US3873280A

US3873280A - Descaling of steel strip

Info

Publication number: US3873280A
Application number: US484574A
Authority: US
Inventors: Aleck Block; Edward H Frank; Robert H Shoemaker
Original assignee: Merit Abrasive Products Inc
Current assignee: Merit Holding Inc
Priority date: 1972-08-22
Filing date: 1974-07-01
Publication date: 1975-03-25
Anticipated expiration: 1992-03-25

Abstract

Steel strip is descaled by a combination of steps including molten salt bath treatment followed by flailings with abrasive leaves whereby descaling is effected without acid bath treatment and without deleterious effects upon the basic stock. Optionally, mechanical scale breaking by flailings with abrasive leaves may precede the salt bath treatment.

Description

United States Patent I [19-] Block etal.

[ Mar. 25, 1975 Assignees DESCALING or STEEL STRIP Inventors: AleckBlock, Los Angeles,-Calif.;

Edward H. Frank,'Washington, Pa; RobertH. Shoemaker, Detroit, Mich.

Merit Abrasive Products, Inc., Compton, Calif.

Filed: July 1, 1974 App]. No.: 484,574

Related U.S. Application Data Continuation of Ser. No. 282,873, Aug. 2-2 1972, abandoned.

u.s. Cl 11 517322, 51/328, 134/2,"

Int. Cl 1324b 1/00, B08b 7/02, C23b 1/06 Field of Search 51/281 R, 322, 323, 326, 51/328; 29/81 R, 529; 134/1, 2, 3, 6, 7, 9,

, References Cited UNITED STATES PATENTS 10/1934 MacGregor 51/328 Steel strip is descaled by a combination of steps in- 5/1939 Broadfield 51/323 X 2,710,502 6/1955 Lyon 51 322 I 3,121,026 2/1964. Beigay 134/2 3,166,841 1/1965 Reinhold 29/529 3,254,011 5/1'966 Zaremski 204/1405 3,260,619 7/1966 Shoemaker 134/3 3,506,487 4/1970 Lenz 134/2 3,625,900 12/1971 Shoemaker 134/2 X 3,699,726 10/1972 Turner 51/322 X Primary Examiner-Donald G. Kelly Attorney, Agent, or Firm-Ellsworth R. Roston ABSTRACT eluding molten salt bath treatment followed by flailings with abrasive leaves whereby descaling is effected without acid bath treatment and without deleterious effects upon the basic stock. Optionally, mechanical scale breaking by flailings with abrasive leaves may precede the salt bath treatment.

13 Claims, N0 Drawings DESCAL ING OF STEEL STRIP This is a continuation of application Ser. No. 282,873, filed Aug. 22, 1972, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for the removal of foreign substances, including oxide .passed from a bar mill to a strip mill in which hot rolls reduce the plates in thickness to gauges of the order of .0625 inch to .125 inch. Further reduction in thickness is conventionally effected, following annealing, by a cold mill in which the sheet may, for example, be reduced to a thickness in the range of 14 to 32 gauge.

Removal of the hot mill scale from the surface of the stock following hot rolling and annealing, and following the annealing of cold rolled strip prior to further reduction, has been effected by a variety of methods.

Chemical methods of scale removal have included immersion in molten alkali or salt baths followed by immersion in acid baths for periods sufficient to effect a chemical reaction between the scale and the constituents of the bath to dislodge the scale from the basic metal. In the case of hot rolled carbon steel the hot rolled scale has been removed by acid and blasting; and the formation of annealing scale has been avoided by annealing in a nonoxygen atmosphere.

Mechanical methods of scale removal have included sand or shot blasting and abrasion, as by wire brushes, sometimes preceded or accompanied by bending of the scale-bearing material to fracture the scale and thus assist in its removal by such blasting or abrading. Aqueous cleaning solutions at temperatures below their boiling points have been employed to some extent in connection with such mechanical methods.

All of these methods of scale removal have disadvantages, particularly in connection with the descaling of stainless steel sheet; the scale formed on the stainless steels being tenacious and relatively impervious and, consequently, more difficult to remove than scale on ordinary carbon steel. Alkali or salt bath descaling alone has been found incompletely effective on stainless steel unless followed by an acid pickling with a resultant risk of pitting or etching the basic metal. Ac'id presents the same problem with hot rolled carbon steel. Mechanical blasting effects an undesirable increase surface hardness on any steel, and may embed particles of foreign material in the metal surface, while wire brushing, if sufficiently harsh to accomplish descaling, adversely effects surface finish.

SUMMARY OF THE INVENTION According to the present invention the tenacious and relatively impervious scale formed on stainless steel during hot rolling and annealing, and during the annealing of cold rolled strip prior to further reduction, and the hot rolled scale on carbon steel, is rapidly and effectively removed by immersing the steel in a molten bath of salt of one of the compositions heretofore employed for descaling, and then, after a water quench,

flailing the surface of the strip with leaves of abrasive material each having one end attached to a rapidly rotating mandrel.

Where heavy scale is present, an additional, similar flailing operation may, desirably, precede immersion of the strip in the molten bath. This appears to fracture the scale to a sufficient extent to expedite the action of the molten bath on the scale.

It is of critical importance to the successful employment of the method of the present invention that the surfaces of the steel strip be flailed, that is, impacted by flexible leaves or strips of abrasive material attached at one end only to a propelling means such as a mandrel so as to swing freely. This avoids the hardening of the metal observed following descaling by means such as shot or sand blasting and also the impairment of the surface finish observed following abrasive descaling by other means. A means for flailing surfaces with flexible leaves of abrasive material which is suitable for employment in the method of the present invention is disclosed in U.S. Pat. No. 3,058,269.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hot mill and annealing scales consist of lower and higher oxides. When metal bearing such scale is immersed in a suitable molten salt bath which is at a tem perature of 300 to 900 F., both metal and scale are heated rapidly, and the unequal expansion of metal and scale causes the scale to crack. The salt then penetrates the cracks in the scale and, in the case of an oxidizing salt, oxidizes the lower oxides to form soluble salts and water causing the scale to swell and become soft and porous. In the case of a reducing salt, chemical reduction of metal oxides to the metallic state or to lower oxides takes place with similar disruption of the physical state of the scale.

The selection of the material composing the molten salt bath is dependent upon the composition of the metal to be treated and the nature of the processing to which it has been subjected, and the criteria for selection of the salt bath material are the same for the method of the present invention as for chemical-only descaling.

Examples of salt bath materials which may be selected for use in the method of the present invention are those of U.S. Pat. No. 3,260,619, marketed as Kolene DGS, or U.S. Pat. No. 3,121,026, marketed as ALKO, or U.S. Pat. No. 3,625,900, marketed as ALKO-N, or U.S. Pat. No. 3,254,0ll relating to ALKO, used with an electric current passed through the metal and the salt bath as described in said patent.

When quenching takes place, a violent activity occurs. Steam causes cracks in the scale to open further. Some scale is rinsed away and the remainder is loosened and conditioned for rapid removal by the flailing action of the abrasive leaves which, being flexible, are capable of removal of scale particles from pits in the metal surface, embedded carbon steel particles and other foreign substances from the metal surface. Opposite faces of the metal strip preferably are simultaneously flailed in the manner described by separate oppositely disposed abrasive ribbon assemblies.

The period of exposure of the scale-bearing steel to the molten salt bath, which has been up to 20 minutes for resistant scales removed by chemical treatment alone, need be only sufficient, according to the present invention, to bring the material up to the bath temperature and partially oxidize or reduce the lower oxides 7 present. The method of the present invention, theremoved continuously through the molten salt bath, the

water quench and the flailing machine.

Some examples, by way of illustration, follow.

FIRST EXAMPLE AlSl Type 304 stainless steel strip .070 gauge, 37 inches wide, annealed at a furnace annealing temperature of 2,050 E, may be passed, in continuous movement at a rate of about 10 feet per minute, first through a molten bath of Kolene DGS salt maintained at a temperature of about 860 F., then through a water quench bath maintained at a temperature of about 100 F., then through a water bath at room temperature, and finally its surfaces may be brought into contact with a pair of rapidly rotating abrasive leaf assemblies of the kind disclosed in Block US. Pat. No. 3,058,269, loaded with leaves of 80 grit, cloth carried, abrasive material, whereby both surfaces of the steel strip are flailed by said leaves of abrasive material extending radially from the mandrels disposed adjacent the steel strip.

Aluminum oxide resin bonded is a good example of the abrasive recommended. Another suitable abrasive is silicon carbide.

SECOND EXAMPLE Metal strip of the same description may be moved at an increased rate of about feet per minute through the above described molten salt bath, water quench, water bath and abrasive leaf assemblies, after which its surfaces may be passed, in the same continuous movement, into contact with a second pair of rapidly rotating abrasive leaf assemblies of the same construction but loaded with leaves of 150 grit, cloth carried abrasive material. After passage through the first of these abrasive ribbon assemblies, removal ofthe scale should be accomplished. A light haze may be visible on the surface of the metal strip, but after passage through the second of these abrasive ribbon assemblies, the surfaces of the metal strip should be clean.

THIRD EXAMPLE The opposite surfaces of steel strip having scale as a result of hot rolling and annealing, or hot rolling, or annealing, may be first passed into contact with rapidly rotating abrasive ribbon assemblies ofthe character described above, loaded with 80 grit, cloth carried, abrasive material. The strip may then be passed in a continuous movement, at a rate of 15 feet per minute, first through a molten salt bath ofKolene DGS salt maintained at a temperature of about 860 F., then through a water quench bath maintained at about 100 F., then through a water bath at room temperature. Finally, the strip may be passed in the same continuous movement through a second pair of rapidly rotating abrasive ribbon assemblies of the character described above, to complete the descaling and cleaning process.

FOURTH EXAMPLE The same, or generally equivalent salt bath and'treatment described in the foregoing examples, when utilized on some metallic formulations, and particularly in removing hot rolled scale from hot rolled carbon steel,

may be improved in action by introducing an electrical current into the salt bath to obtain rapid modification of the scale to make it more amenable to removal by the flailing system.

GENERAL The foregoing examples are by way of illustration, and not of limitation.

As previously mentioned, other molten salt baths may be used, and with differing temperatures. The time of immersion, and speed of travel, may vary depending upon the salt used, the temperature maintainied, and the character of the scale. In most cases the scale will be softened and loosened sufficiently for the flailing step more quickly than when an acid treatment accomplishes the final removal. The important thing is the elimination of an acid treatment and employment of the abrasive flailing step or steps to complete the removal of the softened and loosened scale.

Of course the flailing may be performed on only one surface at a time, but it is preferable to utilize the flailing mechanism in pairs, one above and one below, to perform the operation simultaneously.

Different grits of abrasive may be used, and a series of units may be utilized, with identical, or graduating, grit size, starting with a heavy grit and reducing down to a fine grit to achieve a polished surface.

In some applications it is advantageous to maintain the surface of the metal strip with a flushing action.

The flexibility of the abrasive leaves enables them to adapt to irregularities in the surface presented by scale until its complete removal. The nature of the attack by the abrasive particles is in the form of impact initially directed angularly toward the surface worked on followed by a yieldable wiping action.

The size and close spacing of the abrasive particles, coupled with the rapid succession of the leaves as they make sequential impact, assures a complete surface coverage, and penetration into any pits which may be present in the metal surface.

If any grind line appearance results from use of coarse grit, this may be reduced by oscillating the units. Moreover, a smoother finish, if desired, may be achieved by a follow up flailing with light grit.

Although we have herein shown and described our invention in what we have conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of our invention.

What is claimed is:

1. A method of descaling stainless steel or hot rolled steel or hot rolled carbon steel strip conta ning a scale on the surface of the steel or the hot rolled steel strip comprising the steps of immersing such strip in a bath of molten material at a temperature of the order of 300 to 900 F. and passing the strip continuously through the bath at a substantially constant rate of movement; the material of said bath being chemically inert with respect to the metal of said strip but chemically reactive with respect to constituents of scale carried by said strip and the immersion of such strip in said bath being for a sufficient period of time to crack the scale, thereafter quenching said strip at a temperature of approximately F. while said strip is moving at a substantially constant rate, and thereafter flailing the surface of said strip with flexible leaves of abrasive ma terial to remove the scale from the surface of said strip withouthardening the surfaceof the steel while said strip is moving at the substantially constant rate.

2. A method according to claim 1 in which said steps v of immersing the strip in the bath of molten material,

5 surface of said strip with flexible strips of abrasive material to mechanically crack the scale without hardening the surface of the steel.

4. A method according to claim 2 in which the molten material used is a salt.

5. A method according to claim 2 in which an electric current is introduced into the bath of molten material while the strip is immersed therein to facilitate the rate at which the scale becomes cracked.

6. A method of descaling a metal bearing on its surface scale created during rolling or annealing of the metal and without the use of acid or of work-hardening mechanical treatment, comprising the steps of immersing the metal in a bath of molten material which is chemically inert with respect to the metal but chemically reactive with respect to the constituents of the scale on the surface of the metal to soften and loosen the scale, passing the metal continuously through the bath of molten material at a substantially constant rate, thereafter quenching the treated surface of the metal to facilitate the loosening of the scale on the surface of the metal while the metal is being moved continuously at the substantially constant rate, and thereafter flailing the surface thus treated with captive particles of abrasive material on flaps moved past the surface to complete the removal of the scale from the surface of the metal without hardening the surface of the metal while the metal is being moved continuously at the substan tially constant rate.

7. A method according to claim 6 in which the flailing is done by providing a wheel of flexible abrasive leaves and disposing the flexible surfaces of the leaves in contact with the scale on the surface of the metal and rapidly rotating the wheel to cause the flexible surfaces of the layers sequentially to engage said surface of the metal while the metal is being moved continuously past the wheel at the substantially constant rate and in which the substantially constant rate of movement of the metal is at least 10 feet per minute.

8. A method as set forth in claim 1 in which the strip is exposed to the bath of molten material only for a sufficient period of time, as a result of the continuous movement of the strip through the bath at the substantially constant rate, to bring the material to the temperature of the bath and partially oxidize or reduce the lower oxides present.

9. A method as set forth in claim 7 in which the immersion of the metal in the bath is preceded by the steps of providing a wheel of flexible leaves and disposing the flexible surfaces of the leaves in contact with the scale on the surface of the metal and rapidly rotating the wheel to cause the flexible surfaces of the leaves sequentially to engage said surface of the metal while said metal is being moved continuously past the wheel at the substantially constant rate of at least 10 feet per minute.

10. A method as set forth in claim 7 in which the molten material is a molten salt.

1 l. The method set forth in claim 7 in which the flexible surfaces of the leaves engage the surfaces of the metal initially at an angle to the surface of the metal and then yieldingly wipe the surface of the metal as the metal is being moved continuously past the leaves at the substantially constant rate of at least 10 feet per minute.

12. The method set forth in claim 1 in which the flexible leaves initially engage the surface of the strip at an angle to the surface of the strip and then yieldingly wipe the surface of the strip as the strip is being moved continuously past the leaves at the substantially constant rate of at least 10 feet per minute.

13. A method as set forth in claim 7 in which an elec tric current is introduced into the bath of molten material during the immersion of the surface of the metal in the bath to facilitate the rate at which the molten material acts upon the scale.

* l: l= l l

Claims

1. A METHOD OF DESCALING STAINLESS STEEL OR HOT ROLLED STEEL OR HOT ROLLED STEEL STRIP CONTAINING A SCALE ON THE SURFACE OF THE STEEL OR THE HOT ROLLED STEEL STRIP COMPRISING THE STEPS OF IMMERSING SUCH STRIP IN A BATH OF MOLTEN MATERIAL AT A TEMPERATURE OF THE ORDER OF 300* TO 900*F. AND PASSING THE STRIP CONTINUOUSLY THROUGH THE BATH AT A SUBSTANTIALLY CONSTANT RATE OF MOVEMENT; THE MATERIAL OF SAID BATH BEING CHEMICALLY INERT WITH RESPECT TO THE METAL OF SAID STRIP BUT CHEMICALLY REACTIVE WITH RESPECT TO CONSTITUENTS OF SCALE CARRIED BY SAID STRIP AND THE IMMERSION OF SUCH STRIP IN SAID BATH BEING FOR A SUFFICIENT PERIOD OF TIME TO CRACK THE SCALE, THEREAFTER QUENCHING SAID STRIP AT A TEMPERATURE OF APPROXIMATELY 100*F. WHILE SAID STRIP IS MOVING AT A SUBSTANTIALLY CONSTANT RATE, AND THERE AFTER FLAILING THE SURFACE OF SAID STRIP WITH FLEXIBLE LEAVES OF ABRASIVE MATERIAL TO REMOVE THE SCALE FROM THE SURFACE OF SAID STRIP WITHOUT HARDENING THE SURFACE OF THE STEEL WHILE SAID STRIP IS MOVING AT THE SUBSTANTIALLY CONSTANT RATE.

2. A method according to claim 1 in which said steps of immersing the strip in the bath of molten material, quenching the strip and thereafter flailing the surface of the strip are performed during continuous movement of said strip at a rate of at least 10 feet per minute.

3. A method according to claim 2 in which said immersing step is preceded by a preliminary flailing of the surface of said strip with flexible strips of abrasive material to mechanically crack the scale without hardening the surface of the steel.

4. A method according to claim 2 in which the molten material used is a salt.

6. A method of descaling a metal bearing on its surface scale created during rolling or annealing of the metal and without the use of acid or of work-hardening mechanical treatment, comprising the steps of immersing the metal in a bath of molten material which is chemically inert with respect to the metal but chemically reactive with respect to the constituents of the scale on the surface of the metal to soften and loosen the scale, passing the metal continuously through the bath of molten material at a substantially constant rate, thereafter quenching the treated surface of the metal to facilitate the loosening of the scale on the surface of the metal while the metal is being moved continuously at the substantially constant rate, and thereafter flailing the surface thus treated with captive particles of abrasive material on flaps moved past the surface to complete the removal of the scale from the surface of the metal without hardening the surface of the metal while the metal is being moved continuously at the substantially constant rate.

11. The method set forth in claim 7 in which the flexible surfaces of the leaves engage the surfaces of the metal initially at an angle to the surface of the metal and then yieldingly wipe the surface of the metal as the metal is being moved continuously past the leaves at the substantially constant rate of at least 10 feet per minute.

13. A method as set forth in claim 7 in which an electric current is introduced into the bath of molten material during the immersion of the surface of the metal in the bath to facilitate the rate at which the molten material acts upon the scale.