Method for removing wustite scale

Abstract

Wustite scale is removed from iron or steel products by blasting fine grit from a centrifugal blast wheel which rotates at low speed, against the product at an angle thereto with the grit having sharp edges.

Claims

1. A method of removing wustite scale from an iron or steel product including the steps of feeding fine grit into a centrifugal blast wheel with the grit having sharp edges, rotating the blase wheel at low speed, projecting the grit at a nonperpendicular angle against the product, and utilizing the sharp edges of the grit to chip the wustite scale. 2. A method as set forth in claim 1 wherein the grit is projected at an angle between 45* and 75* with respect to the product. 3. A method as set forth in claim 2 wherein the angle is about 60*. 4. A method as set forth in claim 1 wherein the blast wheel is rotated at a speed of 1,500 rpm to 2,000 rpm. 5. A method as set forth in claim 4 wherein the blast wheel is rotated at a speed of about 1,750 rpm. 6. A method as set forth in claim 1 wherein the grit is less than 0.015 inches long. 7. A method as set forth in claim 6 wherein the grit is between 0.004 and 0.015 inches long. 8. A method as set forth in claim 6 wherein the grit is projected at an angle between 42* and 75* with respect to the project, and the blast wheel being rotated at a speed between 1, 500 rpm and 2,000 rpm. 9. A method as set forth in claim 8 wherein the grit is projected at an angle of about 60*, the blast wheel is rotated at a speed of about 1,750 rpm, and the grit is between 0.004 and 0.015 inches long. 10. A method as set forth in claim 8 wherein the wustite is formed by conversion from magnetite by heating under inert conditions in an inert atmosphere followed by rapid cooling. 11. A method as set forth in claim 6 wherein the product is passed under the blast wheel and is cleaned at about 1/2 HP/ft.2/min. 12. A method of removing wustite scale from an iron or steel product including the steps of feeding a grit having sharp edges into a centrifugal blast wheel, rotating the blast wheel at a speed sufficient to project the grit at a low velocity against the surface to be treated, projecting the grit at a non-perpendicular angle against the surface to be treated, and utilizing the sharp edges of the grit to chip the wustite scale. 13. A method as set forth in claim 12 wherein the grit is a fine grit and is projected at an angle between 45* and 75* with respect to the product. 14. A method as set forth in claim 13 wherein the angle is about 60*. 15. A method as set forth in claim 12 wherein the velocity of the projected grit is from about 160 to about 210 feet per second. 16. A method as set forth in claim 15 wherein the velocity of the projected grit is about 180 to about 200 feet per second. 17. A method as set forth in claim 12 wherein the grit is less than 0.015 inch long. 18. A method as set forth in claim 17 wherein the grit is between 0.004 and 0.015 inch long. 19. A method as set forth in claim 17 wherein the grit is projected at an angle between 45* and 75* with respect to the product, and the grit is projected at velocity of about 160 to about 210 feet per second. 20. A method as set forth in claim 19 wherein the grit is projected at an angle of about 60* at a velocity of about 180 to about 210 feet per second and the grit is about 0.004 and 0.015 inch long. 21. A method as set forth in claim 19 wherein the wustite is formed by conversion from magnetite by heating followed by rapid cooling. 22. A method as set forth in claim 13 wherein the grit has a hardness between about 58 and about 64 Rockwell (C scale).
Carpenter, Jr. et al. METHOD FOR REMOVING WUSTITE SCALE Inventors: James H. Carpenter, Jr.; Don B. Nalley, both of l-lagertown, Md. Assignee: The Carborundum Company, Niagara Falls, NY. Filed: Sept. 22, 1972 Appl. No.: 291,497 Related US. Application Data Division of Ser. No. 215,616, Jan. 5, 1972, Pat. No. 3,731,432, which is a continuation-in-part of Ser. No. 46,969, June 17, 1970, abandoned. [ 1 Sept. 3, 1974 [56] References Cited UNITED STATES PATENTS 2,162,139 6/1939 Unger 5l/9 2,345,942 4/1944 Lehman 51/321 2,605,596 8/1952 Uhri 51/321 2,692,458 10/1954 Lawrence 51/14 3,031,802 5/1962 Leliaert 51/9 3,166,841 1/1965 Gebhard 51/322 UX 3,368,308 2/1968 Physioc 51/9 Primary Examiner-Donald G. Kelly Attorney, Agent, or Firm-David E. Dougherty; William H. Holt [5 7] ABSTRACT Wustite scale is removed from iron or steel products by blasting fine grit from a centrifugal blast wheel which rotates at low speed, against the product at an angle thereto with the grit having sharp edges. 22 Claims, 11 Drawing Figures METHOD FOR REMOVING WUSTITE SCALE CROSS-REFERENCE TO RELATED APPLICATIONS Ser. No. 46,969, filed June 17, 1970, and now abanl doned. BACKGROUND OF THE INVENTION The descaling of hot rolled steel or iron strip or plate is normally done by pickling operations which utilize sulfuric and hydrochloric acid. Although the pickling method is quite effective and gives a good clean surface for cold rolling or die forming, the disposal of acid presents a serious problem. This problem is particularly acute considering the present emphasis on ecology. Because of the disadvantages of the pickling method, those skilled in the art have attempted to find satisfactory replacements which eliminate the acid disposal problem. A significant advancement in descaling steel is described in [1.8. Pat. No. 3,166,841. The method of the patent utilizes a scale conversion process wherein the sheet is heated to approximately 1,200F. At this temperature the magnetite (Fe O converts to wustite (FeO). The sheet is then rapidly cooled in air or in a neutral atmosphere so that the wustite will not reconvert to magnetite. This process is particularly advantageous since although magnetite is difficult to remove, wustite is brittle and may be more readily removed. For example, US. Pat. No. 3,166,841 suggests various means of wustite removal. These removal means include subjecting the converted scale to sandblasting after a bending operation. Although the above patent suggests the use of conventional sandblasting equipment for wustite removal, the actual use of such equipment in a conventional manner does not completely clean the wustite scale. By conventional manner is meant the use of a centrifugal blast wheel rotating at high speed, such as 2,250 rpm, to propel small shot perpendicularly against the plate to be cleaned. It is, therefore, desirable to achieve some means of wustite removal to render the process of U.S. Pat. No. 3,166,841 commercially feasible. SUMMARY OF INVENTION An object of this invention is to provide a method and apparatus for descaling to replace the conventional pickling process and apparatus. A further object of this invention is to provide such a method and apparatus which utilizes improved centrifugal blasting techniques for the removal of converted wustite. In accordance with this invention wustite scale is removed from iron or steel products by blasting fine grit from a centrifugal blast wheel which rotates at low speed against the product at an angle thereto, with the grit having sharp edges. Preferred parameters for the novel method is to project the grit at an anglebetween 45 and 75 and preferably 60 with the blast wheel being rotated at a speed of 1,500 rpm to 2,000 rpm and preferably at 1,750 rpm. The grit is also preferably between 0.004 and 0.015 inch long. THE DRAWINGS FIG. 1 is a block diagram showing the general process steps of this invention; FIG. 2 is a schematic showing of a prior art type arrangement wherein the blast particles are directed perpendicular to the plate; FIG. 3 is a schematic showing similar to FIG. 2 of the inventive method; FIG. 4 is a schematic illustration of a prior art type arrangement wherein spherical shot is used as the blast particles; FIG. 5 is a schematic arrangement similar to FIG. 4 of the inventive method which utilizes sharp angled grit; FIG. 6 is a side elevational view showing a portion of the apparatus of the present invention in section; FIG. 7 is an end elevational view of the apparatus of FIG. 6 in section; FIG. 8 is a partial end elevational view of the apparatus of FIG. 7 in section; FIG. 9 is a detailed view along line 99 of FIG. 8; FIG. 10 is a detailed view in section of a portion of the apparatus; and FIG. 1 1 is a schematic view showing the blast pattern. DETAILED DESCRIPTION FIG. 1 illustrates the general process steps utilized in this invention. As indicated therein the general sequence of steps is similar to that suggested by US. Pat. No. 3,166,841, the details of which are incorporated herein by reference thereto. The iron or steel product containing magnetite scale is first conveyed into a furnace 10 wherein, in accordance with the patent, the magnetite is converted to wustite. The product is then quickly cooled in cooling chamber 12 so that the wustite will not reconvert. Thereafter, the wustite scale is removed at blast station 14. The blasting techniques utilized at blast station 14 are the particular subject matter of this invention. Conventional blasting methods would be to employ a centrifugal blast wheel which projects spherical shot perpendicular to the plate 16 with the blast wheel being rotated at a speed of about 2,250 rpm. The inventive method differs from conventional practices in the utilization of a low speed centrifugal blast wheel which projects sharp fine grit at an angle to the plate 16. Each of these features in the inventive method contributes to obtaining the overall effectiveness desirable in a method which would supplant the conventional pickling operation. Accordingly, FIGS. 2 and 4 are included to illustrate the results which would be obtained if certain of the prior art practices were attempted to be included in the novel method. FIG. 4, for example illustrates the affects obtained from using spherical shot rather than the fine sharp angled grit. As indicated therein the shot 18 strikes the wustite scale 20 but tends to embed a portion 22 of the scale at the bottom of the crater 24 formed by the shot. Because of this embedding action subsequent impacts from other shot does not remove this scale. FIG. 5 shows how with the inventive method the disadvantages illustrated in FIG. 4 are obviated. As indicated therein sharp angular grit 26 is utilzied for the wustite removal. Tests have shown that this sharp angular grit hitting with sufficient force to remove the wustite scale 28 was effective in getting the sheet or plate 16 100 percent clean. Although the theory behind this invention is not completely understood apparently the sharp grit digs the scale out instead of driving it into the surface. It has also been found that the grit should have a fineness of about 0.004 inch to 0.015 inch long and should be propelled with just enough force to remove the scale. Accordingly, the present invention recognizes that since wustite is more easily removed than magnetite, it is possible to drastically reduce the wheel from 2,250 rpm to a speed in the range of 1,500 to 2,000 rpm and preferably 1,750 rpm. Having determined the impact energy required, the clean-rate is thus dependent upon the number of impacts. By slowing the rotation the number of impacts per horsepower is thereby increased by the ratio of (2,250/1,750)? Thus, for example, if the impacts per square inch per second is 1,000 for 2,250 rpm, at a given horsepower the impacts at 1,750 is (1,000) (2,250/1,750) or 1,650 impacts/in. /sec. Accordingly, the theoretical cleaning rate for the wustite would be increased approximately at 1.6 times the rate at standard wheel speed. This, coupled with the fact that wustite is easier to remove than magnetite gives a cleaning rate of four times faster than the standard rate for removing magnetite scale. Although the chipping action of the wustite by use of the fine sharp angled grit and the lower speed of the centrifugal blast wheel solved certain of the problems, an additional problem remained. Specifically, throwing 1.6 times more fine grit particles per HP created a blinding problem. This problem is particularly troublesome if the conventional practice were employed such as illustrated in FIG. 2 wherein the blast wheel 30 projects the particles perpendicular to the plate 16. The problem, however, was solved by utilizing thearrangement illustrated in FIG. 3. As indicated therein, the blast wheel 30 is positioned for projecting the grit at an angle with respect to the sheet or plate 16, with the direction of plate travel being indicated by the arrow 36. The specific angle is in the range of 4575 and preferably at the 60 illustrated therein. By this arrangement the grit is projected in a wave 32 which then ricochets as designated by the reference numeral 34 clear of the succeeding waves of abrasive from the vanes. The attainment of this angular projection is preferably accomplished by utilizing a recently developed direct drive wheel which is illustrated and described in copending application Ser. No. 687,701, filed Dec. 4, 1967, and now US. Pat. No. 3,521,406, the details of which are incorporated herein by reference thereto. This universally mounted wheel is readily adapted for obtaining the proper angular projection of the grit. It has been found that with the improved wustite removal method it is possible to completely clean converted steel at a rate of k HP/ft /min. Additionally, as noted above by slowing down the wheel speed to 1,750 rpm the number of impacts per HP is increased by 1.6 times, and angling the wheel 60 to the horizontal results in preventing abrasive blinding. If smut remains on the cleaned plate it can be removed by high pressure water spray although a dry method of removal is preferable. Among the various advantages with the inventive method is the ecological advantage of eliminating the acid disposal problem. Additionally, less floor space is required, as well as reduced costs as compared with a pickle line. Moreover, the direct wheel drive enables less expense mounting and movement mechanism and thus less expense installation. By locating the wheel 30 inside a cabinet in combination with the reduces speed, results in significantly less noise. Finally the maintenance is less at reduced shot velocity. As hereinbefore discussed, it was discovered that it is possible to reduce the speed of a conventional or standard blasting wheel, such as the type having about a 19-k inch diameter, from a normal speed of 2,250 rpms which corresponds to a velocity of projected grit of 240 to about 260 feet per second to a low speed in the range of 1,500 rpms to 2,000 rpms which correspond to a velocity of projected shot from about 160 feet per second to 210 feet per second. Preferably, the wheel is rotated at about 1,750 rpms which corresponds to a speed of projected shot of about 180 to 200 feet per second. Of course, it is to be realized that a decrease in the diameter of the wheel requires a proportional faster rotation to attain the desired velocity of projected shot while an increase in the diameter of the wheel requires a proportional decrease in speed of rotation of the wheel to obtain a proper velocity of projected particles. Thus, it is possible to rotate a centrifugal throwing wheel of very large diameter at a slow speed to obtain a high velocity of projected particles. In the process of the present invention, it is intended that the velocity of projected particles be low. It is also preferred to have grit particles which are harder than conventional shot. It is believed that the harder grit cleans better due to sharper and harder edges. The soft grit seems to pick up microscopic particles of scale and redeposit them on the plate; whereas, the hard grit does not seem to pick up the scale so that redepositing of the scale is precluded. Abrasive particles which have been found suitable for the practice of the present invention range in hardness between and Rockwell (C scale), and preferably between 58 and 64 Rockwell. FIGS. 6 and 7 are illustrative of apparatus that can be used to carry out the process of the present invention. A treating chamber or housing 42 of an elongated rectangular shape is sufficiently wide to accommodate a workpiece 44 such as strips of sheet metal of various widths. Each end wall of the housing 42 is provided with an opening 46 so that workpiece 44 may be continuously fed in a longitudinal direction through the housing 42. Sealing means can be conveniently provided around each opening to prevent the escape of abrasive particles. The workpiece 44 is supported by suitable means such as a plurality of rollers 48 within the housing 42. As illustrated in FIG. 6, the workpiece 44 is a coil 50 of sheet metal which is mounted for rotation. The coil 50 is unwound and drawn through the treating chamber 42 by suitable means (not shown) such as a conventional coiling apparatus at the exit end. A plurality of elongated support members 52 extend across the treating chamber 42 in a direction transverse to the movement of the workpiece 44 therethrough. Support members 52 are provided above and below the path of travel of the workpiece 44 through the treating chamber 42 for supporting blast wheels 54. As illustrated in detail in FIG. 8 and FIG. 10,'each of the support members 52 is slidably supported by housing 42 for back and forth movement in a direction transverse to the direction of movement of the workpiece 44 through the treating chamber 42. Each mounting member 52 includes portions extending exterior to respective side walls 58 of housing 42 through suitable openings 60. Outer housings 62 which support rollers 56 provide closures at either end of the support member 52. A plurality of rollers 56 which are rotatably mounted exterior to the housing 42 support respective exterior portions of supporting members 52. It is also contemplated that the support members 52 can be mounted in a cantilevered fashion at one end exterior to the housing 42. As illustrated in detail in FIG. 8, suitable means such as hydraulic power device 64 is provided for moving the support member 52 back and forth. One end of the power device 64 is mounted to an end wall 66 of one of the outer housings 62 while the other end is mounted to the support member 52. Other conventional means can be used to effect the movement of the support member 52 such as a driven pinion gear connected to a rack mounted on an exterior portion of the support member 52. Seals 65 are conveniently provided for preventing the escape of ricocheting abrasive particles from the treating chamber 42 and reduce the wear of parts which would otherwise be exposed. Some of the support members 52 are positioned above and others below the path of travel of the workpiece 44. Each support member 52 is spaced from and lies in a plane substantially parallel to the surface of the workpiece 44 to be treated. A blasting wheel 54 which projects abrasive particles toward the workpiece 44 is a path transverse to the direction of movement of the workpiece 44 is mounted on each support member 52. The blasting wheels 54 are mounted to the support members 52 in such a manner that combined blasting patterns of the blasting wheels 54 provide a substantially even treatment of both the top and bottom working surface of workpiece 44. As illustrated in FIG. 11, the blasting patterns or paths of a pair of blasting wheels 54 overlap to give complete coverage of the work. An upper pair of blasting wheels 54 are preferably rotated in opposite directions to effect an overlapping blast pattern and even treatment of the workpiece 44. Also, the lower pair of the blasting wheels 54 are similarly positioned and rotated. The coverage of the working surfaces of the workpiece 44 can be maximized while the amount of projected abrasive wasted or not striking the working surfaces can be minimized by moving the support members 52 back and forth to widen or narrow the combined paths of the projected abrasive. To narrow the combined paths, a pair of the blasting wheels 54 directed at a particular surface being treated, either upper or lower, are moved toward each other. To widen the combined paths, the same blasting wheels 54 are away from each other. FIG. 8 illustrates the movement of a blasting wheel 54 from a solid line to a dotted line position and the resulting shift in blast pattern to accommodate workpiece 44 of a smaller width. It is contemplated that this adjustment could be automatically made by providing means at an opening 46 through which the workpiece 44 is fed to sense the width of the workpiece 44. The sensing means would transmit the information to a controller which would actuate the hydraulic power devices 64 to move the support member 52 to adjust the blasting pattern. Each of the blasting or centrifugal throwing wheels 54 rotate about an axis within a plane substantially vertical and parallel to the direction of movement of the workpiece 44 through the treating chamber 42 on a shaft of a motor which is enclosed in a motor housing 70. During treatment the axis of rotation is non-parallel to the surface of the workpiece 44 to be treated, preferably at an angle between 15 and 45, and even more preferably at an angle of about 30 with respect to the surface. As illustrated in detail in FIG. 9, this mounting arrangement is achieved by pivotably mounting the motor or motor housing on brackets 72 which depend from the support member 52. The motor 72 is mounted for movement about a substantially horizontal axis which is traverse or substantially normal to the direction of travel of the workpiece 44. Means are provided for releasably securing the motor 70 to the brackets 72 to fix the angle at which the abrasive particles are thrown at the work surface of the workpiece 44. As illustrated in FIG. 9, this is in the form of a nut 74 which is tightened to hold the motor 70 in a fixed position. Other means such as a notched disc and plunger arrangement for manual operation or spaced stops and a limit pin for power actuation can be conveniently provided. As illustrated in FIG. 6, the treating chamber or housing 42 includes a plurality of compartments each having a pair of blasting wheels 54 mounted above and below the workpiece 44. Each compartment includes a main hopper 76 which delivers abrasive particles to conduits 78 which are connected to the blasting wheels 54. Spent abrasive particles are collected in the tapered portion 80 at the bottom of the treating chamber 42 and ultimately recycled back to hopper 76 through conventional elevating device 82 and separator 84. The separator 84 removes an unusable portion from a usuable portion of abrasive particles which is recycled. Preferably a plurality of devices 84 for blowing debris and spent abrasive off the top surface of the workpiece 44 are provided. These devices 84 discharge air under pressure onto the top surface so the debris and spent abrasive fall into the tapered portion 80. The abrasive particles projected toward the bottom surface of the workpiece 44 and fall into the tapered portion 80. While preferred embodiments of this invention have been described and illustrated, it is to be recognized thatmodifications and variations thereof may be made without departing from the spirit and scope of this invention as described in the appended claims. What is claimed is: 1. A method of removing wustite scale from an iron or steel product including the steps of feeding fine grit into a centrifugal blast wheel with the grit having sharp edges, rotating the blase wheel at low speed, projecting the grit at a nonperpendicular angle against the product, and utilizing the sharp edges of the grit to chip the wustite scale. 2. A method as set forth in claim 1 wherein the grit is projected at an angle between 45 and with respect to the product. 3. A method as set forth in claim 2 wherein the angle is about 60. 4. A method as set forth in claim 1 wherein the blast wheel is rotated at a speed of 1,500 rpm to 2,000 rpm. 5. A method as set forth in claim 4 wherein the blast wheel is rotated at a speed of about 1,750 rpm. 6. A method as set forth in claim 1 wherein the grit is less than 0.015 inches long. 7. A method as set forth in claim 6 wherein the grit is between 0.004 and 0.015 inches long. 8. A method as set forth in claim 6 wherein the grit is projected at an angle between 42 and 75 with respect to the project, and the blast wheel being rotated at a speed between 1,500 rpm and 2,000 rpm. 9. A method as set forth in claim 8 wherein the grit is projected at an angle of about 60, the blast wheel is rotated at a speed of about 1,750 rpm, and the grit is between 0.004 and 0.015 inches long. 10. A method as set forth in claim 8 wherein the wustite is formed by conversion from magnetite by heating under inert conditions in an inert atmosphere followed by rapid cooling. 11. A method as set forth in claim 6 wherein the product is passed under the blast wheel and is cleaned at about /z HP/ft. /min. 12. A method of removing wustite scale from an iron or steel product including the steps of feeding a grit having sharp edges into a centrifugal blast wheel, rotating the blast wheel at a speed sufficient to project the grit at a low velocity against the surface to be treated, projecting the grit at a non-perpendicular angle against the surface to be treated, and utilizing the sharp edges of the grit to chip the wustite scale. 13. A method as set forth in claim 12 wherein the grit is a fine grit and is projected at an angle between 45 and with respect to the product. 14. A method as set forth in claim 13 wherein the angle is about 60. 15. A method as set forth in claim 12 wherein the velocity of the projected grit is from about to about 210 feet per second. 16. A method as set forth in claim 15 wherein the velocity of the projected grit is about to about 200 feet per second. 17. A method as set forth in claim 12 wherein the grit is less than 0.015 inch long. 18. A method as set forth in claim 17 wherein the grit is between 0.004 and 0.015 inch long. 19. A method as set forth in claim 17 wherein the grit is projected at an angle between 45 and 75 with respect to the product, and the grit is projected at velocity of about 160 to about 210 feet per second. 20. A method as set forth in claim 19 wherein the grit is projected at an angle of about 60 at a velocity of about 180 to about 210 feet per second and the grit is about 0.004 and 0.015 inch long. 21. A method as set forth in claim 19 wherein the wustite is formed by conversion from magnetite by heating followed by rapid cooling. 22. A method as set forth in claim 13 wherein the grit has a hardness between about 58 and about 64 Rockwell (C scale).

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