US3908432A

US3908432A - Process for producing a high magnetic flux density grain-oriented electrical steel sheet

Info

Publication number: US3908432A
Application number: US451231A
Authority: US
Inventors: Tadashi Ichiyama; Takashi Sato; Tsuyoshi Kikuchi
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1973-03-20
Filing date: 1974-03-14
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30
Also published as: GB1456445A; FR2222442A1; SE409472B; BE812533A; FR2222442B1; JPS49119817A; IT1009683B

Abstract

Process for producing a high magnetic flux density grainoriented electrical steel sheet comprising; subjecting silicon steel ingot containing 0.005 to 0.1% of carbon, 2.5 to 4.0% of silicon, 0.01 to 0.10% of selenium and one of 0.01 to 0.15% of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and 0.02 to 0.2% of antimony with a sulfur content low enough to be nonharmful to desired magnetic properties to cold rolling in which the reduction of the final cold rolling step is not less than 70%, and then to decarburization and final annealing.

Description

United States Patent lchiyama et al.

[451 Sept. 30, 1975 PROCESS FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY GRAIN-ORIENTED ELECTRICAL STEEL SHEET Inventors: Tadashi lchiyama, Sagamihara;

Takashi Sato; Tsuyoshi Kikuchi, both of Kawasaki, all of Japan Assignee: Nippon Steel Corporation, Japan Filed: Mar. 14, 1974 Appl. No.: 451,231

Foreign Application Priority Data Mar. 20, 1973 Japan 48-31452 U.S. C1 72/365; 148/111 Int. C1. B2113 3/02 Field of Search 148/110, 111; 72/700, 366,

References Cited UNITED STATES PATENTS 10/1967 Dctcrt 148/111 3,802,936 4/1974 Goto et a1 148/111 Prinulry ExaminerLowell A. Larson Attorney, Agent, or Firm-Toren, McGeady and Stanger nium and one of 0.01 to 0.15% of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and 0.02 to 0.2% of antimony with a sulfurcontent low enough to be non-harmful to desired magnetic properties to cold rolling in which the reduction of the final cold rolling step is not less than 70%, and then to decarburization and final annealing.

2 Claims, 2 Drawing Figures PROCESS FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY GRAIN-ORIENTED ELECTRICAL STEEL SHEET The present invention relates to commercial production of grain-oriented electrical steel sheets having a very high degree of integration of the so-called Goss structure having the magnetizable axis 100 along the rolling direction of the sheet and the (1 l) plane in the surface of the steel sheet.

Grain-oriented electrical steel sheet has been used mainly for iron cores for electrical appliances such as, transformers and other, wherein good excitation characteristics and good watt loss properties are required. The excitation characteristic is expressed by the value of the magnetic flux density in a magnetic field of 800A/m, and the watt loss properties are expressed by the electricity loss at a magnetic flux density of 17 K Gauss.

Methods for producing the high magnetic flux density, grain-oriented electrical steel sheet having good excitation and watt loss characteristics as well as good magnetostriction characteristics were disclosed by the present inventors in Japanese Patent Applications Sho 45-92277 and Sho 46-82442. According to the proposed methods, silicon steel ingots are produced by adding one or more of arsenic, bismuth, lead and antimony together with small amount of selenium using the conventional steel making and ingot casting methods. The hot rolled steel sheet obtained from the ingot is subjected to a cold rolling process including a final cold rolling step of more than 80% reduction so as to produce a high magnetic flux density grain-oriented electrical steel sheet having B characteristics of more than 1.85 wb/m The present invention relates to improvements of the above methods disclosed in the Japanese Patent Applications Sho 45-92277 and Sho 46-82442. Namely, the present invention defines additional conditions for the constituent elements added in the starting materials, and by these additional conditions, the present inventors have succeeded in improving the excitation and watt loss characteristics and in reducing fluctuation in the excitation characteristics of the final products.

The present invention will be described referring to the attached drawings.

FIG. I shows the relation between the sulfur contents and the excitation characteristics in a 3% silicon steel containing one of four elements from the group of arsenic, bismuth, lead and antimony, in addition to selenium in case of the reduction of not less than 70% in the final cold rolling step.

FIG. 2 shows similarly the relation between the sulfur contents and the B values when the final cold rolling step is performed at a reduction rate between 50 and 60%.

In FIG. 1, the values of magnetic flux density B at a 'magnetic field of 800A/m are plotted against the sulfur than 0.01%, the B value becomes remarkably high and the fluctuation is also improved. When a special treatment is applied, a normal sulfur content in the steel ingot is usually between 0.015 and 0.025%. Therefore, in order to maintain the sulfur content to not more than 0.01% and obtain this remarkable improvement of the magnetic properties, it is necessary to lower the sulfur content by using special methods, such as, the rotary kiln method, the DM converter method and the shaking ladle method.

I In FIG. 2, the relation between the sulfur contents and the value of the magnetic flux density B when the reduction of the final cold rolling step is between 50 and 60% which is a normal reduction in the production of grain-oriented electrical steel sheets is shown. It is understood from FIG. 2 that the lowering of the sulfur content does not improve the magnetic flux density value, but rather increases the fluctuation of the value. This means that the new additionalcondition of the sulfur content among the constituent elements is effective when the reduction of the final cold rolling step is higher than the normal reduction of 50 to 60%.

The lowering of the sulfur content to not more than 0.01% improves the B characteristics of the magnetic flux density not only in the case of the combination addition of one of arsenic, bismuth, lead, and antimony with selenium, but also in the case of the combination addition of two or more of arsenic, bismuth, lead and antimony with selenium as disclosed in Japanese Patent Application Sho 46-82442. Based on the above facts,

the chemical composition of the starting material used in the present invention has been defined as follows:

Carbon 0.005 to 0.1%,

Silicon 2.5 to 4.0%,

Sulfur not more than 0.01%, and Selenium 0.01 to 0.1%, with one element selected from the group of Arsenic 0.01% to 0.15%, Bithmus 0.02 to 0.3%, Lead 0.02 to 0.3%, and Antimony 0.02 to 0.2%.

When two or more of the last four elements are added, the total amount should be between 0.02 and 0.5%, with the balance being iron and traces of impurities which are harmless to the magnetic properties.

The reasons for the above limitations of the various elements in the steel are as follows. When the silicon content is less than 2.5%, the a 'y transformation takes place irrespective of the carbon content whatsoever, so that growth of the secondary recrystallization grains take place during the final annealing which is normally done at a temperature not lower than 1000C. On the other hand, silicon contents of more than 4.0% tend to cause crackings due to embrittlement during the cold rolling. Thus the silicon content is defined to 2.5 to 4.0% in the present invention.

Carbon is necessary for promoting the presence of a precipitated dispersion containing arsenic, bismuth, lead or antimony, and when the total amount of the elements constituting the precipitated dispersion is small, the carbon content may be in the higher side in the range and when the total amount is large, it may be in the lower side. 7

Each of arsenic, bismuth, lead and antimony, when present together with selenium, forms a fine dispersion and is necessary for the secondary recrystallization, after the final annealing, to be attained with a strong reduction of at least 70% in the final cold rolling. Sufficient dispersion can not be obtained if the contents of these elements are lower than the lower limit of these ranges. Although their contents may be safely beyond the upper limits, such larger contents cause increased production cost.

Although no theoretical explanation can be given for the fact that the excitation and watt loss characteristics of the final products can be remarkably improved when the sulfur content in the starting silicon steel ingot is maintained at not more than 0.01%, the present inventors have the following comments.

The excitation and watt loss characteristics of a grain-oriented electrical steel sheet depend mainly on the integration degree of the Goss structure formed by the secondary recrystallization which takes place during the final annealing.

Thus, a higher degree of the Goss structure assures better excitation and watt loss characteristics. In order to obtain a high degree integration of the Goss structure, it is necessary that the reduction in the final cold rolling step is higher than the normal reduction of 50 to 60% and that the dispersion which shows special thermal behavior near the recrystallization temperature is utilized. For obtaining this dispersion, the combined addition of arsenic, bismuth, lead and antimony with selenium is effective as disclosed in Japanese Patent Applications Sho 45-92277 and Sho 46-82442. But in this case, sulfur, such as, MnS, which forms dispersion showing a different thermal behavior is harmful. Thus, in order to permit extraordinary growth of only the primary recrystallization having the ideal Goss orientation produced by the high cold rolling reduction, it is considered to be necessary that the state of the dispersion is changed sharpy near the starting temperature of the secondary recrystallization. For this purpose, it is not desirable to use a plurality of dispersions having different thermal behaviors.

According to the present invention, silicon steel ingots having a chemical composition falling within the above defined range are produced by a conventional method and hot rolledinto hot rolled steel sheets of 1.5 to 5.0 mm thickness. In this case, the magnetic properties of the final products are considerably improved when the steel material is retained for 60 to 360 seconds within a temperature range of l200 to 850C in the course of the continuous hot rolling step. The hot rolled steel sheet, after acid pickling, is subjected one time or more than two times to the cold rolling step in such a-manner that the reduction of the final cold rolling is not less than 70%. In this case, when the sheet is subjected more than two times to the cold rolling step, the sheet is subjected to an intermediate annealing for l to 30 minutes at a temperature between 750 and 1200C in a neutral or reducing gas between the cold rollings. When the final sheet thickness is to be obtained by a single cold rolling step, it is desirable to subject the hot rolled steel sheet to a preliminary annealing for l to 30 minutes at a temperature between 750 and l200C in a neutral or reducing gas prior to the cold rolling.

The reason for defining the reduction of the final cold rolling as not less than 70% is to obtain a high magnetic flux density B of more than 1.85 wb/m However it has been found that a reduction rate of not less than 60% is enough due to the considerably improved level of the magnetic properties by the improvement of the hot rolling conditions as described above.

The steel sheet which has been reduced into the final thickness by the cold rolling is subjected to a short-time annealing by a conventional method for the purpose of decarburization and primary recrystallization. For example, the sheet is annealed in a gas mixture of wet hydrogen and nitrogen at a temperature between 800 and 850C for a few minutes. Finally the sheet is annealed in a reducing gas at a temperature of more than 1000C, preferably more than ll00C, for more five hours. During the annealing, elements harmful to the watt loss characteristics, such as, carbon, selenium, and the remaining sulfur are removed and the secondary recrystallization proceeds so that a grain-oriented electrical steel sheet having very high magnetic flux density is obtained in a consistent and stable manner.

Examples of the present invention will be set forth under.

EXAMPLE 1 A silicon steel ingot containing 0.038% of carbon, 2.98% of silicon, 0.006% of sulfur, 0.03% of selenium, and 0.1% of arsenic was broken down into 200 mm thick slabs. The slabs were heated to l250-"-C and then subjected to continuous hot rolling to obtain hot rolled steel coils of 2.3 mm thickness. During the cooling step of the above hot rolling, adjustments were made so as to maintain the steel coils in a temperature range of l200 to 850C for 200 seconds. The hot rolled steel coils were acid pickled and some of them was subjected to preliminary annealing at 900C for 5 minutes and the remainder were not subjected to the preliminary annealing. Then all of the coils were reduced to 0.30 mm thickness by a single-step cold rolling and subjected to decarburization annealing and finishing high temperature annealing.

The magnetic properties along the rolling direction of the products thus obtained are as follows.

1. Product not subjected to preliminary annealing L935 wb/m L18 watt/kg Magnetic flux density:

Watt loss:

2. Product subjected to preliminary annealing 1.965 wb/m 1.10 watt/k Magnetic flux density:

Watt loss:

EXAMPLE 2 Silicon steel ingot containing 0.045% of carbon, 3.01% of silicon, 0.004% of sulfur, 0.05% of selenium and 0.05% of arsenic was broken down into slabs of 200 mm thickness. The slabs were heated to 1300C and subjected to continuous hot rolling to obtain hot rolled steel coils of 2.3 mm thickness. During the cooling step, the hot rolling adjustments were made so as to maintain the steel coils in a temperature range of l200 to 850C for 300 seconds. The thus obtained steel coils were acid pickled, cold rolled to an intermediate thickness of 1 mm, subjected to intermediate annealing at 850C for 5 minutes in hydrogen stream and then secondary cold rolling to obtain the final thickness of 0.30

mm. The cold rolled steel sheet thus obtained was subjected to decarburization annealing and finishing high temperature annealing. The magnetic properties along the rolling direction of the product are as follows.

1.92 wb/m l.l2 watt/kg Magnetic flux density: Watt loss:

u w l'llbll carbon, 2.5 to 4.0% of silicon, 0.01 to 0.10% of selenium and one of an element selected from the group consisting of 0.01 to 0.15% of arsenic, 0.02 to 0.3% of bismuth, 0.02 to 0.3% of lead, and 0.02 to 0.2% of antimony with a sulfur content not more than 0.01% to cold rolling in which the reduction of the final cold rolling step is not less than then decarburization and final annealing.

2. The process according to claim 1 in which the silicon steel ingot contains two or more than two of an element selected from the group consisting of arsenic, bismuth, lead and antimony in a total amount of 0.02 to 0.5%.

Claims

1. PROCESS FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY GRAINORIENTED ELECTRICAL STEEL SHEET COMPRISING: SUBJECTING SILICON STEEL INGOT CONTAINING 0.005 TO 1% OF CARBON, 2.5 TO 4.0% OF SILICON, 0.01 TO 0.10% OF SELENIUM AND ONE OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF 0.01 TO 0.15% OF ARSENIC, 0.02 TO 0.3% OF BISMUTH, 0.02 TO 0.3% OF LEAD, AND 0.02 TO 0.2% OF ANTIMONY WITH A SULFUR CONTENT NOT MORE THAN 0.01% TO COLD ROLLING IN WHICH THE REDUCTION OF THE FINAL COLD ROLLINGG STEP IS NOT LESS THAN 70%, THEN DECARBURIZATION ANDFINAL ANNEALING.

2. The process according to claim 1, in which the silicon steel ingot contains two or more than two of an element selected from the group consisting of arsenic, bismuth, lead and antimony in a total amount of 0.02 to 0.5%.