US3846187A

US3846187A - Slab and plate cooling method for producing grain oriented electrical steel

Info

Publication number: US3846187A
Application number: US00299308A
Authority: US
Inventors: A Sakakura; F Matsumoto; K Ueno; K Takashima; K Kuroki
Original assignee: Nippon Steel Corp
Current assignee: NIPPON STEEL CORP US; Nippon Steel Corp
Priority date: 1971-10-22
Filing date: 1972-10-20
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05
Also published as: JPS4847425A; JPS5026495B2

Abstract

1. IN A METHOD FOR PRODUCING A HIGH MAGNETIC FLUX DENSITY, GRAIN ORIENTED ELECTRIC STEEL OR STRIP OF THE TYPE WHEREIN A STEEL INGOT PRODUCED BY CONVENTIONAL METHODS IS BROKEN DOWN INTO A SLAB, THE SLAB IS HOT ROLLED TO FORM A PLATE, THE PLATE IS COLD ROLLED IN AT LEAST ONE STEP INCLUDING A FINAL COLD ROLLING AT A REDUCTION RATE BETWEEN 65 TO 95% TO FORM A SHEET AND THE SHEET IS DECARBONIZED AND FINALLY ANNEALEED AT A TEMPERATURE ABOVE 800*C., THE IMPROVEMENT WHEREIN THE STEEL INTO CONSISTS ESSENTIALLY OF NOT MORE THAN 4.0% SI, NOT MORE THAN 0.085% CARBON 0.010 TO 0.065% ACID SOLUBLE AL, AND NOT MORE THAN 0.012% N, THE SLAB IS HEATED TO ABOVE 1200*C. TO DISSOLVE THE AIN AND THEN HOT ROLLED SUCH THAT THE SLAB IS COOLED TO A TEMPERATURE BETWEEN 1000 TO 1250*C. IN LESS THAN 200 SECONDS FROM THE TIME THE SLAB IS REMOVED FROM THE HEATING STEP, AND THE RESULTING PLATE IS COOLED FROM 1000-1250*C. TO 600*C. IN LESS THAN 200 SECONDS.

Description

, Nov. 5, 1974 Filed Oct.

IIQO

AKIRA SAKAKURA ETAL 3,846,187 SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED

ELECTRICAL STEEL

20, 1972 6 Sheets-Sheet 1 Temperature Held for I200C x I50 sec.

Hel'd for \l 0 0C x 50 sec.

Time (sec.)

I FIG. I

Nov. 5, 1974 AKlRA SAKAKURA ETAL 3,845,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL Filed Oct. 20, 1972 6 Sheets-Sheet 2..

Be ("b/m2) 2.3% Si' l l L85 unable for secondary recrystallization b IO O 2 00 (seconds) Holding Time at Various Temperatures FIG.2

Temperature 0) Nov. 5, 1974 AKIRA SAKAKURA ETAL 3,846.,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL s Sheets-Sheet s Filed Oct. 20, 1972 e (Wb/m Extrusion v (Slob thickness 40 m/m) 2.95 /a Si 9 i200- IOOO' L95 Cooling Curve 600 During Hot Rolling Water Cooling Cooled 1 I85 50() "1 Air Time from Finishing of Hot Rolling to Start of Cooling (Seconds) FIG. 5 g

Nov. 5, 1974 AK|RA SAKAKURA ETAL 3,846,187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL 6 Sheets-Sheet t Filed Oct. 20, 1972 vdE 09 On 0- m o c u ll l c ul' eo i 6 .-v&%\\no\\\o l 1 1 Q mvo ON o mm o O- Om Om 007 ON m2: @500 owl o Q A 9m o 2 00m 00v 00m 00m 009 OON OOE (1%) emuuadwa Nov. 5, 1974 AKIRA SAKAKURA ETA!- 3.345.187

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL .6 Sheets-Sheet 5 Filed 001:. 20, 1972 and 9 A35 25 s 8.: @ii 8 mmd YQQE

Nov. 5, 1974 AKIRA SAKAKURA F-TAL 5. 87

SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL 6 Sheets-Sheet 6 Filed Oct. 20, 1972 86 $252; 33 E: w m

3,846,187 SLAB AND PLATE COOLING METHOD FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL Akita Sakakura, Fumio Matsumoto, Kiyoshi Ueno, Kunihide Takashima, and Katuro Kuroki, Kitakyushu, gapan, assignors to Nippon Steel Corporation, Tokyo,

apan

Filed Oct. 20, 1972, Ser. No. 299,308 Claims priority, application Japan, Oct. 22, 1971, 46/ 83,736 Int. Cl. H01f 1/04 US. Cl. 148-112 4 Claims ABSTRACT OF THE DISCLOSURE A method for producing a high magnetic flux density grain oriented electrical steel sheet, comprising breaking down and hot rolling a steel ingot containing not more than 4.0% of Si, not more than 0.0085% of C, 0.010- 0.065% of acid soluble Al, and not more than 0.012% of N, cold rolling the plate by at least one step including a final cold rolling with 65-95% reduction rate depending on the silicon content and an intermediate annealing, decarbonizing the sheet and finally annealing the sheet at a temperature above 800 C., said hot rolling comprising heating the slab at a temperature above 1200 C. to dissolve AlN and rolling the slab into a desired plate thickness under the following conditions of heating cycle (1) from slab extraction from the heating step, and during hot rolling to produce a plate, the time for cooling the plate to a temperature between 1000-1250 C. depending the silicon content is less than 200 seconds; and

(2) after the above cooling during hot rolling to 1000 to 1250" C., the time for cooling the plate to 600 C., is less than 200 seconds.

The present invention relates to a process for producing so-called grain oriented electrical steel sheets having an easy magnetization in the rolling direction of the steel sheet.

A grain oriented electrical steel sheet is commonly used as soft magnetic material primarily for the iron cores of electric appliances, such as, transformers, and it is important, with reference to the magnetic properties to have a good magnetization characteristic as well as a good iron loss characteristicv Recently the minimization of size of the electric appliances has been increasingly important, and for this purpose, it is necessary to reduce the weight of the iron cores. In general, in order to reduce the iron core weight, it is necessary to utilize high magnetic flux density so that magnetic materials having good magnetization characteristic, namely the B characteristic (magnetic flux density at magnetization of 8 AT/cm.) are required (AT means ampere-turn). Also when the high magnetic flux density is utilized, the iron loss value also increases. In this connection, as compared with a magnetic material having low B characteristic, a magnetic material having high B characteristic shows much better iron loss at hi hly magnetic field and shows low increasing rate of the iron loss accompanying the increase of the magnetic flux density.

In View of the above requirements, improvements of the magnetic flux density which are necessarily required with an increased size of electric appliances will be realized only by developments of high magnetic flux density grain oriented electrical steel sheets.

Further, as a means for reducing the iron loss, it is necessary to obtain a thin sheet product in order to reduce the eddy current loss which occupies a large part of the iron loss.

"United States Patent Patented Nov. 5, 1974 Conventionally, the grain oriented electrical steel sheet was directed to products having 05-030 mm. thickness, but in recent years, products having a plate thickness of 12 mil (0.30 mm.) 11 ml, 9 mil and 6 mil have been increasingly used for the iron cores of electric equipment and appliances.

The production of such a thin product can not be at tained stably by the conventional two-step cold rolling, and the production technics have been developed, for example, by utilizing the sulphur or selenium permeation process as disclosed in Japanese Patent Publication Sho 43-5966. However, the thin products produced by this process show remarkably increased hysteresis loss by the thickness reduction because the B characteristic is about 1.8 wb./m. so that the reducing effect on the eddy current loss is killed and thus a remarkable improvement of the iron loss can not be obtained.

On the other hand, the present inventors have developed high magnetic flux density grain oriented electrical steel sheet from steels containing a small amount of acidsoluble Al (hereinafter called Al), and based on this development the present inventors have further developed, through improvements of grain orientation, low-iron-loss products having remarkably high B characteristic as high as more than 1.9 wb./rn. and showing a very little increase in hysteresis loss even in a thin product by effectively utilizing the balance of AlN.

One of the objects of the present invention is to provide a high magnetic flux density grain oriented electrical steel sheet which shows, as compared with the conventional electromagnetic steel sheet, remarkably excellent magnetization characteristic in the rolling direction, namely B characteristic showing at least 1.88 Wb./II1.

Another object of the present invention is to produce stably a grain oriented electrical steel sheet having the high B characteristic as mentioned before as well as low iron loss value.

Still another object of the present invention is to produce a grain oriented electrical steel sheet which shows high B characteristic and low iron loss value even in a thin product.

Other objects of the present invention will be clear from the following description, examples and attached drawings.

FIG. 1 shows the cooling curve during the hot rolling operation, FIG. 2 is a graph showing the holding temperature and time prior to the finishing rolling in the hot rolling, and the magnetic characteristics of the products, FIG. 3 is a graph showing the starting temperature of rapid cooling after the completion of the hot rolling and the magnetic characteristics of the products, FIG. 4 is a graph showing the cooling curve during the hot rolling and the precipitation of aluminum nitride, and FIG. 5, FIG. 6 and FIG. 7 show the macro-structure and the magnetic characteristic of the products according to Examples 4, 5 and 6 respectively.

The present invention will be explained in details.

The starting material used in the present invention is an ordinary steel or silicon steel which may be made by any known steel making process and casting method, but the chemical composition of the starting material should satisfy the following condition.

'c0.0ss% by weight Si4.0% by weight Al 0.0l00.065% by weight (A1 means acid-soluble Al which will be referred to simply as Al hereinafter) Balance Fe and unavoidable impurities For example, the steel material disclosed in Japanese Patent Publications Sho 40-15644 and Sho 46-23820 may be used as the starting material in the present invention.

Explanations will be made on the contribution by the impurities coming during the production of the grain oriented electrical steel sheet as well as on the eifects of Al.

Generally, in the production of grain oriented electrical steel sheets, secondary recrystallization of the socalled cube on edge having {ll} 001 orientation takes place in the final annealing so that products having excellent magnetic characteristics in the rolling direction can be obtained. In this case the precipitates produced by the impurities such as nitrides, sulfides and oxides play an important role.

Conventionally it has been considered that these precipitates are dispersed in fine particles and precipitate into the matrix to prevent the grain growth of the matrix. The present inventors have found that some of the precipitates which precipitate in a special orientational relation to the matrix have also ability of selectively bringing up only the grains having special orientation so that the orientation of the secondary recrystallization is closely controlled, and thus products having excellent B characteristic can be obtained.

AlN formed by the addition of Al in the present invention is a precipitate of the latter type, and the present invention is based on the formation of such AlN, and it is supposed that precipitates formed by the addition of other elements do not show such ability but merely prevent the growth of the primary recrystallization grains of the matrix.

As above explained, for the production of the grain oriented electrical steel sheet, the presence of precipitate forming elements is absolutely necessary, and it is very important for the final characteristics of the products to precipitate these elements in effective size and distribution. In the present invention, the efiective size of the precipitates which can contribute to the growth of the secondary recrystallization grains is roughly estimated to be less than 0.1g.

On the other hand, when the formation process of these precipitates in connection with the production process of the grain oriented electrical steel sheets, it is necessary that these precipitates of the effective size have been already produced in the cold matrix before the final cold rolling. Therefore, the precipitate forming process include the solidifying step from the molten metal, the cooling step at the time of the break-down rolling, the cooling step during the hot rolling, and the annealing and cooling step of the hot rolled plate or intermediate thick plate before the final cold rolling. However, in the solidifying step before the hot rolling or in the cooling step of the breaking down rolling, the mass of the material is so large that the cooling rate is relatively slow and the size of most of the precipitates formed during these steps is larger than the eifective size. Therefore, the present inventors have found that a grain oriented electrical steel sheet having excellent magnetic characteristics can be obtained by reheating the steel slab in the hot rolling step to redissolve the precipitates again into the matrix so as to obtain a hot rolled steel sheet having precipitates of eifective size by appropriate cooling at the time of rolling, and if necessary, by subjecting the steel sheet to heat treatment before final cold rolling to give the desired precipitate condition of the impurities to the material.

As explained above, the role played by the hot rolling step is most important in the production of the grain oriented electrical steel sheet. Therefore, the present inventors have completed the present invention through various experiments on hot rolling using various materials satisfying the required condition of the chemical composition.

A silicon steel containing 0.034% of Al and 2.3% of silicon was used as the starting material, and three specimens of 40 mm. thick small piece were prepared therefrom and held at 1300 C. for minutes to completely dissolve AlN into the matrix and left in the outdoor to temperatures of 1200 C., 1100 C. and 1000 C. respectively, and immediately held for 50-250 seconds in furnaces held at 1200 C., 1100 C. and 1000 C. respectively, then hot rolled to 3.2 thickness by two passes, and cooled in the air. The thus hot rolled plates were cold rolled into products of 0.35 mm. thickness. The relation between the B characteristic of the products, the holding temperature and the holding time before the hot rolling is shown in FIG. 2. One example of the cooling curve of the material during the hot rolling is shown in FIG. 1 (A) in FIG. 1 shows the cooling curve obtained when the rolling was conducted immediately after the slab extraction. (B) and (C) show cooling curves obtained respectively by holding the material at 1100 C. for 50 seconds and at 1200 C. for 150 seconds.

As clearly understood from FIG. 2, in case of holding temperature of 1000 C., the characteristic is deteriorated by a 50 second holding, and when the holding time is more than seconds, the secondary recrystallization itself is unstable. In case of holding times of 1100 C. and 1200 C., similar tendencies appear as in case of 1000 C., but a higher holding temperature tends to increase the holding time allowable for the characteristic deterioration and the appearance of secondary recrystallization.

However, if the material is held at 1100 C. for 200 seconds, the secondary recrystallization is prevented from taking place and held at 1200 C. for more than 200 seconds, the magnetic property deteriorates. This phenomenon may be attributed to the assumption that most of AlN which has been dissolved into the matrix by the holding at 1300 C. for 30 minutes precipitates during the holding between 1000 and 1200 C. so that the amount of AlN of effective size precipitating during the cooling in the hot rolling step becomes relatively small, and the latter precipitation progresses rapidly at a relatively low-temperature holding as at 1000 C., while it progresses slowly at a high-temperature holding.

Similar tendencies are observed by the results of similar experiments as in FIG. 2 made on materials having different silicon contents in respect to the holding temperature and time. In this case, however, the allowable temperature and time ranges vary in correspondence to the silicon contents. Namely, in case of 1.0% silicon content, the deterioration of the characteristics is observed as the holding time becomes longer in case of the holding temperature of 1000 C., but the occurence of the secondary recrystallization grains is stable even with the holding time of seconds. In the case of a holding temperature of 1100 C., some deterioration of the characteristic is observed as the holding time become longer, but the occurence of the secondary recrystallization grains is stable even when the holding time exceeds 200 seconds.

When the holding time is at 1200 C., the holding time has no influence on the deterioration of the characteristic and on the occurence of the secondary recrystallization grains in case of a 1.0% silicon content. This indicates that the temperature of 1200 C. itself is satisfactory for the solid dissolution of AlN in the slab. On the other hand, when the silicon content in the material is increased, the allowable holding temperature and time before the rolling becomes extremely narrow. For example, in case of 3.15% silicon content, the occurence of the secondary recrystallization grains becomes unstable regardless the holding time, and a holding'temperature of 1150 C. at the minimum is necessary and the holding time is also desirably within 50 seconds in order to avoid the deterioration of the characteristic.

FIG. 4 shows the results of observations of the relation between the cooling cycles of hot rolling and the amount of AlN precipitate by changing the silicon content. In case of 2.8% silicon content, AlN starts to precipitate about 1250 C. and the precipitation progresses rapidly below 1200 C., while in case of 1.1% silicon content, AlN does not substantially precipitate at a temperature down to 1000 C., and starts to precipitate at a temperature below 1000 C. This is considered to be due to the fact that the oc'y transformation zone of the material increases or decreases in correspondence to its carbon and silicon contents, and the behaviour of AlN precipitation is related closely to the amount of 7 phase.

Therefore, the phenomenon that the slow cooling zone before the hot rolling changes in accordance with the silicon content is also quite reasonable from the results in FIG. 4. Also careful consideration should be given to the precipitation of AlN after the completion of the hot roll ing. FIG. 3 shows the relation between B characteristic and the starting temperature of water quenching in case when the product was obtained by heating 3% Si steel at 1350 C. for 30 minutes and immediately rolling it to a finished thickness of 3.5 mm., and water quenching the plate from the temperature immediately after the hot rolling, and finally subjecting the hot rolled plate to the production process for a grain oriented electrical steel sheet. It is understood from the results that better product characteristics are obtained when the material is cooled rapidly from the possible earliest stage after the hot rolling to 600 C. at which most of AlN precipitate is completed, namely, cooled as rapidly as possible wiihin a range below 200 seconds. The effect of the silicon content of the material at this stage is similar to that on the slow cooling zone beofre the hot rolling and at a higher silicon content, it is necessary to rapidly cool the material from a higher temperature zone. At a low silicon content, the desired characteristics are obtained even by slow cooling from a relatively low temperature zone, and it is understood that the amount of the a-v transformation of the material is relevant in this stage also. i

As for the cooling cycle in the rot rolling of grain oriented electrical steel sheet containing a very small amount of Al, the cycle as shown in FIG. 1 (A) is most desirable. Namely the heating of the slab before the hot rolling should be done at a temperature and time sufficient for redissolving fully the AlN into the matrix and the hot rolling should be done by holding the material temperature after the slab extraction up to the starting of the rolling (finish rolling) at a highest temperature for the shortest time possible, and it is necessary that the material is cooled as rapidly as possible to room temperature immediately after the completion of the rolling.

In the case of hot rolled steel plates obtained by the above most desirable cooling cycle, the high temperature heat treatment for AlN redissolution and recrystallization may be omitted and yet excellent product characteristics can be obtained as shown in Example (1).

From the above, the hot rolling conditions for production of a grain oriented electrical steel sheet having excellent directional properties utilizing the etfect of AlN for preventing grain growth maybe defined as follows. After the slab material is heated to a temperature above 1200 C. in accordance with the silicon content to dissolve AlN into the matrix, the heat cycle in rolling the slab to a desired plate thickness should satisfy the following condition.

(l) The time after the slab extraction until the material is cooled to a temperature of 1000-1250" C. depending on the silicon content is not more than 200 seconds.

(2) After the above cooling to 1000-1250 C., the time for cooling the plate to 600 C. is not more than 200 seconds.

As explained before, when the steel sheet is treated according to the present invention, a secondary recrystallization structur of very high orientation is caused by Al (more strictly AlN), but in a certain range a higher B characteristic namely {1l0} 100 secondary recrystallization structure having higher accumulation degree can be obtained as the Al content increases. Thus, when such a material is used to produce a final product of thickness more than about 0.35 mm. (14 mil), very excellent characteristics can be obtained stably. However, in case of the production of a final product of thickness less than 0.3 mm. (12 mil), the product is more susceptible to the 6 influence of the other elements such as C, Si and N and production conditions as the Al content increases, and thus unless these factors are closely controlled, the amount, size and distribution of AlN becomes unbalanced to cause incomplete secondary recrystallization.

In order to correct such conditions, it is necessary to keep AlN effectively in good balance, thereby it is possible to increase stability in the chemical composition and the production conditions and to expand the allowable range of Al to a higher Al content so that stable final products having remarkably improved characteristics, particularly thin products of thickness less than 0.3 mm. can be obtained.

Namely, in the production of high magnetic flux density grain oriented electrical steel sheet from Al-containing steels, the present inventors have succeeded to commercially produce final products, particularly thin final products having more perfect secondary recrystallization and very excellent characteristics by nitricling the hot rolled material as mentioned before in a continuous annealing treatment.

The reasons why the final products having further improved characteristics can be obtained by an appropriate nitriding may be considered as follows. Although the magnetic properties become very excellent as the Al content increases, but the secondary recrystallization becomes slightly unstable due to the unbalance of the AlN content.

As disclosed in the Japanese Patent Publication Sho 46-23820, the precipitated AlN of specific fine size is present in a specific amount. AlN has such ability that the grain growth of the matrix along the growth of the secondary recrystallization nuclei is prevented, but only the grains having a specific orientation relation to the precipitating direction of the fine AlN are selectively allowed to grow, thus the orientation of the secondary recrystallization is closely controlled so that very sensitive l00 orientation can be obtained.

When the AlN content is high, it is favourable to the {l00} 100 orientation and the selection of the secondary recrystallization grains because of a large amount of the specific AlN, but the growth of the secondary recrystallization nuclei is prevented by the strong preventive effects due to the great amount of AlN, thus causing incomplete secondary recrystallization.

When the nitriding treatment is applied, the enriched nitrogen reduces the amount of fine AlN into an appropriate amount and eliminates the factor preventing the growth of the specific secondary recrystallization nuclei so that the secondary recrystallization is made stable.

Also, nitrogen combines with Al in the steel and makes it possible to provide an appropriate amount of the specific fine AlN as necessity arises. Thus, even if the Al content is high, the selectivity of 100 secondary recrystallization grains can be maintained and their growth is effectively controlled.

In the nitriding treatment which is done to facilitate the formation of the effective AlN, the required amount of enriched nitrogen varies between 0.0005 and 0.004% depending on the chemical composition of the material, particularly the A1 content and the N content, the working history before the nitriding treatment and the thickness of the final product, and in this way it is desirable that the total amount of nitrogen in the steel plate is 0.005- 0.012%. After the amount of nitrogen has been adjusted to the above range, the precipitation treatment which forms the effective AlN is applied to form 0.0005 to 0.0095% of AlN.

If the amount of enriched nitrogen is out of the above range, for example below the lower limit, the secondary recrystallization becomes unstable, and above the upper limit, the very high B characteristic which is the feature of a high magnetic flux density steel sheet can not be obtained any more, and the stability of the secondary recrystalization becomes poor.

From the aspect of enriching nitrogen, it may be considered to add all necessary amounts of nitrogen during the steel making, but it is desired that the components, particularly the Al content is increased in order to obtain the thin products of thickness less than 0.30 mm. (12 mil), having very excellent characteristics, and thus the amount of nitrogen required for the formation of effective AlN is naturally increased. However, if a large amount of nitrogen is added during the steel making, blisters are more apt to occur in the final product, thus lowering the yield of the final product.

On the other hand, if the nitriding is done during the continuous annealing of the hot rolled steel plate, such an undesirable phenomenon does not take place even if the nitrogen content increases. Also the nitriding during the continuous annealing of the hot rolled plate is very advantageous int hat close control can be done to the material in which the amount and formation of effective AlN is unbalanced due to variations of the components (Al, N) in the steel making and variations of hot rolling conditions.

The nitriding can also be done in the continuous decarburization annealing of the cold rolled steel sheet of final thickness, but in this case it is very difficult to control closely the nitrization amount, and thus the production is unstable although excellent characteristics are obtained in some cases.

Any nitriding source may be used for the nitrization, and for example, gas containing nitrogen compounds such as NH and NO may be added to the furnace gas, or a nitrogen compound may be coated directly to the steel sheet. The use of N gas as nitrogen source is not efiicient because N gas is inert. In any way, activated nitrogen is supplied and enriched for the nitrization.

The nitrogen enrichment to the steel sheet by the nitrogen compound gas, such as, NH and NO is achieved by introducing these nitrogen compound gases either or mixed with the furnace gas into an annealing furnace and supplying them to the steel sheet. In the case when the nitrogen enrichment is done by coating a nitrogen compound onto the steel sheet, the compound is applied before the annealing in front of the furnace in a similar way as the separator is applied before the final annealing. For example, powders of manganese nitride and so on are mixed with water and dropped to the coating roll while stirring and coated on the surface of the steel sheet. The amount of nitrogen to be given is adjusted by the coating amount.

In case of NH gas which is most commonly used,

the amount of active nitrogen source required for the before mentioned range of enriched nitrogen must be more than 0.2% expressed in the mixing proportion to the furnace gas, although it depends on the gas flow rate and time.

The nitriding treatment is desirably done in the continuous annealing step of the hot rolled steel plate, but it may be done before this step if required. In this case, the nitriding is effected at a temperature above 600 C. for 30 seconds to 30 minutes, and the amount of nitriding source to be supplied must be more than 0.2% in the mixing proportion to the furnace gas in case of NH After the nitriding treatment, the steel plate is cooled once to the room temperature and successively subjected to the ordinary continuous annealing, or after the nitriding treatment the steel plate is successively subjected to the ordinary continuous annealing and rapidly cooled to cause effective AlN precipitates.

The material used in the present invention, as mentioned before, is an ordinary steel or a silicon steel containing less than 40% of silicon and 0.0l-0.065% of aluminum and in the form of steel ingots obtained by the conventional steel making and melting method, or in the form of steel slabs obtained by the continuous casting or the pressure casting. Commercially produced steel slabs contain more than 0.0020% of nitrogen, which is well enough for the formation of AlN important to the present invention. The above materials are hot rolled to 1.5-7 mm. thickness, after the break-down rolling to destruct the cast structure, if necessary. The AlN precipitation annealing after the hot rolling but before the final cold rolling is as fully disclosed in the Japanese Patent Publication Sho 46-23820, and its key points are that the material is annealed in the temperature range of 7501200 C. for 30 seconds to 30 minutes and is cooled, and in this cooling step, the material should be rapidly cooled through the temperature range from 950 to 400 C. in 2 to 200 seconds. The cold rolling in the present invention is done in such a way that the final cold rolling is done at a reduction rate of -95% depending on the silicon content. The decarburization annealing and the final annealing after the cold rolling may be done by a conventional method.

EXAMPLE 1 A silicon steel ingot containing 0.050% of C, 3.05% of Si, 0.030% of Al and 0.028% of S was broken down to a slab thickness of 40 mm., which was held at 1350 C. for 30 minutes and immediately subjected to a finish rolling. The starting temperature of the rolling was 1280 C. and the time after the slab extraction to the start of the rolling was 15 seconds. The slab was reduced to a thickness of 3.5 mm. by two passes in the finish rolling, the finishing temperature of the rolling was 1120 C. and the time required after the slab extraction up to the completion of the rolling was 35 seconds. Then the plate was rapidly cooled in water at 20 C. The time required by the rapid cooling was 10 seconds.

The thus obtained hot rolled plate was acid pickled, cold rolled by to a final thickness of 0.35 mm., subjected to a continuous decarburization annealing and a final annealing in H at 1200 C. for 20 hours. The magnetic characteristics in the rolling direction of the product were as follows:

EXAMPLE 2 A silicon steel ingot containing 0.045% of C, 2.3% of Si, 0.025% of Al and 0.013% of S was broken down to a slab thickness of 40 mm., which was held at 1300 C. for 30 minutes and immediately held in a furnace at 1200 C. for seconds, and then subjected to the finish rolling. The time required after the slab extraction to the start of the finish rolling was seconds, and the finish rolling was done by two passes to obtain a plate thickness of 4.0 mm. at a finishing temperature of 1050 C. The time after the slab extraction to the completion of the rolling was 210 seconds. The hot rolled steel plate thus finished was immediately subjected to a rapid cooling in water to room temperature. The time required by the water cooling was about 10 seconds. The hot rolled steel plate was then acid pickled, cold rolled by 25%, annealed in an N atmosphere at 1100 C. for 2 minutes, and quenched in hot water at 100 C. Then the plate was acid pickled, cold rolled by 88% to a final thickness of 0.35 mm., continuously decarburized, and subjected to the final annealing in N at 1200" C. for 20 hours. The magnetic characteristics in the rolling direction of the final product were as follows:

B,=1.9s3 (Wb./m.

W 17/50=1.32 (W./kg.)

The relative low iron loss as compared with the B characteristic is due to the unusually large grain size of the product.

EXAMPLE 3 A silicon steel ingot containing 0.053% of C, 2.80% of S1, 0.032% of Al, and 0.027% of S was broken down to a slab of thickness of 40 mm., which was held at 1350 C. for 30 minutes and immediately subjected to a finish rolling.

The starting temperature of the rolling Was 1280 C., the time required after the slab extraction up to the start of the rolling was 15 seconds. The slab was reduced to a plate thickness of 2.8 mm. by three passes in the finish rolling at a finishing temperature 980 C. The time required after the slab extraction up to the completion of the rolling was 45 seconds. The cooling after the rolling was done in the air and the time required for cooling the plate to 300 C. was 180 seconds. The hot rolled plate was then annealed in N atmosphere at 1150 C. for 2 minutes, and cooled from 1150 C. to the room temperature in 45 seconds by using vapour-water spray. The annealed plate was acid pickled, cold rolled to a final thickness of 0.30 mm., subjected to a decarburization annealing and a final annealing in H at 1200 C. for 20 hours. The magnetic characteristics in the rolling direction of the product were as follows:

B =1.923 (-wb./m.

W 17/50=1.05 (W./kg.)

EXAMPLE 4 The steel ingots A and B having the chemical compositions shown in Table 1 were broken down, hot rolled and subjected to the following treatments shown hereunder. The processes up to the hot rolling were same as in Example 3.

Cold rolling Continuous decarburizatton annealing (850 0.)

Finish annealing (1200 C.)

Table 2 shows the increment AN of nitrogen after the nitriding treatment, the total nitrogen amount TN and the amount of precipitated AlN (N as AlN).

FIG. 5 shows the macro-structure and magnetic characteristics of the product obtained by the above process.

In case of the product of 0.35 mm. thickness excellent characteristics were obtained regardless of the nitriding treatment. On the other hand, in the case of the product of 0.30 mm. thickness, the secondary recrystallization is incomplete and the characteristics are poor when the nitriding treatment is not applied. When the nitriding treatment was applied, the secondary recrystallization was complete and excellent characteristics were obtained. Thus it is clear that the effect of the nitriding treatment is remarkable in the case of thin products.

The steel ingot A used in Example 4 was broken down,

hot rolled under similar conditions as in Example 4 and subjected to the processes as shown hereunder.

Hot rolled plate (1) No coating Coating of nitride (2) Manganese nitride (N fgrigeongl was coated Continuous annealing (1100 C. 2 minutes N2 100%) Acid pickling Cold rolling (0.23 mm.)

Continuous decarburization annealing (850 0.)

Finish annealing (1200 C.)

Table 3 shows the increment AN of nitrogen after the nitriding treatment, the total nitrogen TN and the precipitated AlN.

FIG. 6 shows the macro-structure and the magnetic characteristics of the product obtained by the above processes.

TABLE 3 Process (2) AN 0.0023 TN 0.0079 AlN (N as AlN) 0.0063

EXAMPLE 6 The steel ingot A used in Example 4 was broken down, hot rolled under similar conditions as in Example 4 and subjected to the following processes.

esses. It is understood the effect of the nitriding treatment is remarkable.

TABLE 4 Process (2) AN 0.0030 TN 0.0086 AlN (N as AlN) 0.0069

What is claimed:

1. In a method for producing a high magnetic flux density, grain oriented electric steel sheet or strip of the type wherein a steel ingot produced by conventional methods is broken down into a slab, the slab is hot rolled to form a plate, the plate is cold rolled in at least one step including a final cold rolling at a reduction rate between 65 to to form a sheet and the sheet is decarbonized and finally annealed at a temperature above 800 C., the improvement wherein the steel into consists essentially of not more than 4.0% Si, not more than 0.085% carbon, 0.010 to 0.065%. acid soluble Al, and not more than 0.012% N, the slab is heated to above 1200 C. to diSr solve the AlN and then hot rolled such that the slab is cooled to a temperature between 1000 to 1250 C. in less than 200 seconds from the time the slab is removed from the heating step, and the resulting plate is cooled from 1000-1250" C. to 600 C. in less than 200 seconds.

2. A method according to claim 1 in which the hot rolled plate is subjected to a high temperature continuous annealing in a temperature range of 750-1200 C. for 0.5 to 30 minutes and rapid cooling to precipitate AlN before the final cold rolling.

3. A method according to claim 2 in which the nitrogen is enriched by 0.0005-0.004% in the plate while the plate is subjected to the continuous annealing between 750-1200 C. by contacting the plate with a nitrogen-containing compound and the annealed plate is rapidly cooled from the annealing temperature range of 750-950 C. to 400 C. in 2-200 seconds to obtain 0.0005-0.0085% of precipitated AlN.

4. A method according to claim 2 in which prior to the continuous annealing, the hot rolled plate is treated at a temperature above 600 C. for 30 seconds to 30 minutes by contacting the plate with a nitrogen-containing pound to enrich the nitrogen content in the plate by 0.0005-0.004%, and the subsequent continuous annealing is etfected at a temperature of 750-1200 C. for 30 seconds-30 minutes, and the plate is rapidly cooled from 12 a temperature range of 750-950 C. to 400 C. in 2-200 seconds to obtain 0.0005-0.0095% of precipitated AlN in the plate.

References Cited UNITED STATES PATENTS 3,671,337 6/1972 Kumai et al. 148---12l 3,620,856 11/1971 Hiraoka l48121 3,180,767 4/1965 Easton et al. 148l2 3,165,428 1/1965 Albert et al 148-111 3,632,456 1/1972 Sakakura et al 148l12 3,636,579 1/1972 Sakakura et al. 148-112 FOREIGN PATENTS 726,154 1/1966 Canada 148-111 WALTER R. SA'ITERFIELD, Primary Examiner US. Cl. X.R.