GB1574323A

GB1574323A - Methods of cleaning metal sheets

Info

Publication number: GB1574323A
Application number: GB1028477A
Authority: GB
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1977-03-09
Filing date: 1977-03-09
Publication date: 1980-09-03

Description

(54) METHODS OF CLEANING METAL SHEETS (71) We, NIPPON STEEL CORPORATION, a Japanese Company, of No. 6-3, 2-chome, Ote-machi, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of cleaning cold rolled metal products, such as steel strips, sheets and plates, all of which hereinafter will be referred to as steel sheet.

In the production of a cold rolled steel sheet, the steel sheet is reduced by the cold rolling step to almost the final thickness, and the steel sheet thus cold rolled is very hard. For applications where good working properties, such as bending and drawing, are required it is necessary to anneal such cold rolled steel sheets appropriately, depending on their intended uses, in order to improve the mechanical properties. Such annealing is usually done at a temperature ranging from 550"C to 800"C in a non-oxidizing gas atmosphere.

However. when the steel sheet is cold rolled, a large amount of a mixture of lubricant oil used in the cold rolling and metal powders caused by friction during the cold rolling remains on the surface of the cold rolled steel sheet, and if the steel sheet with this mixture is annealed, the lubricant oil is carbonized and the metal powders are partially carburized and adhere to the steel surface, to cause a dirty surface and a significant degradation of the surface properties. This results in a loss of the commercial value of the final steel products, and in addition can cause a considerable hindrance in subsequent workings.For example, when such dirty cold rolled steel sheets are used as plating substrates, the residues adhering to the steel surface cannot be removed even by usual pre-treatments for the plating and thus cause vital surface defects which hinder satisfactory coating so that the surface appearance is deteriorated and the corrosion resistance of the plated surface is remarkably decreased.

Furthermore, when such dirty cold rolled steel sheets are used as substrates for paint coating, pre-treatments such as phosphate coating cannot be performed satisfactorily.

For the reasons set forth above, it is necessary to remove the surface residues by surface cleaning prior to the annealing of the cold rolled steel sheet. However, previously there have been difficulties in the removal of the mixture of the rolling oil and the metal powders sticking to the steel surface after cold rolling, and the surface cleaning methods which can be applied to cold rolled steel sheet are very limited. This is due to the nature of the surface residues after the cold rolling.

Researches by the inventors of this Application have shown the main components, such as fatty acids and fatty acid esters, of the rolling (lubricant) oil combine chemically in an iron-soap with a thin iron oxide film covering the steel sheet surface and the fine iron powder generated by friction during the rolling, so that the particles of iron powder themselves strongly adhere to each other or to the steel sheet surface by means of the soap thus formed.

The particles of fine iron powder are less than about 0. lit in size and are ferromagnetic, easily magnetized and strongly attracted by the steel surface. These facts are considered to cause the difficulties in removing the mixture.

As is disclosed by "Tekko Seizo Ho" (Steel Production Process) Section III Workings (2) edited by Japan Iron & Steel Association, an electrolytic method has been established as a cleaning method for surface cleaning of the steel sheet after the cold rolling. This method is performed in an electrolytic cleaning line having a total length of about several tens of meters, provided separate from or at the entrance to a continuous annealing furnace. The electrolytic cleaning line normally comprises four stages of processing; that is, alkali-immersion, brush ing, alkali-electrolysis and brushing, wherein the rolling oil is removed by saponification of the alkali while the final iron powders are removed by the brushing.

For this reason, as the rolling speed of the mill to which the electrolytic cleaning line is adjoined increases, a longer cleaning line is required. Maintenance of the brushing section, which performs the mechanical removal, is not easy, requires considerable costs and consumes a great amount of energy.

Furthermore, from the aspect of efficiency in the rolling oil removal, an alkali solution mainly composed of sodium silicate is used, but during the electrolysis in such a silicate solution, a film mainly composed of silicate is formed on the steel surface being treated if one tries to remove the rolling oil satisfactorily, which film can cause a severe hindrance to the properties of the final products, such as solderability which may be required for assembling after press-forming, susceptability to phosphate treatments as pre-treatments for paint coating, and qualities required for plated steel plates, such as a tin-plate.

In order to overcome the above difficulties, trials and proposals for simplifying the surface cleaning step by omitting the electrolytic cleaning or, further, eliminating the step have been made for a long time.

One of the proposed methods uses a brushing treatment alone, omitting the alkali electrolysis treatment. By omitting the alkali electrolysis treatment, the degradation of the steel sheet due to the silicate film formation can be prevented, but in turn, the rolling oil removal efficiency decreases, resulting in a decrease in the iron powder removal efficiency, with a large amount of residue unremoved even after the annealing and, as a whole, degradation of the surface cleanliness as compared with electrolytic cleaning methods.

Another proposed method is based on the principle of mill cleaning or detergent rolling, and uses a detergent mainly composed of a surface active agent and warm water instead of the rolling oil in the final stand of the cold rolling mill. According to this method, various modifications and considerations are given to the rolling oil used in all the rolling stands except the last, and to the detergent used in the last stand, but in spite of these efforts, the surface cleanliness is not improved satisfactorily and the power consumption at the last rolling stand increases. Thus this method has not yet been successful and established commercially.

In spite of various trials and proposals as mentioned above for replacing the electrolytic cleaning method, none of them have been successful.

Therefore, the known techniques have not been successful in overcoming the problem that the residues produced from the rolling oil and the iron powders, sticking to the steel surface after the annealing, cause severe effects on the quality and properties of the final products.

Furthermore, as the surface cleanliness of the cold rolled steel sheet has a definite and clear effect on the corrosion resistance after paint coating, and, an improvement in the surface cleanliness has been always demanded so that substantially all cold rolled steel sheets are produced using electrolytic cleaning lines, in spite of the large power consumption thereof.

The term "shect" used in this specification is intended to include "foil", "plate", "strip" and 'bar" - i.e. products from cold rolling mills.

From the aspect of saving energy consumption, extensive and various studies and experimints have been made to obtain a surface cleanliness similar to or better than that obtainable by the usual electrolytic cleaning process, in spite of eliminating the electrolytic cleaning step.

These have solved the technical incompatability of the prior art that if one tries to simplify the cleaning step the surface cleanliness deteriorates, and if one tries to improve the surface cleanliness the cleaning step becomes complicated and the capital cost and energy consump tion increases considerably.

It has now been discovered that when a surface of steel sheet after the cold rolling is simply wetted with an organic or inorganic wetting agent (hereinafter simply called a "wetting solution") and then a high-pressure water jet is applied thereto, the rolling oil and the iron powders sticking to the steel surface can be easily removed together with the wetting solution by the contact of the high-pressure water jet in a very short time, and yet the surface cleanliness thus obtained is similar to or better than that obtained by cleaning in an electrolytic cleaning line and can be subjected directly to the annealing.Thus. according to this invention. there is provided a method of cleaning cold rolled metal sheet prior to the annealing thereof. which method comprises wetting the surface of the cold rolled steel sheet ',-ith a wetting agent. applying a high-pressure fresh water jet to the wetted surface so as to form a continuous linear water curtain across the width of the steel sheet thereby removing rolling lubricant and iron powders adhering on the sheet surface simultaneously with the wetting agent.

In the method of this invention. a device to apply the wetting solution and a device to direct the high-pressure water jet can be incorporated as compact equipment at the delivery end of the final stand of a cold rolling mill or at the entrance to a continuous annealing furnace. In this wa-. this invention has succeeded in eliminating the usual electrolytic cleaning line.

This invention extends to cold-rolled metal products whenever cleaned by a method of this invention.

By way of illustration only, certain specific embodiments of this invention will now be described, reference being made to the accompanying drawings, in which: Figures 1 and 2 illustrate respectively one embodiment of the production process of a cold rolled steel sheet according to the present invention; Figure 3 shows the relation between the water jet energy received by the steel sheet and the jet nozzle pressure, for comparison of a water jet according to this invention with a conventional alkali spray Figures 4(a , (b) and (c) show schematically various types of water jet curtains; and Figure 5 shows schematically the jet water curtain from a slit nozzle.

Referring to Figure 1, 1 is a pay-off reel, 2 is a guide roll, 3 are rolling mill stands, 3' is a final rolling stand, 4 is a tension reel. The strip 5 is fed from a coil mounted on the pay-off reel and passes through the guide rolls 2. The strip is cold rolled by the work rolls of the rolling stands 3 with the application of rolling oil and then coiled on the tension reel 4. At the delivery of the final stand 3' of the rolling mill stands, there are provided a wetting solution applicator 6, a squeeze roll 7, a high-pressure water jet nozzle 8, a squeeze roll 7' and an air knife 9, which in combination constitutes a cleaning apparatus 10. The surface of the strip 5' coming out of the final stand 3' is contaminated with a large amount of the rolling oil and fine iron powders.

The strip enters the cleaning apparatus via guide rolls 2' , and the wetting solution is applied to the strip by the applicator 6 onto the mixture of the rolling oil and the fine iron powders adhering to the steel surface. Then high-pressure water jet is jetted from the high-pressure water jet nozzle 8 onto the strip surface to which the mixture of the rolling oil and the fine iron powders adhere and wetting solution is applied thereto, and after both of the mixture and the wetting solution have been removed simultaneously, the water on the strip surface is removed by the squeeze rolls 7' and the air knife 9 and the strip is then coiled on the tension reel 4.

Referring to Figure 2 which illustrates another embodiment of the present invention where the cleaning apparatus is provided at the entry of a continuous annealing furnace, the strip 5 from No. 1 and No. 2 coil holders 11 passes through a welder 12, pinch rolls 13 and enters a cleaning apparatus similar to that shown in Figure 1 where the surface of the strip 5 is cleaned fully, and then dried by a dryer 14. The cleaned strip 5' is annealed normally by a continuous annealing apparatus 15. and after ordinary after-treatments (not shown) coiled on No. 1 and No. 2 tension reels 16.

Description will be made on the method according to the present invention.

When the wetting agent to be applied to the cold rolled steel sheet is an inorganic wetting agent, an aqueous solution mainly composed of one or more of sodium silicate, sodium hydroxide (caustic soda), sodium aluminate and sodium phosphate may be used.

The alkali concentration is preferably 5 to 10 wt.%. In addition, in order to improve removal efficiency of the various rolling oils, 0.3 to 1 wt.% of a surface active agent, and further, in the case when the amount of adhering iron powders is large, 3 to 6 wt.% of an iron-chelating agent may be added. As for the iron-chelating agent, citric acid and its derivatives and salts, gluconic acid and its derivatives and salts, carbamic acids, hydroxylamines and amines may be used.

When the wetting agent is an organic wetting agent, organic solvents having a viscosity ranging from 0.5 to 4CP (at 30"C), namely a viscosity similar to that of water, are useful.

In order to attain very efficient removal of the rolling oil and the iron powders from the steel surface during the cold rolling which is normally performed at a high speed, it is necessary that the organic solvent in this case, has a high degree of removability by a high-pressure water jet. For this purpose an organic solvent unsusceptible to emulsification or dissolution in water and having a viscosity similar to or same as that of water is desirable.

To satisfy the above requirement, hydrocarbons and halogenated hydrocarbons are suitable, and kerosene, light oil, solvent naphtha, trichloroethylene, perchloroethylene have a viscosity within the above specified range. Therefore in a preferred aspect of the present invention a mixture solvent of one or more of the above hydrocarbons is used.

Preferably the amount of the wetting solution to be applied on the steel sheet is from 5 to 10 ml/m2, which is attainable by the roll squeezing. When using amounts less than 5 ml/m2, the removal of the rolling oil and the iron powders is not satisfactory. There is no upper limitation on the applied amount of the wetting solution, but amounts more than 10 ml/m2 are not economical.

It is important in the combination of the application of the wetting solution and the high-pressure water jet, in the first place, that all of the rolling oil, the fine iron powders and the wetting agents are removed completely and simultaneously by one coup of the water jet jetted on the steel surface. In this case the work performed by the jet water is determined by the jet water pressure imposed on the steel sheet and the volume of jet water per unit area of the steel sheet surface (hereinafter called flow density), and according to the discoveries of the present inventors, the work can be expressed simply by the jet nozzle pressure and the flow density.As shown in Figure 3, in order to obtain the desired results, that the jet nozzle pressure should be not less than 5 kg/cm2 and preferably not less than 8 kg/cm2 for any portion of the steel sheet surface and the work expressed by (jet nozzle pressure x flow density) should be not less than 0.1 to w hr/m2 per unit area of the steel sheet surface.

According to the results of the extensive experiments conducted by the present inventors, a surface cleanness similar to that obtained by the electrolytic cleaning can be obtained in a short time so far as the above conditions are maintained (see Table 2). Therefore, in an actual operation, merely by adjusting the water jet conditions in accordance with a maximum travelling speed of steel sheet, a surface cleanness similar to that obtained by electrolytic cleaning can be obtained in spite of variations of the travelling speed. Further, the highpressure water jet nozzle may be arranged only in a single stage so as to cover the whole width of the steel sheet.

Secondly, as shown in Figure 4 (a), it is necessary that the jet water curtain 18 is jetted onto the steel sheet surface 17 in a linear form across the steel width. If the jet water curtain is not in a linear form across the sheet width but in a zig-zag form (Figure 4 (b)) or a feather-stitch form (Figure 4(c)) as commonly done, the jet water does not contact the steel sheet surface at the same time all across the sheet width. Consequently, the water jetted on the steel surface flows from the portions of the steel surface which have been already struck with the jet water to the portions which have not been struck with the jet water and dilutes or washes off the wetting solution on the surface so that the rolling oil and fine iron powders are not completely removed when the jet water strikes thereon, leaving stripes 19 parallel to the travelling direction.Thus, it is important that the jet water curtain strikes on the sheet surface at the same time all across the sheet width.

Thirdly, it will be appreciated that it is important to select a nozzle for satisfying the above requirement. In order to maintain the required jet water pressure imposed on the steel sheet surface and the required work on the steel surface performed by the jet water and to bring the jet water curtain into contact with the steel sheet surface at the same time all across the sheet width, a slit nozzle as shown in Figure 5 is most desirable. The slit nozzle has a linear slit of a constant width and a length long enough to cover the whole width of the sheet. By using this slit nozzle, it is possible to jet a fresh water at the same time all across the sheet width with a constant jet water pressure and a constant flow density at any portion of the sheet surface.

In the case of a conventional spray cleaning technique, a number of nozzles are provided in a pressure header so as to spray the fresh water onto the steel sheet surface. However, the cleaning water is sprayed by the nozzles in a divergent form at a certain angle, and four types of nozzles having a spray pattern of "full cone", "hollow cone", "square spray" and "flat spray" are at present commercially available.

When the above commercially available nozzles are used in the present invention, it is necessary that they satisfy the first and second requirements mentioned hereinbefore. The commercially available nozzles of "full cone", "hollow cone" or "square spray" type cannot be used satisfactorily in the present invention because it is difficult for these nozzles to form a jet water curtain in a linear form across the sheet width and to bring the jet water into contact with the sheet surface at the same time all across the sheet width, and the jetted water from these nozzles spreads over a wide area and thus the flow density decreases so that the first requirement cannot easily be satisfied by maintaining the required jet water pressure imposed on the steel sheet.

On the other hand, the commercially available "flat spray" nozzles can be used in the present invention if a number of them are arranged on a header in such a manner that the divergent angle of the each water jets are more identical so as to form jet water curtains in a linear form across the sheet width. In this case, however, it should be noted that both the flow rate of Jet water and the jet water pressure imposed on the steel sheet are at their maximum at the centre portion of diverging water jet, thus exhibiting their distributions in a mountain-like curve. Therefore, when a number of "flat spray" nozzles are arranged on a pressure header, it is necessary to adjust the pitch between individual nozzles so as to avoid too great a pitch which would prevent the first requirement from being satisfied at an intermediate portion between the nozzles. This is the fourth important point.

The fifth important point is the surface temperature of the steel sheet just prior to the application of the Jet water thereon and the temperature of the wetting solution applied thereon.

Oils, such as palm oil and beef tallow, which freeze at ordinary temperatures are usually employed as the cold rolling oil, and in order to remove the rolling oils adhering to the steel Surface by a fresh water Jet it is necessary to maintain the adhering rolling oils in a melted state. This is particularly important in cases where inorganic agents are used as the wetting agents. Results of studies from the above view point have revealed that it is desirable to maintain the surface of the steel sheet and the wetting solution at temperatures not lower than 70"C. If they are maintained below 70"C, the desired results in respect of the surface cleanness cannot be obtained.

Regarding a method for maintaining the surface of the steel sheet and the wetting solution to be applied thereto at 700C or higher, it is possible to utilize the heat generated in rolling by performing the method of the present invention immediately after the cold rolling, because the steel sheet coming out of the cold rolling mill is usually at 100"C or higher. Alternatively, it is possible to pre-heat the steel sheet (as is done for cold strip coils) in cases where the present invention is applied at the entry of a continuous annealing furnance, etc.

As for the pre-heating means, a single conventional pre-heating means such as hot-air, steam, hot water, may be used, or a combination of such means maybe used. However, a more desirable method is to immerse the steel sheet in a heated wetting solution so as to perform both the coating of the wetting solution and the pre-heating of the steel sheet simultaneously.

According to experiments carried out by the present inventors, the required surface temperature of the steel sheet can be attained by immersion in a hot water heated at 950coin a short time of one second, or less in the case of a cold rolled steel sheet of an ordinary thickness.

Sixthly, it is important that the rolling oil and the fine iron powders which have been removed will not adhere to the steel sheet again.

The rolling oil and the fine iron powders adhering to the steel sheet surface are removed completely and simultaneously from the sheet surface by the jet water according to the present invention and flow as a dirty drainage, and this dirty drainage must be removed quickly from the steel sheet surface, otherwise it will remain on the sheet surface and contaminate the surface again.

The drainage can be removed by means of squeeze rolls, or by one or more air knives.

However, in order to remove the drainage quickly to attain the desired results of the present invention, it is desirable that the above drainage removal means is arranged immediately after the train of the fresh water jet nozzle. Apart from the above removal means, a fresh water jet may be used for removing the dirty drainage.

The present invention is basically different from the conventional alkali spray cleaning method, which is similar to the present invention in respect of the use of alkali.

The surface cleaning according to the alkali spray cleaning method is performed by the combination of an alkali spray stage and a water rinsing stage. In this case, the surface dirts are removed in the alkali spray stage and the water rinsing stage is necessary to remove the alkali remaining on the steel surface from which the dirts have been removed. Whereas according to the present invention, the wetting solution is merely applied on the surface and thus the dirts are not removed during the stage of the wetting solution application. The removal of the surface dirts or stains, according to the present invention, is performed only by the et blowing of a high-pressure water after the application of wetting solution, and the wetting solution is also removed together with the dirts.Thus, according to the present invention, the removal of dirts and water rinsing are effected together in a single stage.

Further, in the conventional alkali spray cleaning method, a spray chamber in which a number of spray nozzles are arranged is used, but there is no specific limitation in the selection and arrangement of the nozzles as required in the present invention. Thus in the present invention, the first and second requirements as mentioned hereinbefore must be satisfied, while in the alkali spray cleaning method there are no such requirements, and the nozzles are arranged in the spray chamber in such a manner that the steel sheet surface is brought into contact with the spray liquid repeatedly until the dirts on the steel sheet surface are removed.

The conditions of the water jet in the present invention are quite different from the conventional art. For example, according to the disclosure of "Cleaning of Metal" of Modern Engineering Library published by Chijin Shokan, Tokyo, Japan, the spray pressure ranges from 8 to 30 psig, or 2.1 kg/cm2 at maximum, and the spray amount ranges from 60 to 100 l/m2 per minute, and the cleaning is done in about 3 minutes. These conditions may be expressed by the nozzle pressure and the work of water given to the steel sheet as shown in Figure 3 from which it is very clearly understood that the conventional art is done in a completely different zone from that of the present invention. Further, in the present invention, the cleaning is completed in a time shorter than about one second, and thus the cleaning effect is much higher.

The reason why a high-pressure jet, as in the present invention, is not used in the conventional alkali spray method is that there is a problem of foaming due to spraying of the alkali solution. For example, excessive foaming will cause overflow of the solution from the cleaning device. In case of the conventional alkali spray cleaning method, a nozzle pressure exceeding 20 psig (1.5 kg/cm2) will cause foaming and thus the operation is normally performed with a nozzle pressure below 20 psig. If a higher nozzle pressure is to be applied, it is necessary to use a cleaning agent of less foamability, but such a cleaning agent is also low in cleaning power.

This fact has been an important limitation in the conventional alkali-spray cleaning method.

Thus the present invention is based on a completely different technical concept from that of the alkali spray cleaning method, and is completely free from the foaming problem because the wetting solution is merely applied and the water is jetted under a high pressure. Therefore in the present invention the wetting solution can be selected without limitations from the foaming problem.

Regarding conventional cleaning methods similar to that of the present invention in that an organic solvent is used for the surface cleaning, there is a method in which adhering oil or grease and metal particles are removed simultaneously by the dissolving power and jet pressure of the solvent, and also a "Petroleum Solvent Spray Method" (old U.S. Army Specification MIL-116C).

These methods are limited in the kind of the solvents used because of danger of flaming and explosion induced by the spraying. The present inventors have conducted these spray methods with solvents selected mainly from the aspect of cleaning effect, but the results as shown in Example 4 have revealed that their cleaning effects are far inferior to that obtained by the present invention.

Still further, it has been conventionally practised in various fields to jet high-pressure water. However, in the field of steel surface cleaning, a water jet has been used only as a low-pressure spray-rinse for the water washing in the finishing step after the surface cleaning such as alkali cleaning and emulsion cleaning. This will be clearly understood if one refers to the surface cleaning methods specified by JIS Z 0303 or MIL-P-11 6D according to which a surface cleaning method only by water jet has not be classified or specified.

Naturally, as the water itself is a polar solvent, it is possible to remove water-soluble dirts by a water jet and some surface cleaning effect can be attained thereby. However, the mixture of rolling oil and fine iron powders sticking to the steel surface after cold rolling to which the present invention is directed cannot be removed only by water jet as will be clearly understood in the examples hereinafter set forth. The present invention is based on the discovery that when the wetting solution is applied to the steel sheet as cold rolled and the high-pressure of the wetting solution, the surface stains or dirts can be easily removed, and this discovery is novel in view of the conventional steel surface cleaning art.

One may consider combining the conventional alkali immersion cleaning, the conventional alkali spray cleaning or the solvent degreasing method with a high-pressure water jet but it leads only to the results that surface stains are removed by the alkali immersion cleaning or alkali spray cleaning and then the spray rinse is done under a high pressure. Such a type of a high-pressure water jet has nothing to do with the present invention as will be clearly understood from the foregoing descriptions.

Description of Preferred Embodiments: The present invention will be more clearly understood from the following embodiments: Example 1: A cold rolled steel strip of 200 mm width, 0.8 mm thickness was passed through an experimental cleaning apparatus at a travelling speed of 100 m/min. The cleaning apparatus comprised a wetting agent solution application device of a spray and roll squeezing type, a single stage high-pressure water jet nozzle and an air knife. The amount of rolling oil and iron powders sticking to the steel surface before the test was in average 2 g/m2 and 900 mg.iron/m2 respectively.The surface cleaning was carried out by applying to the steel surface 5 ml,'m2 of 6% sodium ortho-silicate solution and jetting a high-pressure water jet at 60"C onto the surface with a nozzle pressure of 15 kg/cm and a flow density of 1 l/m2 according to the present invention (the work done by the jet water was 0.41 w.hr/m2) (present invention in Table 1).

For comparison tests were done on five cases. In the first case (comparison 1 in Table an alkali solution was applied and water at 60"C was jetted with a nozzle pressure of 2 kg/c and at a flow density of 1 l/m2. Thus the work was 0.05 whr/m2. In the second case, no alkali solution was applied, but only water at 600c was jetted with a nozzle pressure of 15 kg/cm2 and a flow density of 1 l/m2. Thus the work of the jet water was 0.41 w hr/m2 (comparison 2 in Table 1). In the third comparison, no alkali solution was applied, but only a 3% sodium ortho-silicate solution at 60"C was jetted with a nozzle pressure of 15 kg/cm2 at a flow density of 1 l;m2. Thus the work was 0.41 w-hr/m2 (comparison 3 in Table 1). In the fourth comparison, an alkali solution was applied and then 3% sodium ortho-silicate solution was jetted with a nozzle pressure of 2 kg/cm2 at a flow density of 11 1m2. Thus the work was 0.05 w.hr/m2 (comparison 4 in Table 1). In the fifth comparison, no alkali solution was applied but 3% sodium ortho-silicate solution at 60"C was jetted with a nozzle pressure of 15 keg/ cm and a flow density of 1 l/m.The work was 0.41 w.hr/m (comparison 5 in Table 1). The surface cleanness of the steel strips was tested and evaluated into five classes by the wiping method (JIS Z0303) and the water wetting method. The results are shown in Table 1. Table 1 Application of Water Jet Work of Nozzle Surface Cleanness Wetting Agent Water Jet Pressure W.hr/m Kg/cm Test by Test by Wiping-off Wetting Present yes 60 C High 0.41 15 Invention Pressure Water Comparison 1 yes 60 C Low- 0.05 2 xx Pressure Water Comparison 2 no 60 High- 0.41 15 xx xx Pressure Water Comparison 3 no 60 C High- 0.41 15 Pressure Alkali Comparison 4 yes 60 C Low- 0.05 2 x Pressure Alkali Comparison 5 no 60 C High- 0.41 15 Pressure Alkali Remarks: Test by Wiping-off: The steel surface is rubbed by white flannel and the amount of residual iron powder is estimated by the degree of blackening of the flannel.

Test by Wetting The steel sheet is immersed in a distilled water and the amount of residual rolling oil is estimated by the degree of water adhering to the steel surface when the sheet is taken out of the water.

Results: - Excellent, - Good, - fair, x - poor, xx - bad.

Surprisingly, the surface cleanness results obtained by the present invention were best. In cases where alkali was use d as the jet water, the removal of the residual iron powders was not satisfactory.

Example 2: The cleaning according to the present invention was performed by a cleaning device arranged at the delivery side of the final stand of a four-stand tandem cold rolling mill designed for recirculating rolling lubricant of a beef tallow based oil, as shown in Figure 1.

The effective length of the device was 3.5 m.

During cold rolling a steel strip of 2.3 mm thickness and 1050 mm width into a tin-plate substrate of 0.30 mm thickness at a rolling speed of 700 m/min., 10% sodium ortho-silicate solution was applied at a rate of 7 ml/m2 onto the surface of the strip from the spray-roll squeezing device provided at the delivery side of the final stand, then fresh water at 60"C was jetted on the top and bottom surfaces of strip from high-pressure slit nozzles extending to the total width of the steel strip at a nozzle pressure of 10 kg/cm2 and flow density of 0.5 l/m2 (work of the jet water: 0.13 w.hr/m2) and the water was removed by the roll squeezing and the air knife.The surface cleanness of the steel strip thus obtained was 14.1 mg/m2 of residual iron powders as iron, and 4.1 mg/ m2 of residual rolling oil (present invention I in Table 2).

Table 2 Nozzle Work of Cleaning Method Treat- Surface Surface Performance of Tin Plate Pres- Water ing. Cleanness Appearance Cold Rolled Sheet Quality sure Jet Time after Kg/cm W.hr/cm (sec.) Iron Rolling Annealing Compati- Corro- Appea- Corro Powder Oil bility sion rance sion mg/m mg/m for Resist- Resist Phosphate ance ance Coating after Paint Coating Alkali Application Present 10 0.13 + 0.3 14.1 4.1 Beautiful Invention 1 High-Pressure Water Jet Compari- 10 0.13 Alkali 0.3 134 53 Black xx x x xx son 1 Application Stain + Alkali Jet Chelate-Contain Present ing Alkali Invention 2 10 0.11 Application 0.25 8.0 3.0 Beautiful + High-Pressure Water Jet Compari- Electrolytic 6 17.0 4.6 Beautiful son 2 - - Cleanning Compari- Detergent Black son 3 5 0.2 Rolling - 113 170 Stain x xx xx xx Key: -Excellent, -Good, -fair, x - poor, xx - bad For comparison, only 3 wit.% sodium ortho-silicate solution was jetted to the strip surface under the same conditions as in the above example, the results were that the amount of the residual iron powder was 134 mg/m2 as iron and the amount of the residual rolling oil was 53 mg/m2 (comparison 1 in Table 2).

Then, the rolling speed was increased to 850 m/min. and a solution of 10% sodium ortho-silicate and 1% sodium gluconate was applied to the strip surface in an amount of 5 maim2, and a high-pressure fresh water at 600C was jetted at a nozzle pressure of 10 kg/cm2 and flow density of 0.41/ m2. The result was that the amount of the residual iron powders was 8.0 mg/m2, and the amount of the residual rolling oil was 3.0 mg/m2. The work of the jet water was 0.11 w /m2 (present invention 2 in Table 2).

Meanwhile when the cleaning according to the present invention was not performed. the resultant surface cleanness after the cold rolling was that the amount of the residual iron powder was 870 mg/m2 as iron and the amount of the residual rolling oil was 2.4 g/m2 at a rolling speed of 700 m/min. The cold rolled steel strip was passed at a speed of 600 mpm through an electolytic cleaning line having an effective length of 60 m, and subjected to hot alkali immersion, brushing. alkali electrolysis and brushing for surface cleaning, and the resultant surface cleanness was that the amount of the residual iron powder was 17 mg/m2 as iron and the amount of the residual rolling oil was 4.6 mg/m2 (comparison 2 in Table 2).

Further for comparison, the final stand of the cold rolling mill was blocked off so as to be insulated from the recirculation system. and a hot solution at 600C containing 1 wt.% of non-ionic surfactant was jetted to the rolling rolls and the strip with a nozzle pressure of 0.5 kg/cm2 and a flow rate of 2 m3/min. so as to effect the detergent rolling. At a rolling speed of 600 m/ min., the roll wearing became severer as the power consumption required for the final stand increased about 20%. The resultant surface cleanness, despite the large amount of detergent, was that the amount of the residual iron powders was 113 mg/m- as iron and the amount of the residual rolling oil was 170 mg/m2. The work of the jet water was 0.2 w m2 (comparison 3 in Table 2).

The steel strips thus cleaned by the above treatments were annealed and evaluated for their qualities as an electrolytic tin-plate, and the results as shown in Table 2 revealed that all of the strips treated by the comparative treatments except for the electrolytic cleaning (comparison 2 in Table 2) were contaminated by dirts after the annealing and their surface properties as a cold rolled sheet and as well as a tin-plate substrate were unsatisfactory.

Whereas the results obtained by the present invention were equal to those obtained by the electrolytic cleaning.

Example 3: A cold rolled steel strip of 0.8 mm thickness was divided into eight sheets. Five sheets of them were coated with the wetting solvent specified in the present invention by immersion.

and the remaining three sheets for comparison. were coated with wetting solvent outside the scope of the present invention by immersion. The amount of the solvent applied was all 10 mli m2 and the application was done at 40"C.

Onto the surfaces of the wetting solution coated eight steel sheets, water was jetted at a nozzle pressure of 8 kg/cm2 with a rate of 20 1/min. and a work of the jet water of 0.12 whr/m to remove the smudges from the surfaces. The sheet travelling speed was maintained at 200 mpm. The results thus obtained are shown in Table 3.

Table 3

Wetting Viscosity Removal of Agents CP (300 C) oil and iron powder (No) Spindle oil 8.3 40 C: 0 " Hydrocarbon Commercially 39.6 Hydrocarbon available cleaning oil o A B " B 4.9 20 Kerosene 1.18 98 Light oil 2.86 90 ,O Hydrocarbon c Solvent 0.56 95 c naphtha c Trichloro- 0.52 95 Halogenated ethylene hydrocarbon a ethylene As will be clearly understood from Table 3, even when the jetted condition of the jet water is within the scope of the present invention, the removal of smudges is very low as 40% or less if the organic solvent is outside the scope of the present invention. According to the present invcntion, a smudge removal as high as 90% or higher can be obtained.

Example 4: A cold rolled steel strip of 0.8 mm thickness was divided into four sheets, and one sheet was treated for removal of smudges according to the present invention and the remaining three sheets were treated by a comparative method. The results are shown in Table 4. The sheet pass speed and the jet water conditions are the same as in Example 3.

Table 4 Wetting Jet Removal of oil Agents medium and iron powder Present invention Kerosene Water 98 none Kerosene spray 40 Comparison Kerosene Steam 55 none Water 5 Example 5: The cleaning treatment according to the present invention was performed using an ordi nary electrolytic cleaning equipment consisting of an alkali hot-dip tank, a first brush scrubber, a hot-alkali electrolysis tank, and a second brush scrubber. A cold rolled steel strip of 0.8 mm and 1,200 mm width was passed through the equipment at a speed of 600 m/sec.

The surface condition before the cleaning was that the amount of beef tallow based oil adhering to the surface was 500 to 600 mg/m2, and the amount of iron powders was 300 to 350 mg/m2 as iron.

For starting the treatment, all of the brush rolls of the brush scrubbers were taken off and the hot alkali electrolysis tank was emptied. The alkali hot-dip tank was filled with fresh water, and heated to 950c by steam. The steel strip passing through the heated fresh water in 0.7 seconds was heated to a surface temperature of 75"C.

A coating device of spray roll squeeze type was provided immediately after the squeeze roll at the delivery side of the hot-dip tank, and a solution of 5% caustic soda, 5% sodium ortho-silicate, and 2% gluconic acid was coated at 700C on the steel strip in an amount of 10 ml/m2, and a high-pressure water jet was jetted from a fresh water jet nozzle provided at the central portion of the first scrubber. Then the dirt drainage was removed from the strip surface by means of the squeeze rolls and the air wiper provided at the outlet side of the scrubber.

Present Invention 1: a slit nozzle having a linear slit of 1.3 m length extending in the direction of the strip width was provided on a pressure header, and a high-pressure fresh water jet was jetted with a header pressure of 8 kg/cm2 and a flow density of 0.5 l/m2. The resultant surface was very beautiful and the amount of the residual rolling oil was 2.8 mg/m2 and the amount of the residueal iron powders was 7.3 mg/m2. The work of the water jet was 0.11 whr/m2.

Comparison 1: The similar slit nozzle as used in the "Present Invention 1" but having an expanded slit spacing was used and a fresh water jet was jetted with a header pressure of 1.5 kg/cm2, a flow density of 2.7 l/m2. The work of the jet water was 0.11 w-hr/m2, but the strip surface thus obtained showed dirts scattered all over the surface, and the amount of the residual oil was 83 mg/m2 and the amount of the residual iron powders was 133 mg/m2, indicating very unsatisfactory cleaning.

Comparison 2: A plurality of flat spray nozzles having a diverging angle of 42 was provided with a pitch of 70 mm therebetween in a pressure header, so as to form a jet water curtain in a linear form across the strip width, and a fresh water jet was jetted with a header pressure of 15 kg/cm2 and a nozzle flow rate of 26 1/ min. per nozzle at a distance of 100 mm from the strip surface.

The average flow density was 0.65 l/m2, and the work of the jet water was in average 0.26 w-hri m2. The resultant surface showed light strip-like dirts of about 10 mm width with a pitch of 70 mm. This was caused by the improper arrangement of the nozzles in respect of the pitch length which decreased the iet water pressure and the flow density at the intermediate portions between the adjacent two nozzles.

Present Invention 2: Similar flat spray nozzles as in the above Comparison 2 were arranged with 50 mm pitches in the pressure header, and a high-pressure fresh water jet was jetted with a header pressure of 15 kg/cm and a flow density of 0.9 l/m2. The work of the jet water was in average 0.37 w-hrxm . The resultant surface was beautiful with 3.5 mg/m2 residual rolling oil and 8.7 mg/m2 residual iron powders.

Next, a high-pressure fresh water eft was jetted with a lowered header pressure of 8 kg/cm2 and a nozzle flow rate of 19 1/min. The average flow density was 0.65 l/m2 and the average work of the jet water was 0.14 whr/m2. The resultant surface was beautiful with 3.0 mg/m2 residual oil and 9.3 mg/m2 residual iron powders.

Comparison 3: Two pressure headers each having flat spray nozzles arranged with 100 mm pitches as in Comparison 2 above, and the headers were arranged so as to place the nozzles in a triangle zig-zag arrangement with 100 mm pitches, and a high-pressure of 15 kg/cm2 and a nozzle flow rate of 26 1/min. per nozzle. The average flow density was 0.9 l/m2 and the average work of the et water was 0.37 w hr/m2. The resultant surface showed strip-like dirts of about 40 mm width with 100 mm pitches.Thus is caused by the fact that the jetted water from the first pressure header flowed around the surface and removed the wetting agent coated on the surface portion onto which the water jet is jetted later from the second pressure header, which was caused by the delay of contact of some of the water et with the strip surface across the strip width.

As can be clearly understood from the foregoing descriptions and examples, the present invention has made it possible to remove rolling lubricant and iron powders adhering on the steel sheet and cold rolling with a high degree of efficiency by means of a very compact cleaning device arranged with a cold rolling mill or a continuous annealing equipment, and the steel sheet cleaned according to the present invention shows a surface cleanness equal to or better than that of an electrolytically cleaned steel sheet, and can be directly annealed into a final product. Thus according to the present invention, it is possible to eliminate the electrolytic cleaning step which has been indispensable in the conventional art, and save the power consumption to a great degree.

It should be also understood that the present invention can be applied to sheets of metals other than steel.

WHAT WE CLAIM IS: 1. A method of cleaning cold rolled metal sheet prior to the annealing thereof, which method comprises wetting the surface of the cold rolled metal sheet with a wetting agent, applying a high-pressure fresh water jet to the wetted surface so as to form a continuous linear water curtain across the width of the metal sheet thereby removing rolling lubricant and iron powders adhering on the sheet surface simultaneously with the wetting agent.

2. A method according to claim 1, in which the high-pressure fresh water is applied with a jet nozzle pressure of not less than 5 kg/cm and under the condition of (jet nozzle pressure) x (volume of jetted water per unit area of sheet surface) > 0.1 w hr/m2 (work done by water jet per unit area of sheet surface) across the whole of the width of the steel sheet.

3. A method according to claim 1 or claim 2, in which the wetting agent is an alkali solution.

4. A method according to claim 3, wherein the alkali solution is a solution of sodium ortho-silicate, caustic soda, sodium phosphate, or sodium aluminate.

5. A method according to claim 3 or claim 4, in which the wetting agent contains from 5 to 10 wt.% of the alkali.

6. A method according to claim 1 or claim 2, in which the wetting agent is a hydrocarbon or halogenated hydrocarbon having a viscosity from 0.5 to 4CP at 300c.

7. A method according to any of the preceding claims, in which the wetting agent contains 1 to 6 wt.% of an iron-chelating agent.

8. A method according to claim 7, in which the iron-chelating agent is one or more of gluconic acid and its derivatives and salts, citric acid and its derivatives aand salts, carbamic acids, hydroxylamines and amines.

9. A method according to any of the preceding claims, in which wetting agent contains 1 to 3 wt.% of sodium gluconate as an iron-chelating agent.

10. A method according to any of the preceding claims, in which the wetting agent contains 0.3 to 1 wt. % of surfactant.

11. A method according to any of the preceding claims, in which the water is jetted through a slit nozzle having a slit length at least as great as the steel sheet width.

12. A method according to any of claims 1 to 10, in which the water is applied by a plurality of jet nozzles so arranged that a single jet curtain from one jet nozzle overlaps the curtain from the adjacent iet nozzle.

13. A method as claimed in claim 1 and substantially as hereinbefore described, referring to the accompanying drawings.

14. A method of producing metal sheet from pickled cold rolled sheet, comprising cleaning the metal sheet in accordance with a method according to any of claims 1 to 13, followed by an annealing treatment.

15. Metal sheet whenever produced by a method according to any of claims 1 to 14.

16. Metal sheet according to claim 14, wherein the metal is steel.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. power consumption to a great degree. It should be also understood that the present invention can be applied to sheets of metals other than steel. WHAT WE CLAIM IS:

1. A method of cleaning cold rolled metal sheet prior to the annealing thereof, which method comprises wetting the surface of the cold rolled metal sheet with a wetting agent, applying a high-pressure fresh water jet to the wetted surface so as to form a continuous linear water curtain across the width of the metal sheet thereby removing rolling lubricant and iron powders adhering on the sheet surface simultaneously with the wetting agent.

16. Metal sheet according to claim 14, wherein the metal is steel.