US2859145A - Cold rolling of steel - Google Patents

Description

United States Patent M corn ROLLING OF STEEL Richard I. Somers, Northville, Mich, and Paul L. Petty, Fort Thomas, Ky; said Somers assignor to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan, and said Petry assignor, by mesne assignments, to Acme-Newport Steel Company, Newport, Ky., a corporation of Illinois a No Drawing. Application January 25, 1956 Serial No. 561,355

Claims. (Cl. 148-6.15)

The present invention relates to a method for improving the continuous cold rolling of ferrous strip stock and more particularly relates to a process which permits the elimination of the conventional oiling step prior to rolling and produces a smoother, flatter rolled strip than has been heretofore obtainable. The process includes the provision of a phosphate coating on steel strip stock moving through a conventional strip steel pickle line under conventional conditions and then cold rolling that treated strip.

It was early suggested in U. S. Patent No. 2,105,015 that the deformation of metal was facilitated by providing a phosphate coating on the metal surface prior to the deforming operation. It has also been suggested that the drawing of wire, bar stock, tubes and the like is made easier by phosphate coating the part to be drawn. To the best of our knowledge there has been no proposal of a process suitable for use in continuous cold rolling of steel strip stock.

Present day cold rolling processing of steel strip stock comprises preparing the roll surface for cold rolling and then cold rolling in a reversing mill or a multiple stand mill. Low carbon steel, silicon steel or other steel strip, which emerges from a conventional hot rolling mill usually has some residual rolling scale and corrosion products on its surface. The preparation for cold rolling normally includes processing the hot rolled strip to clean the surface by passing it through a pickle line of conventional form which may, for example, include in sequence one or more mineral acid dip stations, one or more water rinse stations and an oiling station. In certain instances Where the mill scale is heavy, the pickle line may include mechanical abrading stations or alkali stations prior to the acid dip stations. The steel strip is continuously passed through these sequential stages and is recoiled after oiling to await the cold rolling operation. The rolling mill, whether reversing or multiple stand, operates at speeds and under conditions which make it, impossible to feed directly from the pickle line to the rolling line. Because of this necessary but variable length delay between thepickling of the strip and its rolling, the corroding of the pickled strip becomes an important problem. It is for this reason that most pickle line's include a final oiling station The oiling step is highly undesirable. from the dual standpoints that it drips from the pickled coils, makes the working area slick, dirtyand unsafe for workmen and that-after cold'rolling when the cold rolled strip is annealed the oil residue produces a surface coating of undesirable smut. i

It will be apparent that in order to produce sufiicient pickled steel strip to keep a cold rolling mill in continuous production, the pickle line must necessarily operate continuously and at relatively rapid speeds in order to be economic. Although actual speeds nec'essarily'varyv in dilferent installations, pickle line speeds in the range of l 150 feetper minute to 450 feet per minute are common.

One of the difficulties in supplying a process for continu- 2,859,145 Patented Nov. 4, 1958 2 ously forming a phosphate coating on the'strip steel moving through a pickle line is to provide a solution which will form the necessary coating in the limited time which is available for contact of the strip surface with the phosphate coating solution. For example, if the pickle line moves at feet/minute a phosphate coating tank having a 25 foot length provides only about 10 seconds contact time for the strip. It will be apparent that as the pickle line speed increases, either the phosphate coating tank must be greatly lengthened or the coating solution must be adapted to act in a much shorter time than 10 seconds, for example in 3 to 8 seconds.

in the rolling of strip through a reversing mill or in a multiple stand mill, the most important objective is to produce rolled strip of uniform thickness or gauge having good surface finish. The speed of travel of a steel strip through a cold rolling mill will vary, reversing mills being generally slower than multiple stand mills. In order to be economic, the strip speed should be above about 600 feet/minute and preferably above about 1000 feet/minute with speeds as high as 50007500 feet per minute being obtainable on present day multiple stand rolling mill equipment. It will be appreciated that variations in gauge of the resulting strip can only be accomplished by varying the spacing between the rolls and at economic cold rolling speeds only extremely minor adjustments of the roll spacing can be made while the strip is passing through the rolls. In accordance with this invention it has been found that when variations in the weight or thickness of the phosphate coating on the surface of the strip exist, the rolling mill operator experiences difiiculty in maintaining gauge. When the phosphase coating weight varies widely from point to point along the strip, attempts by the operator to correct for these variations may cause side slipping or ultimate ripping and tearing of the strip. When a thick portion is observed by the operator the roll pressure is increased in an attempt to hold gauge and as the thicker portion of the strip passes through the rolls the extra roll pressure causes the rolls to grab the following portion of the strip and thus cause ripping or tearing. This problem represents the most formidable difliculty which must be overcome in providing a continuous phosphate coating process for steel strip. It was further found that when using conventional phosphate coating solutions and normal delays in processing in the pickle line occurred, that portion of the steel strip in contact with the phosphate coating solution during the delay received a heavier, and thusthicker phosphate coating than other portions ofthe strip which passed through the solution at the normal pickle line speed. Normal operation of a pickle line necessarily involves variation in strip speed to permit the tacking together of the ends of coils and of course there are unexpected mechanical delays which accompany any continuous operation. It was thus found tobe necessary to provide phosphate coating solutions which would function to produce the necessary quantity of phosphate coating in a short time, e. g., less than 15 seconds, but which would nevertheless avoid the deposition of widely variant weight coatings even though a pickle line stoppage, measured in minutes, occurred.

The primary objective of this invention is to provide a process which enables the continuous coating of steel strip with a phosphate coating of suflicient uniformity in thickness to facilitate cold rolling, the uniformity enabling the holding of gauge during continuous cold rolling. A further object of this invention is to provide a process for treating ferrous strip stock continuously in a pickle line which produces a phosphate coating which aids in the subsequent cold rolling and which leaves on the sur: face a suflicient quantity of phosphate residue to prevent sticking when the cold rolled strip is annealed either in stacked sheet or coil form.

Another important objective ofthis invention is to provide phosphate coating solutions capable of producing on a continuously moving ferrous strip a coating of the necessary Weight or thickness and of suflicient uniformity to permit the attainment of the desired gauge tolerance in conventional cold rolling.

Another object of this invention is to provide a process for continuously forming a corrosion-resistant phosphate coating on the surface of ferrous strip subsequent to an acid pickle which enables the elimination of the application of an oil coating after acid pickling and prior to cold rolling, which heretofore has been conventional in this art.

Still another object of this invention is to provide a process for continuously applying a phosphate coating to a ferrous strip moving through a pickle line which is of sufficient uniformity to facilitate subsequent continuous cold rolling while maintaining the desired gauge tolerance and which reduces the power required to effect a predetermined amount of reduction; to provide such a phosphate coating as to enable the cold rolling of ferrous sheet of unusual flatness.

Another object is to provide such a phosphate coating to enable the elimination of the use of a roughened surface roll such as a Pangborn roll in a reversing mill or in the last stand of a multiple stand mill and yet produce a cold rolled sheet of suitable surface condition to present sticking on annealing.

Yet another object is to provide such a phosphate coating as to enable the annealing of sheets of such cold rolled strip in stacks up to 100% higher than heretofore used practice and at temperatures up to about 1750 F.; to provide such a phosphate coating as to enable the subsequent stamping, in the hardened cold rolled condition of various parts such as electric motor laminations, which after assembly may be annealed without sticking at temperatures as high as about 1750 F.

In accordance with the present invention it has been found that the above objectives may be realized if certain phosphate coating solutions are used under relatively closely controlled conditions in operation. It has been found that certain other phosphate'coating solutions are either of minor benefit or are unsatisfactory for the purposes of this invention. Broadly stated the method of this invention comprises the steps of removing scale and corrosion products from the surface of ferrous strip stock, contacting the cleaned and continuously moving surface of the strip with an aqueous acidic zinc phosphate coating solution. comprising as the essential coating producing ingredients in weight percent, 0.18 to 0.9 zinc, 0.5 to 2.0 P0 0.7 to about 2.5 C10 and having a total acid of 10-.60 points, and thereafter cold rolling the coated strip- Somewhat more desirable results are obtained if the solutions include as optional ingredients 0.1 to 2.4 N0 and 0.17 to 0.75% calcium by weight. By total acid of 10- 60 points is meant that a 10 ml. sample of the solution requires 10-60 ml. of one-tenth normal sodium hydroxide to reach a phenolphthalein endpoint. It is satisfactory to employ a solution as above defined but containing the proportions of zinc and N0 which would result from adding zinc nitrate to the solution to reach the maximum total acid of 60 points so long as the ratio of calcium to zinc is maintained in the range of 0.421 to 1.111.

When the above proportions of constituents and the coating weight does not exceed 200 mg./sq. ft. from the materially improve the rolling characteristics and after rolling do not leave a sufiicient quantity of phosphate residue to prevent sticking on annealing, whereas coating Weights above about 1000 mg./sq. ft., even though substantially uniform, make it difficult to maintain the desired gauge upon rolling. When the coating is substantially uniform and in the preferred weight range, the coated strip may be rolled in a series of passes to reduce the thickness to one-sixth the original thickness and yet sufficient phosphate residue remains after rolling to, prevent sticking when sheets stacked fifty inches high areannealed at temperatures as high as about 1400 F. It has been observed that the presence of the residual phosphate on the surface after continuous cold rolling permits annealing at 100 F.200 F. higher than temperatures heretofore utilizable so long as the pressure on the adjacent surfaces is comparable.

Due to the above referred to uncontrollable and unexpected stoppages which occur in the pickle line during normal operation, the relative proportions of ingredients in the above solutions have been found to be critical. When the proportions fall outside the limits given, the variation in coating weight which is obtained is so great that on subsequent rolling, difficulty is experienced in maintaining the desired gauge tolerance, or side slipping, ripping or tearing of the strip is encountered.

The best over-all results have been obtained from the use of solutions containing calcium. When calcium is present the ratio of calcium to zinc in the solution: should be maintained within the range of 0.4 to 1 up to 1.1 to 1. Moreover, in these calcium-containing solutions, while it is possible to use C10 concentrations .as low as 0.15% it is desirable to maintain the chlorate concentration above'about .3% and preferably between 0.4-0.6. The calcium containing solutions appear to produce a finer grain coating and this coating provides the maximum aid during rolling while concurrently producing the best surface finish.

After the coated strip emerges from the phosphate coating solution, it may, optionally, be rinsed in.a conventional dilute aqueous solution of chromic acid or the equivalent. It is then recoiled on a receiving mandrel in a conventional manner. Rolling may then be effected on a reversing or multiple stand mill by employing the conventionally used lubricants during the rolling operation. Any of a number of satisfactory lubricants may be employed including oils, such as mineral oil, palm Lubricants for this purpose are 1 preferably used in aqueous form, either as a solution 1 or emulsion. Dilute aqueous solutions consisting of, for

oil, rape seed oil, etc.

example, 1% to 3% oil and the balance water, of a water soluble mineral oil, or a lard base oil together with an emulsifying agent, have been satisfactorily em ployed. One specific aqueous base lubricant which has been used with unusual success is a mixture of lard oil and a fatty acid and has the following analysis: Fatty acid (calculated as oleic acid), 12.9% by weight, iodine number 36.9 and a saponification number of 32.4. A small amount of an emulsifying agent to make this mixed oil water soluble is added to a solution containing'1% 2% of this mixed oil, balance water.

Hot rolled strip may be satisfactorily pickled in a pickling line consisting only of an aqueous sulfuric acid solution followed by a water rinse. Conventional acid pickling solutions, rinsing procedures and the like may be employed but it is desirable to process the hot rolled strip through an acidic pickling solution prior to its introduction into the above disclosed phosphate coating solutions. It'is important to avoid the contamination of the phosphate coating solution by the pickling acids and is therefore desirable to thoroughly rinse the strip emerging from the sulfuric acid pickle before its introduction into the phosphate coating solution. Where heavy mill scale is present on the hot rolled strip or other strip to .be phosphate coated, conventional mechanical abrading means or molten alkali baths may be employed, prior to the acid pickling.

One unexpected advantage which accrues from the practice of the method of this invention is that it is unnecessary to employ shot blasted or roughened rolls in a reversing type mill to attain a surface finish that will not stick during coil or stack annealing It is also unnecessary to employ a similar roll as the last position roll in a multiple stand mill. 7 p

The invention is illustrated in greater detail in the examples given below. It is to be understood that the examples given are intended to illustrate operative conditions only and are not intended to limit the invention to the specific proportions of ingredients set forth since the critical limits thereof have been hereinbefore.

Example I An aqueous acidic phosphate solution tank was installed as the last station in a sulfuric acid pickling line comprising a sulfuric acid pickling station, a water rinse and the phosphate coating solution station. An aqueous acidic zinc phosphate solution was prepared in the phosphate station and analyzed -to contain in percent w./v.; chlorate0.17, nitratel.7, zinc-0.5, PO 1.3, calcium-0.5; the bath operated at a total acid of 25-30 points and a free acid of 4-5. The bath was maintained at a temperature of about 150 F.-180 F. and hot rolled low carbon steel strip was fed throughthe sulfuric acid pickle, water rinsed and through the above solution at the rate of about to 120 feet per minute which provided a contact time in the coating solution of about 10-12 seconds. This processing was continued for about 8 weeks during which time about 22,500,000 square feet of steel strip was processed through the solution. The coating weights obtained during this period had average weights between about to mg./ sq. ft. with Variations therefrom ranging from about 50 mg./sq. ft. to about 200 mg./sq. ft. The strip emerging from the phosphate coating solution was coiled in conventional manner. In some cases the coils were immediately rolled and in others a time delay of one or more days occurred before the rolling was done. stock was rolled under conditions which reduced the initial thickness from 0.1" to 0.068" in one pass, to 0.05" in two passes, to 0.038" in three passes, to 0.029" in four passes and 0.025" in five passes. rolled on a Bliss rolling machine of the reversing type and during the rolling there was a continuous flooding of the rolls and the work with an aqueous base lubricant. During this period of operation there were shutdowns in the pickle line travel for periods exceeding ten minutes and yet the variation in coating weight was insuflicient to prevent the coil from being continuously cold rolled. Some of the coated stock was stamped directly into small horsepower motor laminations in the cold rolled hardened condition and it was observed that there was less wear on the dies when the stamping was done in the cold rolled hardened condition than otherwise. Other portions of this rolled strip were annealed in air and deoxidizing atmosphere furnaces in coils and in stacked sheet form at temperatures ranging from 1200 F. to as high as 1650 F. without encountering sticking between successive layers of the coil or between the sheets. Some of these stacks were increased to as high as 60 inches, from the normal 36" height and annealed at 1275 F. without sticking.

During the operation of the above solution, sodium Most of this treated strip The strip was a chlorate was added to raise the chlorate content to 0.32. The coating resulting from this solution was uniform in appearance and appeared to increase in weight less on line stoppage than solutions containing the lower chlorate percentages. Additional chlorate was added to raise the concentration to 0.38% and an even greater uniformity Another solution, which does not contain calcium, which was found to be satisfactory was prepared in the phosphate coating station and analyzed to contain in percent w./v.: chlorate-'-0.43%, zinc0.7%, P0,; 1.1%, total acid 26.2%. Low carbonhot rolled strip operating in the same line as that described in Example I was processed through the 'bath of the above composition at a temperature of about F. F. until approximately 370,000 square feet of surface had passed through the bath. The coatings obtained in a contact time in the bath of 10-12 seconds were observed to be of conventional gray appearance and uniformly distributed over the surface. Typical coating weights obtained were about 188-205 mg./sq. ft. With chlorate concentration above 0.7% a substantially uniform coating was obtained even though the line stopped for times measured in minutes.

The coated strip was rolled while an aqueous base lubricant was flowing over the surface of the rolls and the strip. No trouble was encountered during the rolling, and the rolled strip upon examination revealed a uniformly flat surface having a smooth surface finish and a clean metallic appearance.

Example III has been processed through the bath. Coating weights which resulted from this treatment varied from 264- 370 mg./sq. ft. During the operation of this bath the chlorate content of the solution was increased in steps to .6%, .7% and gradually to 2%. As the proportion of chlorate exceeded 0.7% it was noted that line shutdowns had less' effect on the increase in the resulting coating weight. The resulting coils were rolled in accordance with the procedure stated in Example I and satisfactory rolling conditions were obtained with the resulting rolled strip having uniformly smooth surface, being flat and having the appearance of clean metal.

Example IV An aqueous acidic zinc solution was prepared, placed in the phosphate coating station and analyzed to contain in percent w./v.: zinc-0.55, PO 0.82, N0 1.17. Sodium nitrite was added to the solution to produce a concentration of 0.0005% and maintained ator above that concentration during use. The bath had a total acid of 20 points. The bath was maintained at a temperature of F.- F. and hot rolled low carbon steel strip was processed through the bath in the line and under similar conditions described in Example I. When the coil was moving through the bath at the regular speed of 100-420 feet/minute, the coating weights obtained varied from about 150-200 mg./'sq. ft. When the pickle line Was stopped'for, a period of ten minutes the coating weight on'the strip which was emerged in the tank during this sto'ppage'was in the range of 800- 1000 ing/sq. ft. Upon rolling this coil it was found that this heavy phosphate coating weight area caused slipping and tearing of the strip as it passed through the rolls. i

What is claimed is:

1. A method for treating ferrous the steps of pickling the strip, contacting the cleaned strip while the same is moving with an aqueous acidic zinc phosphate solution comprising as the essential coating producing ingredients in weight percent, L0.18-0.9 zinc, 0.5-2.0 P0 and 0.7-2.5 ClO said solution having a total acid in the range of 10-60 points, maintaining said strip in contact with said solution until a phosphate coating is formed on the surface. of said strip, said coating having a Weight in the range of about 50 mg./per sq. ft. to about 1000 mg./per sq. ft. and varying from the average coating Weight prevalent on said strip by not more than about 200 rug/per sq. ft., and thereafter continuously cold rolling the said strip.

2. A method for treating ferrous strip which comprises the steps of pickling the strip, contacting the cleaned strip while the same is moving with an aqueous acidic zinc phosphate solution comprising as the essential coating producing ingredients in weight percent, 0.18-0.9 zinc, 0.5-2.0 P0,, 0.1s 2.5 010,, 0.1-2.4 N0 0.17 0.75 calcium, said solution having a total acid in the range of 10-60 points, controlling the temperature so as to produce a coating having a weight in the range of 50-1000 mg./per sq. ft. and varying from the average weight prevalent on said strip by not more than about 200 mg./per sq. ft., and thereafter continuously cold rolling the said strip.

3. A method for treating ferrous strip which comprises the steps of pickling the strip, contacting the cleaned strip while the same is moving with an aqueous acidic zinc phosphate solution comprising as the essential coating producing ingredients in weight percent, 0.18- 0.9 zinc, 0.5-2.0 P0 0.15-2.5 C10 0.1-2.4 N0 0.17- 0.75 calcium, said solution having a total acid in the range of 10-60 points, controlling the temperature so as to produce a coating having a weight in the range of 50-1000 mg./per sq. ft. and varying from the average coating weight prevalent on said strip by not more than about 200 mg./per sq. ft., rinsing said coating strip in a dilute aqueous chromic acid solution and thereafter cold rolling said strip.

4. A method in accordance with claim 2 wherein said chlorate is present in a concentration between 0.3 and 0.6%.

5. A method in accordance with claim 1 wherein the strip which comprises continuousrolling is performed while continuously flooding said strip with an, aqueous base lubricant.

6. A method in accordance with 'claim2 wherein said rolling is performed lv'vhile; said strip is continuously flooded with a dilute aqueous lubricant consisting essentially of 97%"99%. water and 1-3% of a mixture of fatty acid and an emulsifying agent, said fatty acid containing the equivalent of 12.9% by weight of 'oleic acid,

having an iodine number of 36.9 and a .saponifica'tion' number of 32.4. r

7. A method in accordance with claim 2 wherein said coating has a weight in the range of about 150-400 mg./per sq. ft. 1

8. A method in accordance with claim 2 wherein the ratio of calcium to zinc in the solution is within the range of 0.4-1 and 1.1-1. i

9. A ferrous strip which may" readily be continuously cold rolled and which has an acidic phosphate coating thereon, said coating having a weight in the range of a pickling thereof, with an aqueous acidic zinc phosphate". solution comprising as the essential coating producing;

ingredients in weight percent, 0.18-0.9 zinc, 0.5-2.0 P0

and 0.7-2.5 C10 0.1-2.4 N0 0.17-0.75 calcium, said solution having a total acid in the range of 10-60 points! said phosphate coating having a weight in the range of about 50 mg./per sq. ft. to about 1000 mg./per sq. ft. and varying the average coating weightprevalent'onjsaid; strip by not more than about 200 mg./per sq. ft.

References Cited in the file of this patent UNITED STATES PATENTS '1940 2,191,435 Ballardet a1. Feb. 27, 2,230,319 -Canzler Feb. 4, 1941 2,293,716 Darsey Aug. 25,1942 2,540,314 Amundsen Feb. 6 1951 2,724,668 Russell Nov. 22, .1955 2,743,204 Russell Apr. 24,

OTHER REFERENCES Streicher: Phosphatization of Metallic Surfaces, Metal Finishing, pages 61-69, August 1948 (page 66 re-:. lied on).

Claims (1)

1. A METHOD FOR TREATING FERROUS STRIP WHICH COMPRISES THE STEPS OF PICKLING THE STRIP, CONTACTING THE CLEANED STRIP WHILE THE SAME IS MOVING WITH AN AQUEOUS ACIDIC ZINC PHOSPHATE SOLUTION COMPRISING AS THE ESSENTIAL COATING PRODUCING INGREDIENTS IN WEIGHT PERCENT, 0.18-0.9 ZINC, 0.5-2.0 PO4 AND 0.7-2.5 GLO3, SAID SOLUTION HAVING A TOTAL ACID IN THE RANGE OF 10-60 POINTS, MAINTAINING SAID STRIP IN CONTACT WITH SAID SOLUTION UNTIL A PHOSPHATE COATING IS FORMED ON THE SURFACE OF SAID STRIP, SAID COATING HAVING A WEIGHT IN THE RANGE OF ABOUT 50 MG./PER SQ. FT. TO ABOUT 1000 MG./PER SQ. FT. AND VARYING FROM THE AVERAGE COATING WEIGHT PREVALENT ON SAID STRIP BY NOT MORE THAN ABOUT 200 MG./PER SQ. FT., AND THEREAFTER CONTINUOUSLY COLD ROLLING THE SAID STRIP.