excellent watt loss value by the addition of B. It has been found however, that when steel sheet having a very low C content (0.015% or less) is produced by the method the product is not entirely uniform in its magnetic properties.

Accordingly, it is an object of the present invention to provide a non-oriented silicon steel sheet with uniform magnetic properties which has B content adjusted to obtain a defined ratio of B to N and further contains at least one rare earth element.

The present invention is directed to a non-oriented silicon steel sheet having uniform magnetic properties containing < 0.015% C, 1.5-3.5% Si, 0.05-1.0% Mn, 0.005%-0.08% Sol Al, ~0.015% S, ~0.010% 0, ~0.005% N, B in an amount to obtain a ratio of B to N in the range of 0.4-2.0 and 0.0010.02% of at least one rare earth element, the balance being Fe and unavoidable impurities.

Other objects of the invention will be better understood from the following detailed description with reference to the accompanying drawings, in which: Figure 1 is a diagram showing the relationship between the Si content and the ratio of precipitated nitride to total nitrogen; Figure 2 is a graph showing the relationship between the Ce content and the watt loss value; and Figure 3 is a diagram explaining the variation in watt loss values.

As a result of detailed investigations conducted by the inventors on the precipitation behaviour of nitride in the very low C zone (0.015% or less), it has been found that in steel containing 1.5% or more Si, particularly, 2.0% or more Si and 0.010% or less C, the amount of N precipitated as nitride in the hot rolled sheet is very small. Figure 1 is the graph showing the relationship between the ratio (%) of nitride precipitate to the total nitrogen and the Si content (%). The chemical composition of the sample was such that C < 0.010% Sol Al = 0.008-0.050%. N < 0.005%, and B=0.0010-0.0050%. Thus, it was apparent that the insufficient precipitation of nitride in the hot-rolled steel was the cause of the variation in watt loss value in the very low C zone.

Following this finding regarding low nitride precipitation, it was further discovered that the tendency toward slight variation in the watt loss value could be eliminated and the magnetic properties could be made uniform by adding b to silicon steel having a very low C content and a low Sol al content so as to be in the ratio of B to N within a defined range, and further by causing a small quantity of a rare earth element to coexist therewith.

The steel used for producing a magnetic material in accordance with the invention is melted in a steel refining furnace (eg. an electric furnace, an open hearth furnace, a converter or the like) and then, if required, is further refined in a vacuum refining furnace to reduce its C content to 0.01 5% or less, after which the required amounts of Al, Si, Mn, B and rare earth element are added thereto.

The C content of the steel is adjusted to not more than 0.015% be decarburizing in the postprocessing step since C exerts a harmful effect on the magnetic properties and magnetic ageing. If the C content exceeds 0.015%, the time required for decarburization will be prolonged, giving rise to an economic disadvantage.

The Si content is adjusted in accordance with the required watt loss level. A Si content above 3.5% however, is detrimental to the cold rolling property of the steel. As the present invention aims at the production of high-grade non-oriented silicon steel sheet, the Si content is specified as falling in the range of 1.53.5%.

Al is added to deoxidize the steel and a Sol Al content of less than 0.005% results in insufficient deoxidation. Moreover, an insufficient Sol Al content causes problems in the casting of ingots, results in an increase in silver on the surface of the steel product, and causes poor yields of B and the rare earth element. Therefore, the Al content is specified as being not less than 0.005%. But a large Al content will not only result in increased cost but also will cancel out the effect of the addition of B and the rare earth element. Therefore, the Al content should be not more than 0.08%. The preferable range of the Sol Al content is 0.015-0.050%.

Mn is added in an amount of not less than 0.05% in order to prevent brittle fracture in the hot rolling step and to compensate for the specific resistance of the steel due to the low content of Al so as to reduce this watt loss. On the other hand, the upper limit of Mn is set at 1.0% because the magnetic properties of the steel are degraded by an excessive Mn content.

As S is injurious to the magnetic properties, it is specified as being not more than 0.015% and preferably not more than 0.007%.

In general, since 0 is detrimental to the magnetic properties and induces useless consumption of B and the rare earth element, it is limited to not more than 0.010%.

If N exceeds 0.005%, it is detrimental to the magnetic properties and causes an economic disadvantage by increasing the required B content described hereinbelow. Hence the upper limit of the N content is fixed at 0.005%.

The B content of the steel is adjusted so that the ratio by weight of B to N falls within the range of 0.4 - 2.0 since the watt loss increases when this ratio falls above or below this range. When thrarare earth element described hereinbelow is also present, the preferred range of the B/N ratio from t! ie point of reducing the watt loss is 0.6 - 1.5.

Another important element in addition to the above B is the rare earth element. Not less than 0.001% of at least one rare earth element is required in order to prevent the variation in magnetic properties observed to some extent in very low C low Sol Al silicon steel. If the content of the rare earth element exceeds 0.02%, the quantity of inclusions increases sufficiently to be detrimental to the magnetic properties. Therefore, the content of the rare earth element is specified as falling in the range of 0.001 - 0.020%.

The results of an investigation conducted on the watt loss values very low C - low Sol Al silicon steels containing Ce as a rare earth element together with B are shown in Figure 2. In addition to Ce and B, the sample steels contained 0.003 - 0.008% C, 0.005-0.080% Sol Al and 2.2% Si.

In Figure 2: X indicates : No B added.

A indicates : B/N under 0.4 or above 2.0 o indicates : B/N in the range of 0.4 - 2.0 As clearly shown in Figure 2, the silicon steels which had a B/N ratio in the range of 0.4-2.0 and also contained Ce showed a low watt loss value. Particularly, the silicon steels with 0.001 - 0.020% Ce had a small watt loss value as well as excellent magnetic properties.

The effect of the coexistence of B and Ce is remarkable in steels having a very low C content of not more than 0.005%. But the watt loss value was high in the case of silicon steels containing Ce alone or having a B/N ratio under 0.4 or above 2.0.

As the rare earth elements there can be used a mixture of metals having atomic numbers between 57 and 71. Mischmetal which contains about 50% Ce, the remainder being chiefly lantha um and neodymium, is an inexpensive example falling in the class. Ce is preferred.

The rare earth element may conveniently be added when deoxidizing has been completed and the regulation of the composition has been completed.

In the present invention, the steel melt having the appropriately adjusted composition is cast to form a slab by the continuous casting process or is cast in a mould to form an ingot, which is further bloomed into a slab. Otherwise, a slab obtained by sizing rolling a continuous cast slab may be used.

The steel slab is usually heated to a temperature in the range of 1050 to 1 2500C in a heating furnace, and rolled to a thickness in the range of about 1.5 to 3.0 mm. After the oxidized surface layer of the hot rolled sheet has been removed, it is rolled to the thickness of the final product. Then, the hot-rolled sheet may be subjected to annealing or to double cold-rolling, in which case annealing is performed between the two rolling operations, or to a combination of these steps. The cold-rolled sheet is subjected to continuous annealing and coated with an insulating film to obtain a final product.However, the method of casting the steel melt into a slab or ingot, and of further processing the slab or ingot may be selected in any way desired without departing from the spirit and object of the invention whose scope is defined in the claims.

The invention will now be further illustrated in the accompanying Examples: EXAMPLE 1 Steel slabs having the compositions shown in Table 1 were produced by preparing steel in a converter, refined in a vacuum degassing vessel, and formed into slabs by the continuous casting process. These slabs were heated to 11 500C in a heating furnace, hot-rolled to 2.3 mm thickness, subjected to annealing at 9000C as required, pickled, and then cold-rolled to 0.5 mm thickness.

Thereafter the cold-rolled sheets were annealed at 9000 or 9500C for 60 seconds. The magnetic properties of an Epstein test sample of each of the above steels are shown in Table 2.

EXAMPLE 2 Samples Nos. 3 and 11 of Example 1 were chosen for their very low C content and 20 hot-rolled sheets of each sample were prepared, subjected to pickling, cold rolling to 0.5 mm thickness, and then to annealing at 9000C for 60 seconds. The average value of the watt loss and the variation therein was determined for the sheets. Figure 3 is a diagram explaining the variation in watt loss values for sample Nos. 3 (white) and 11 (hatching). As is clear from the Figure 3, the sample according to the present invention was superior to the reference sample especially in the range of watt loss values W1 5/50 between 3.6 and 4.0 W/kg.

Analytical values, average values of the watt loss, and the standard deviation of the samples are listed in Table 3.

As is clear from the results of Examples 1 and 2, nonoriented silicon steel sheet having a low watt loss as well as small deviation can be manufactured by the present invention.

TABLE 1

Sample Classification C Si Mn SolAi S N B B/N Ce O % % % % % % % % % % 1 This invention 0.003 2.20 0.25 0.025 0.007 0.0022 0.0020 0.91 0.004 0.003 2 " 0.004 2.22 0.30 0.065 0.007 0.0018 0.0030 1.67 0.008 0.002 3 " 0.003 2.15 0.41 0.028 0.006 0.0026 0.0020 0.77 0.014 0.002 4 " 0.003 2.25 0.66 0.018 0.007 0.0021 0.0030 1.43 0.017 0.004 5 " 0.004 2.88 0.30 0.022 0.005 0.0026 0.0022 0.85 0.005 0.002 6 " 0.005 2.93 0.62 0.043 0.005 0.0023 0.0031 1.35 0.009 0.002 7 " 0.003 2.92 0.22 0.032 0.004 0.0023 0.0020 0.87 0.016 0.002 8 " 0.004 2.83 0.18 0.044 0.004 0.0020 0.0024 1.20 0.011 0.002 9 Reference material 0.004 2.30 0.26 0.031 0.007 0.0026 0.0022 0.85 0.026 0.004 10 " 0.004 2.16 0.20 0.022 0.006 0.0031 - - 0.009 0.003 11 " 0.003 2.27 0.36 0.024 0.007 0.0022 0.0022 1.00 - 0.002 12 " 0.001 2.19 0.33 0.021 0.006 0.0033 0.0029 0.88 0.010 0.004 13 " 0.004 3.01 0.22 0.033 0.004 0.0024 0.0020 0.83 0.024 0.002 14 " 0.004 2.88 0.22 0.038 0.004 0.0028 - - 0.013 0.002 15 " 0.003 2.95 0.26 0.035 0.004 0.0023 0.0024 1.04 - 0.002 16 " 0.025 2.83 0.21 0.041 0.004 0.0026 0.0020 0.77 0.008 0.002 TABLE 2

Final Magnetic propartias Sample Processes to which the sheet was subjected following annealing No. Classification hot rolling temperature W15/50 B50 W/Kg T 1 This invention Pickling-Cold Rolling-Final Annealing 900 C 3.89 1.67 2 " " " 3.95 1.66 3 " " " 3.98 1.65 4 " " " 4.05 1.66 5 " Annealing-Pickling-Cold Rolling-Final Annealing 950 C 3.25 1.68 6 " " " 3.19 1.67 7 " " " 3.30 1.68 8 " " " 3.31 1.68 9 Reference material Pickling-Cold Rolling-Final Annealing 900 C 4.52 1.66 10 " " " 4.68 1.67 11 " " " 4.18 1.66 12 " " " 4.42 1.67 13 " Annealing-Pickling-Gold Rolling-Final Annealing 950 C 3.88 1.69 14 " " " 3.94 1.69 15 " " " 3.55 1.69 16 " " " 3.69 1.68 Note : Trefers to TESLA TABLE 3

Chemical Analytical Value (%) Watt Loss Value Sample Sample Standard No. C Si Mn SolAi S N B B/N Ce Average Dev.

W/Kg W/Kg 8 0.003 2.15 0.41 0.028 0.006 0.0026 0.0020 0.77 0.014 3.93 0.15 11 0.003 2.27 0.36 0.024 0.007 0.0022 0.0022 1.00 - 4.18 0.25 Note : Dev. stands for Deviation

Claims (9)

1. A non-oriented silicon steel sheet having uniform magnetic properties comprising < 0.01 5% of carbon, 1.5-3.5% of silicon, 0.05-1.0% of manganese, 0.005-0.08% acid-soluble aluminium, < 0.01 5% of sulphur, #0.010% of oxygen, < 0.005% of nitrogen and boron in such amount that the ratio of boron to nitrogen is in the range of 0.4-2.0,0.001-0.020% of at least one rare earth element, the remainder of Fe and unavoidable impurities.

2. A steel sheet according to claim 1, wherein the soiuble Al content is 0.015-0.050%.

3. A steel sheet according to claim 1 or 2, wherein the sulphur content is not more than 0.007%.

4. A steel sheet according to any preceding claim wherein the boron:nitrogen ratio is 0.6-1.5.

5. A steel sheet according to any preceding claim, wherein the amount of rare earth element is 0.001-0.020%.

6. A steel sheet according to any preceding claim, wherein the rare earth element is cerium.

7. A steel sheet according to any preceding claim, wherein the rare earth element is mischmetal.

8. A steel sheet according to Claim 1 containing < 0.005% c,0.005-0.02% acid-soluble Al and 0.001-0.020% Ca.

9. A non-oriented steel sheet having uniform magnetic properties substantially as hereinbefore described in any of the Examples.