Low-carbon steel sheets temper-rolled after the final anneal to improve magnetic properties

Abstract

Low-carbon sheet steel having inproved magnetic properties is produced by hot rolling the steel to about 0.050 to 0.100 inch thick sheet such that the temperature thereof is 1430.degree. to 1620.degree.F when the steel is finished, and 900.degree. to 1200.degree.F when the steel is coiled. The steel is then pickled and cleaned, coldrolled to effect a thickness reduction of 40 to 80 percent, annealed to effect recrystallization, and temper-rolled to effect a plastic elongation of 6 to 10 percent.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of application Ser. No. 136,805 filed Apr. 23, 1971, now abandoned.

Description

BACKGROUND OF THE INVENTION

Because of their superior magnetic properties, silicon sheet steels are widely used in the production of magnetic core components in electrical equipment such as motors, generators, transformers, and the like. These favorable magnetic properties, namely high magnetic permeability, high electrical resistance and low hysteresis losses, will minimize wasteful conversion of electrical energy into heat, and will therefore permit manufacture of electrical equipment having greater power and efficiency. In order to effect and optimize the desired magnetic properties, however, the silicon sheet steels must be produced under carefully controlled and exacting processing parameters. Silicon sheet steels are therefore substantially more expensive than other more conventional flat rolled steel products.

In the high volume manufacture of small electrical equipment for consumer appliances, toys and the like, unit cost is perhaps the most important consideration, far outweighing equipment efficiency and power considerations. For these applications, therefore, electrical equipment manufacturers frequently utilize the less expensive, more conventional low-carbon sheet steels for magnetic core components. Hence, there is a considerable market for low-carbon sheet steels having acceptable magnetic properties for magnetic core applications.

In the course of producing low-carbon sheet steels for magnetic applications, economic considerations have dictated that expensive processing steps be avoided and that even the inexpensive steps be minimized. Therefore, even though elaborate processes have been developed for producing low-carbon sheet steels having exceptional magnetic properties, such processes have not been adapted commercially, because the use of such processes would greatly add to the cost of the product, while not improving the magnetic properties of the resultant sheet to equal those of silicon sheet steels having comparable cost of production. To be of any commercial value, therefore, any new process for improving the magnetic properties of low-carbon sheet steels must be one that will not significantly increase the steel's production cost. Commercially, therefore, low-carbon sheet steels for magnetic applications are produced from conventional low-carbon steel heats having less than 0.1 percent carbon and the usual residual elements at normal levels for cold-rolled products. The rolling procedures are similar to those used for other cold-rolled products. Specifically, the production steps are usually limited to hot rolling a low-carbon ingot to slab form; hot rolling the slab to sheet form; pickling the hot rolled sheet, cold rolling the pickled sheet for a reduction of 40 to 80 percent; and annealing the sheet to effect recrystallization, generally in a box annealing furnace. An optional final temper roll of from 1/2 to 2 percent is sometimes provided for the purpose of flattening the resultant sheet and make it better suited for subsequent slitting and punching operations. To optimize flatness, and hence suitability for slitting and punching, temper rolling elongations are minimized at 1/2 to 2 percent.

The commercially produced low-carbon sheet steels for magnetic applications, when rolled to 18.5 mils thickness, typically exhibit permeabilities in the rolled direction of from 5000 to 6000 at 10 kilogauss, with core losses of from 1.3 to 1.6 watts/lb. For the same thickness at 15 kilogauss, permeabilities in the rolled direction typically range from 2000 to 4000 with core losses of 3.0 to 4.0 watts/lb. Sheets rolled to 25 mils typically exhibit permeabilities in the rolled direction of from 4200 to 4800, with core losses of 1.8 to 2.0 watts/lb. at 10 kilogauss; and permeabilities in the rolled direction of from 2000 to 3000 with core losses of 4.2 to 4.8 watts/lb. at 15 kilogauss.

These relatively wide ranges in magnetic properties reflect an established tendency on the part of industry to deemphasize magnetic properties in low-carbon sheet steel and emphasize low cost of production. Nevertheless, customers have recently begun to demand improved magnetic properties, particularly at 15kilogauss, without an appreciable increase in cost. As noted above, producers have been hard pressed to improve magnetic properties in these steels without substantial increases in production costs.

SUMMARY OF THE INVENTION

This invention is predicated upon my discovery that temper rolling the cold rolled and annealed low-carbon sheet steel between very critical elongation limits of from 6 to 10 percent will very significantly enhance the steel's magnetic properties to values never before attained in non-silicon sheet steels. Since the other process steps may be substantially the same as those presently practiced commercially, the single modification provided by this invention, i.e., the increased temper-rolling elongation, does not significantly increase the cost of the product.

It is an object of this invention to provide a new process for producing low-carbon sheet steel having improved magnetic properties without a significant increase in production costs.

It is another object of this invention to provide a new temper-rolling procedure to be used in the manufacture of low-carbon sheet steel for magnetic applications.

It is a further object of this invention to provide a low-carbon sheet steel having improved magnetic properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are graphs showing test results of one experimental heat described at the end of this specification. The graphs show permeabilities and core losses at 10 and 15 kilogauss as a function of percent temper-rolling elongation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred practice of this invention, the starting steel should have a composition substantially the same as those low-carbon sheet steels presently produced commercially. Specifically, the composition of the steel is usually as follows: 0.02 to 0.10 percent carbon; 0.40-0.60 percent max. manganese; 0.02-0.09 percent max. phosphorus; 0.025 percent max. sulfur; and 0.010 percent max. silicon; the balance iron and other typical unintentional impurities.

To produce the sheet steel in accordance with the preferred practice of this invention, a steel heat, having the above composition, is cast into ingot form and then hot rolled to slab form in accordance with conventional slabbing mill practices, or continuously cast into slab form in accordance with conventional continuous casting practice. The slab is then reheated and hot rolled to sheet having a thickness of about 0.06 inch or between 0.05 and 0.100 inch, such that the finishing temperature upon exit from the finishing roll train is within the range 1430.degree. to 1620.degree.F. The sheet is thereafter cooled with water sprays so that it is coiled at a temperature of from 900.degree. to 1200.degree.F. The preferred hot rolling practice is to attain a temperature within the range 1900.degree. to 1950.degree.F when the steel is about 1 inch thick, and 1460.degree. to 1600.degree.F when the steel is finished.

After cooling, the steel is suitably pickled to remove mill scale and then cold rolled to effect a thickness reduction of from 40 to 80 percent. Thereafter, the sheet is suitably annealed to effect recrystallization. The annealing is preferably performed in a box annealing furnace at a temperature of from 1125.degree. to 1300.degree.F for 3 to 30 hours.

In some commercial operations the annealing of the cold rolled sheet steel completes the process and the steel is sold thereafter. The most common commercial practice however, has been to temper roll the annealed sheet effecting plastic elongations no greater than 2 percent for the purpose of improving the sheet's flatness and thus enhance its slitting and punching characteristics and render it more suitable for laminated end products. Elongations in excess of 2 percent are avoided because such elongations will usually result in distortions of sheet flatness and variations in gage, or thickness, across the sheet width.

The crux of this invention resides in the unexpected discovery that sheet steel produced in accordance with the above process can be provided with very substantially improved magnetic properties if the final temper roll is sufficient to provide a plastic elongation within the critial range of from 6 to 10 percent, and preferably between 7 and 9 percent. As noted above, the prior art low-carbon sheet steel rolled to a thickness of 18.5 mils and tested at 10 kilogauss, typically exhibit permeabilities of 5000 to 6000, with core losses of from 1.3 to 1.6 watts/lb. In contrast thereto, sheet steels produced in accordance with this invention will exhibit optimum permeabilities of about 7200 and core losses of about 1.1 watts/lb. Table I below contrasts the optimum magnetic properties achieved by the practice of this invention with the magnetic properties obtainable in prior art steels for two different thicknesses and tested at two different inductions.

TABLE I __________________________________________________________________________ Magnetic Properties Achieved by Inventive Process Contrasted to Prior Art Thickness Induction Core Losses Steel (Mils) (Kilogauss) Permeability (Watts/lb.) __________________________________________________________________________ Prior Art 18.5 10 5000-6000 1.3-1.6 This Invention 18.5 10 7200 1.1 Prior Art 18.5 15 2000-4000 3.0-4.0 This Invention 18.5 15 5500 2.5 Prior Art 25 10 4200-4800 1.8-2.0 This Invention 25 10 5200 1.5 Prior Art 25 15 2000-3000 4.2-4.8 This Invention 25 15 4200 3.8 __________________________________________________________________________

In considering the above table, it should of course be realized that the magnetic properties finally achieved will vary somewhat from sample to sample even when identical process parameters are employed. Nevertheless, the ranges of magnetic properties shown above for the prior art steels are the usual optimum values, for indeed many low-carbon sheet steels are sold for magnetic applications having magnetic properties inferior to those optimum values shown in Table I for prior art steels. By the same token, not all steels processed in accordance with the process disclosed herein will have the optimum magnetic properties shown in Table I. Nevertheless, if the process is rigourously adhered to and there are no adverse factors to account for, e.g., physically damaged sheet, then magnetic properties superior to the best prior art properties can be consistantly achieved. For example, at 18.5 mils and testing at 10 kilogauss, not all samples of sheet processed according to this invention will have permeabilities as high as 7200 core losses as low as 1.1 watts/lb. Nevertheless, the results should consistantly provide permeabilities in excess of 6000 and core losses less than 1.3 watts/lb. Accordingly, even the more inferior samples of steel produced in accordance with this invention will have magnetic properties superior to the best of the prior art steels.

The following test is presented here to illustrate the critical nature of this invention. For this test, a single heat of steel was prepared having the following ladle analysis:

Carbon 0.07 % Manganese 0.57 % Phosphorus 0.06 % Sulfur 0.021% Silicon 0.003% Copper 0.01 % Nickel 0.01 % Chromium 0.02 % Molybdenum 0.01 % Tin 0.006%

This heat was cast into ingot form and hot-rolled first to slab form and then to 0.060 inch thick sheet. Hot rolling was controlled such that the sheet was at a temperature of 1950.degree.F at 1 inch thickness, and exited the final rolls at 1440.degree.F. Prior to coiling, the hot rolled sheet was cooled to 1180.degree.F with water sprays.

The hot rolled sheet was then segmented into five portions, and cold rolled to various gages, such that the final or temper rolling following annealing, various degrees of deformation could be imposed in reducing the sheets to one of two final thicknesses. The intermediate thicknesses, final thicknesses and degree of temper rolling are shown in Table II below.

TABLE II __________________________________________________________________________ Reduction Schedules and 60-Hertz Magnetic Properties 10 Kilogauss 15 Kilogauss Intermediate Percent Percent Core Loss, Core Loss, Gage, inches Elongation Reduction w/lb. Permeability w/lb. Permeability __________________________________________________________________________ 0.0185 Inch Thick Sheet 0.0188 1.6 1.5 1.52 5618 3.70 2055 0.0195 5.5 5.1 1.45 5629 3.07 5004 0.0200 8.0 7.8 1.12 7246 2.54 5456 0.0204 10.2 9.3 1.32 5905 3.02 4639 0.0222 20.0 16.7 1.48 5391 3.42 3409 0.0250 Inch Thick Sheet 0.0254 1.6 1.5 1.84 4761 4.62 2679 0.0263 5.0 4.9 1.83 4545 4.23 4054 0.0270 8.0 7.8 1.50 5155 3.79 4286 0.0275 10.0 9.1 1.84 4348 4.34 3661 0.0300 20.0 16.7 1.92 4348 4.69 3000 __________________________________________________________________________

Following the reduction to intermediate gage, the sheets were box annealed for 12 hours at 1200.degree.F in a nitrogen atmosphere containing 10 percent hydrogen and having a dewpoint of about 70.degree.F. The sheets were then temper rolled as indicated in the above table and sheared into test strips. The longitudinal test strips were annealed for one hour at 1450 in the above atmosphere to relieve shearing strains, and the magnetic properties thereafter measured at 60 Hertz. The resulting properties are tabulated in the above table and shown graphically in FIG 1-4, which are plots of permeability and core losses as a function of percent plastic elongation at 10 and 15 kilogauss. The superior effect of temper rolling between 6 and 10 percent elongation is clearly demonstrated.

Claims

I claim:

1. A process for producing low-carbon sheet steel for magnetic applications consisting of; forming a steel slab consisting of 0.02 to 0.10 percent carbon, 0.40 to 0.60 percent manganese, 0.02 to 0.09 percent phosphorus, 0.025 maximum percent sulfur, 0.010 maximum percent silicon and the balance iron and residual impurities; hot rolling said slab to a thickness of 0.050 to 0.100 inch with a finishing temperature within the range 1430.degree. to 1620.degree.F; coiling the hot rolled steel at a temperature of 900.degree. to 1200.degree.F; cooling the coiled steel to ambient temperature; cleaning the steel to remove mill scale; cold rolling the cleaned steel to effect a thickness reduction of 40 to 80 percent; annealing the cold rolled steel at a temperature of 1125.degree. to 1300.degree.F to effect recrystallization thereof; and finally temper rolling the annealed steel sufficient to effect a plastic elongation of 6 to 10 percent to improve the steel's magnetic properties such that at 18.5 mils the steel will exhibit a permeability in excess of 6000 with core losses of less than 1.3 watts/lb. when subjected to an induction of 10 kilogauss, and exhibit a permeability in excess of 4000 with core losses of less then 3.0 when subjected to an induction of 15 kilogauss; while at 25 mils the steel will exhibit a permeability in excess of 4800 with core losses less than 1.8 when subjected to an induction of 10 kilogauss and exhibit a permeability in excess of 3000 with core losses less than 4.2 when subjected to an induction of 15 kilogauss.

2. The process of claim 1 in which said temper rolling effects a plastic elongation of 7 to 9 percent.

3. A low-carbon sheet steel for magnetic applications produced by the process consisting of hot rolling a steel slab consisting of 0.02 to 0.10 percent carbon, 0.40 to 0.60 percent manganese, 0.02 to 0.09 percent phosphorus, 0.025 maximum percent sulfur, 0.010 maximum percent silicon and the balance iron and residual impurities to a thickness of 0.050 to 0.100 inch with a finishing temperature within the range 1430.degree. to 1620.degree.F, coiling the hot rolled steel at a temperature of 900.degree.-1200.degree.F, cooling the coil, cleaning the coil to remove mill scale, cold rolling the cleaned hot rolled steel to effect a thickness reduction of 40 to 80 percent, annealing the cold rolled steel to effect recrystallization thereof, and finally temper rolling said annealed steel sufficient to effect a plastic elongation of 6 to 10 percent, said steel characterized by excellent magnetic properties such that at 18.5 mils the steel will exhibit a permeability in excess of 6000 with core losses of less than 1.3 watts/lb. when subjected to an induction of 10 kilogauss, and exhibit a permeability in excess of 4000 with core losses of less than 3.0 when subjected to an induction of 15 kilogauss; while at 25 mils the steel will exhibit a permeability in excess of 4800 with core losses less than 1.8 when subjected to an induction of 10 kilogauss and exhibit a permeability in excess of 3000 with core losses less than 4.2 when subjected to an induction of 15 kilogauss.

4. The steel of claim 3 in which said temper rolling effects a plastic elongation of 7 to 9 percent.